Additively Manufactured Alnico Permanent Magnet Materials—A Review

Abstract

Additive manufacturing offers manufacturing flexibility for intricate components and also allows for precise control over the microstructure. This review paper explores the current state of the art in additive manufacturing techniques for Alnico permanent magnets, emphasizing the notable advantages and challenges associated with this innovative approach. Both the LPBF and L-DED processes have demonstrated promising results in fabricating Alnico with magnetic properties comparable with conventionally processed samples. The optimization of process parameters successfully reduced porosity and cracking in the LPBF processing of Alnico. The review further explored the significance of additive manufacturing process parameter optimization in managing the temperature gradient and solidification rate for a desired microstructure and enhanced magnetic properties. Other potential additive manufacturing methods suitable for the fabrication of Alnico were discussed, along with the challenges associated with the process. The insights provided also highlight how additive manufacturing holds the potential to replace post-processing techniques like solutionization, magnetic annealing, and tempering often necessary in Alnico production.

1. Introduction

Alnico is a permanent magnetic material containing mainly Fe, Co, and Ni along with other alloying elements such as Al, Ti, and Cu [1,2]. The high-temperature stability (up to 400 °C) of Alnico alloys makes them suitable for replacing rare earth permanent magnets in applications such as traction motors [3]. Alnico alloys find applications in electric motors/generators, MRI machines, guitar pickups, and aviation and aerospace for navigational instruments due to their high-temperature stability [3,4,5,6]. Permanent magnets (PM) primarily consist of Alnico alloys, ferrites, and rare earth containing magnets like NdFeB and SmCo [2]. Permanent magnetic materials have high coercivity (H_c) ranging from 0.5 to 35 kOe, a remanence (B_r) of 0.7–1.5 T, and a maximum energy product (BH_max) of 0.75–54.04 [7]. Permanent magnets are widely used in electric motors, wind turbines, generators, and alternators to convert electrical energy into mechanical energy and vice versa [6]. Figure 1 presents a schematic graphical representation describing the magnetic properties of a material showing the relationship between magnetic field strength (H) and magnetic flux density (B), which is termed as hysteresis plot or loop. For a permanent magnet, key parameters include remanence (residual magnetism at zero field), coercivity (field strength needed for demagnetization), and BH_max (maximum magnetic energy storage). A high remanence value ensures sustained magnetization, while high coercivity indicates resilience against demagnetization, and BH_max reflects the magnet’s energy storage capacity. Thus, ideal permanent magnets exhibit high coercivity, remanence, and BH_max for optimal performance.

Rare earth permanent magnets have a high coercivity of 11.30–34 kOe and maximum energy product (15.08–54.04 MGOe) compared with non-rare earth permanent magnets like Alnico and ferrites [9]. Meanwhile, the remanence of Alnico alloys is comparable with that of rare earth magnets. The remanence of Alnico alloys ranges from 0.7 to 1.3 T, comparable with those of rare earth magnets, which typically have remanences of 0.7 to 1.5 T, but higher than those of other permanent magnetic materials like SmCo and ferrites [10,11]. The coercivity of Alnico alloys falls within the range of 0.5 to 1.89 kOe, lower than those of NdFeB and SmCo magnets, which have a coercivity ranging from 15.08 to 54.04 kOe and 15.08 to 32.67 kOe, respectively, but higher than those of ferrites, which typically range from 0.75 to 5.66 kOe [9,11].

Table 1. Magnetic characteristics of various types of permanent magnets fabricated using conventional methods [9].

Permanent Magnetic Alloy Family	Br (T)	Hc (kOe)	BHmax (MGOe)	Curie Temperature (${\mathit{T}}_{\mathit{c}}$) (°C)
Alnico	0.70–1.30	0.50–1.89	1.25–9.40	820–860
Ferrite	0.20–0.46	2.4–4.5	0.75–5.66	450
SmCo	0.70–1.20	12.5–35.0	15.08–32.67	700
Nd-Fe-B	0.70–1.50	11.30–34.00	15.08–54.04	310–370

presents the magnetic properties of various permanent magnets along with their curie temperatures, the temperature above which certain materials lose their permanent magnetic properties [9].

Alnico is a permanent magnetic alloy developed in 1931 by Mishima [12]. Initially, it contained aluminum, nickel, and iron, and later on, cobalt was added to increase the coercivity and maximum energy product. Among all Alnico alloys, Alnico-8H stands out with a high coercivity of 1.9 kOe [13,14,15]. Meanwhile, Alnico-5 boasts a high remanence of 1.28 T, and Alnico-9 exhibits a high BH_max of 9.0 MGOe [16,17]. The composition and magnetic properties of various types of Alnico alloys are listed in

Table 2. Various types of Alnico alloys and their constituent composition with solidification range and magnetic characteristics [12].

Alloy (Cast)	Al (wt%)	Ni (wt%)	Co (wt%)	Ti (wt%)	Cu (wt%)	BHmax (MGOe)	Br (T)	Hc (kOe)	$\mathit{\Delta }$${\mathit{T}}_0$ (Solidification Range) (°C)
Alnico-1	12	21	5	-	3	1.4	0.72	0.47	24
Alnico-2	10	19	13	-	3	1.7	0.75	0.56	27
Alnico-3	12	25	-	-	3	1.35	0.70	0.48	35
Alnico-5	8	14	24	-	3	5.5	1.28	0.64	24
Alnico-6	8	16	24	-	3	3.9	1.05	0.78	26
Alnico-8	7	15	35	5	4	5.3	0.82	1.65	220
Alnico-8H	8	14	38	8	3	5.0	0.72	1.90	283
Alnico-9	7	15	35	5	4	9.0	1.06	1.50	220

[12].

Traction motors require permanent magnets that can operate at temperatures above 180 °C [3]. Rare earth permanent magnets have poor temperature stability (example: Nd-based PM) and require the addition of Dysprosium (Dy) to operate at temperatures above 200 °C, which is used in traction motors [3,18]. The limited availability of Dy resources and the relatively high cost restrict the use of Nd-based PM in traction motors; thus, considerable interest has been developed to seek alternatives to Nd-based PM [7]. On the contrary, magnetic properties of Alnico alloys are unaffected by temperature changes of up to 400 °C [18]. Alnico alloys have a curie temperature of 820–860 °C and have high temperature (>400 °C) stability, but Alnico alloys have lower coercivity (H_c) (0.5–1.89 kOe) and BH_max (1.25–9.40 MGOe) than NdFeB [2,19,20]. Therefore, Alnico can be a potential alloy for replacing NdFeB, if the coercivity (H_c) and BH_max of the alloy are improved. In addition, Alnico has better corrosion resistance than rare earth permanent magnets, which may be a critical requirement in applications like electric motors [21].

At temperatures above 1200 °C, Alnico exists completely as an $\alpha$ phase with a body center cubic (BCC) crystal structure. A schematic of an Alnico equilibrium phase diagram is presented in Figure 2 [22]. Accordingly, upon cooling to below 1200 °C, a face center cubic (FCC) gamma ($\gamma$) phase is formed in the temperature range of 1175–850 °C [23,24,25]. The $\gamma$ phase, which is formed in the temperature range of 1175–850 °C, transforms to ${\alpha }_{\gamma }$ below 860 °C. When it is cooled to below 850 °C, the $\alpha$ phase spinodally decomposes into ${\alpha }_1$ (BCC-B2) and ${\alpha }_2$ (BCC L2₁) [26]. The alloying elements Ti, Co (up to 35 wt.%), and Nb (up to 2 wt.%) improve the coercivity, remanence, and BH_max by increasing the volume fraction of the ${\alpha }_1$ phase [27,28,29,30]. On the other hand, Ti avoids the $\gamma$ phase formation, and Nb reduces the chipping and brittleness [28,30]. The effects of various alloying elements on Alnico are listed in

Table 3. Effects of various alloying elements on Alnico alloy [27,28,29,30].

Alloying Element	Effect on Alnico Alloy
Ti	1. Increases the volume fraction of the ${\alpha }_1$ phase. 2. Avoids the formation of the gamma phase. 3. Supports the formation of Cu-Ni bridges between the ${\alpha }_1$ phase.
Co	1. Coercivity and remanence increased with the increase in Co of up to 35 wt% beyond which it decreases. 2. Increases volume fraction of the ${\alpha }_1$ phase.
Nb	1. Coercivity increased with increase in Nb up to 2 wt% beyond which it decreases. 2. Remanence and BHmax highest at 1.5 wt%. 3. Increase in the ${\alpha }_1$ phase. 4. Reduces chipping and brittleness.

.

Sintering and casting are two processes that are mainly used for manufacturing Alnico alloys [32]. Sintering can produce simple shapes that are in small and medium size [33]. Generally, the parts that are processed by sintering possess a good surface finish. Meanwhile, casting is used to produce complex shapes that are large in size when compared with that produced by sintering, and it is an expensive process [34]. After casting, a rough surface is required to be finished for smoothness by machining to obtain desired magnetic properties [32]. Furthermore, as Alnico alloys are hard and brittle, conventional machining would introduce cracks on the surface [32]. Even though sintering can give relatively good surface finished parts, it cannot fabricate the components of complex shapes and large sizes. On the contrary, the alloys that are manufactured by additive manufacturing (AM) can produce a good surface finish when compared with the parts manufactured by casting [35]. Fabricating Alnico via sintering or casting poses challenges. Sintering limits the component size and intricate designs. On the contrary, although casting is suitable for the fabrication of large components, the process yields rough surfaces prone to cracking during post-casting machining and difficult to create near net shapes with fine details [36,37]. Both processes are associated with totally different thermokinetics as sintering is conducted at high temperatures below the melting temperature of an alloy and mostly involve the solid–solid state diffusion-based consolidation of the material [32,38]. Meanwhile, casting is conducted at a temperature above the melting temperature and involves liquid–solid transformation. Such distinctly different physical processes are associated with distinctly different thermokinetics, thereby generating different microstructures and resultant magnetic properties. As AM can be used to produce near net shape complex geometries of various sizes, it can be adopted to fabricate Alnico alloys at a relatively low cost [39,40]. In addition, Alnico alloys are expected to possess better magnetic properties when a columnar grain morphology is evolved during processing. Although a columnar microstructure in Alnico in conventional process like casting is possible to achieve, it is an expensive process requiring multi-cavity mold casting [3,41]. Meanwhile, the generation of a columnar microstructure elongated along a <100> direction through the optimization of the AM process parameters holds high potential [41].

The primary objective of this review paper is to conduct a thorough exploration of the current advancements in the additive manufacturing of Alnico. This involves the optimization of additive manufacturing process parameters to achieve Alnico materials with high density (7.25 g/cc), free from porosity and cracks. The paper provides a detailed analysis of the microstructural features of additively fabricated samples. Additionally, it examines the influence of grain size and crystallographic texture on magnetic properties. The paper also seeks to discuss the potential benefits of using alternative additive manufacturing techniques. Through this examination, the paper aims to provide insights into the promising future of additive manufacturing in advancing the fabrication of Alnico magnets. Conventional methods (sintering/casting) often face challenges such as design flexibility, size, and material waste. Additive manufacturing offers the opportunity to overcome these challenges by enabling complex geometries and reduced material waste. Furthermore, the paper discusses approaches to control the grain morphology and the grain size by manipulating the temperature gradient and solidification rate through the key process parameters such as laser scan speed and laser power during laser-based additive manufacturing. Additionally, the impact of a laser scanning strategy on the development of crystallographic texture is discussed. The discussion on other possible potential alternative AM techniques like binder jet printing and extrusion-based process for the fabrication of Alnico expands the scope of possibilities for Alnico fabrication. Lastly, the review also provides the impact of incorporating a magnetic field during additive manufacturing processes. This aspect is particularly important as it sheds light on how magnetic properties, grain size, and the overall quality of material can be enhanced through innovative manufacturing approaches. The review provides valuable guidelines to enhance the fabrication process of Alnico magnets, which can improve efficiency, reduce costs, and yield better control over the final product quality. The review highlights the potential for enhancing the magnetic properties of Alnico magnets through an additive manufacturing process. This information is valuable for industries relying on Alnico magnets for various applications such as motors, sensors and magnetic devices. Through its comprehensive analysis and discussion, the review article identifies key areas for future research and development in the field of additive manufacturing of Alnico.

2. Additive Manufacturing

Additive manufacturing is a process of joining materials layer by layer to fabricate parts from 3D model data. The additive manufacturing processes used for the fabrication of a metal-based composition can be categorized into three types based on temperature and time of interaction during the fabrication. Solid-state fabrication takes place at 0.7 times the liquidus temperature, sintering fabrication occurs at the liquidus temperature, and in a fusion-based process, fabrication takes place well above the liquidus temperature [42]. Figure 3 illustrates the various types of additive manufacturing processes classified based on energy source–material interaction time and temperature. Fusion-based metal AM processes utilize various energy sources such as lasers, electron beam, or electric arc, in combination with feedstock materials like powder, wire, or sheets [43]. These energy sources are precisely focused on the selected region of the feedstock, causing localized melting or sintering to build up the desired metal component layer by layer. Additive manufacturing finds applications in various industries, including aerospace, automotive, medical, and tooling [44]. The properties of the metallic parts produced in AM are influenced by various process parameters [10]. Among the various types of AM processes, laser powder bed fusion (LPBF) and laser direct energy deposition (L-DED) are the most commonly used techniques. LPBF, which is also known as selective laser melting (SLM), is a popular AM technique that involves spreading a thin layer of powder on a build platform. A high-power laser (>200 W) then selectively melts and fuses the powder particles together, layer by layer. Through this gradual buildup, the final 3D part is formed [45]. Figure 4 illustrates the laser powder bed fusion process [42]. Powder size distribution and packing directly influence the density of the printed part [46]. The processing parameters like laser power and scan speed play a pivotal role in achieving the desired quality and integrity of the final product. The input energy density (fluence) in J/${\mathrm{mm}}^2$ of LPBF is typically expressed as follows:

$$E_v=\frac{P}{vh}$$

(1)

where

P—laser power (W)

v—scan speed (mm/s)

h—hatch spacing (mm)

Figure 3. Correlation between temperature and the duration of energy source interaction with material in various additive manufacturing processes (Based on the concept) [42].

Figure 3. Correlation between temperature and the duration of energy source interaction with material in various additive manufacturing processes (Based on the concept) [42].

Figure 4. Schematic of the LPBF process [47].

Figure 4. Schematic of the LPBF process [47].

Laser-directed energy deposition (L-DED) involves direct injection of the precursor powder into a laser beam while both are synchronously moved for complex configuration onto a specific area of the substrate [45]. Simultaneously, it melts the feedstock material, which is in the form of powder, and subsequently, the molten material is deposited and solidified onto the substrate, as the laser beam advances [10]. A schematic of L-DED is presented in Figure 5. One notable advantage of L-DED is its compatibility with conventional subtractive processes, facilitating seamless integration with machining operation [45]. This means that, after the deposition process, the resulting part can undergo further machining steps, such as milling or turning, to achieve the desired final shape, dimensions, and surface finish [32]. Both the LPBF and L-DED processes have high cooling rates, typically on the order of 10³–10⁶ K/s, inducing rapid solidification [47]. This rapid solidification process significantly influences the phase formation and microstructure in the fabricated material [48]. Both L-DED and LPBF fabricate the component in a layer-by-layer fashion; there is a difference in the fundamental nature of laser beam–material interaction. In the L-DED process, both the laser beam and substrate onto which powder is being deposited are in motion, whereas in LPBF, only the laser beam is in motion [43]. The LPBF process has rapid cooling rates of the order of ${10}^6$ K/s and ${10}^3$ K/s in the L-DED process; this difference due to thermokinetics associated with the process leads to a finer grain size in the LPBF process and a coarser grain size in L-DED [42,43,47].

An ideal permanent magnet should exhibit highly anisotropic properties to ensure optimal magnetic performance. A columnar grain morphology with a <100> crystallographic texture is particularly desirable for superior magnetic characteristics like coercivity, remanence, and BH_max [23,30] in BCC metals/alloys. The <100> texture aligns the crystal lattice in a specific direction, allowing for enhanced magnetic alignment and coherence [16,34,49]. Consequently, permanent magnets with this crystallographic texture are highly sought after for a wide range of applications. In additive manufacturing, the crystallographic texture and grain morphology of the produced materials are profoundly affected by the thermokinetics of the process [47,50,51,52,53]. White et al. successfully fabricated Alnico-8 by L-DED, and after processing, properties were comparable with the sintered Alnico-8H [4,54]. Rottmann et al. fabricated Alnico-8 using LPBF coupled with a heated stage (no post-processing), and their findings showed magnetic properties equal to the commercially available Alnico [34]. Following section details, the fabrication of Alnico using AM techniques (LPBF and L-DED), and how their grain morphology can be tuned from equiaxed to columnar transition by varying the process parameters (laser power and scan speed) to control the temperature gradient and solidification rate. Thus, a comprehensive grasp of the additive manufacturing process is instrumental in achieving precise control over solidification dynamics. This, in turn, empowers researchers and engineers to exercise command over the grain morphology and crystallographic texture of diverse materials.

2.1. Additive Manufacturing of Alnico

Among various AM processes available, two of the most frequently utilized techniques for producing Alnico components are LPBF and L-DED.

Table 4. Table summarizing AM process parameters for different Alnico variants and their compositions [4,34,54,55,56,57,58,59,60].

Alloy	Composition (wt%)	AM Process	Laser Power	Scan Speed	Notes
Alnico-8	33Fe-7Al-13Ni- 38Co-7Ti-3Cu	L-DED	50–200 W	0.203 mm/s (motion only along Z-axis)	1. Melt pool size is kept constant by varying the laser power. 2. Stainless steel is used as a substrate. 3. Pre-alloyed powders were used for printing.
Alnico-8	30Fe-7Al-14Ni- 38.5Co-7.5Ti-3Cu	L-DED	50–200 W	0.203 mm/s (motion only along Z-axis)	1. Melt pool size is kept constant by varying the laser power. 2. Samples are printed on Alnico-9, and stainless steel is used as a substrate. 3. Pre-alloyed powders were used for printing.
Alnico-8H (Co lean)	32Fe-7.5Al-17Ni- 33Co-8Ti-3Cu	EBM/PBF	3.5–14 mA	200–800 mm/s	1. Pre-alloyed powders were used for printing. 2. Cobalt content is reduced from 38 to 29 wt.% when compared with Alnico-8H. 3. Geometrical density of 7.27 g/cc. 4. Equiaxed microstructure is observed throughout the sample.
Alnico-8H	28Fe-7.5Al-15Ni- 38Co-8Ti-3.8Cu	L-DED	100 W	1500 steps/s	1. Pre-alloyed powders were used for printing. 2. Melt pool size is kept constant by varying the laser power between 70 and 200 W. 3. Geometrical density of 7.25 g/cc. 4. Equiaxed microstructure is observed throughout the sample.
Alnico-8H (Co lean)	32Fe-7.5Al-17Ni- 33Co-8Ti-3Cu	L-DED	100 W	1600 steps/s	1. Pre-alloyed powders were used for printing. 2. Cobalt content is reduced from 38 to 29 wt.% when compared with Alnico-8H. 3. Geometrical density of 7.25 g/cc.
Alnico-35	55Fe-12Ni-25Co- 2Mo-3Cu	LPBF	190 W	600–800 mm/s	1. Pre-alloyed powders were used for printing.
Alnico-5	50.4Fe-24Co-13Ni- 7Al-3Cu	LPBF	110–190 W	600–800 mm/s	1. Pre-alloyed powders were used for printing. 2. Sound sample is obtained at 190 W and 800 mm/s.
Alnico-8	29Fe-38.5Co-7.5Al- 14.5Ni-7.5Ti-3Cu	LPBF	170 W	600 mm/s (hatch distance of 0.06 mm)	1. Pre-alloyed powders were used for printing. 2. Printing is performed on a heated stage at a temperature of around 800 °C. 3. Precipitation of the gamma phase is observed along the grain boundaries.
Alnico-5	48Fe-24Co-12Ni- 14Al-2Cu	LPBF	150–190 W	400–1200 mm/s	1. Pre-alloyed powders were used for printing. 2. Defects like holes and cracks were observed. 3. Maximum density is achieved at 170 W and 800 mm/s.

provides a comprehensive overview of the various Alnico alloys fabricated through L-DED and LPBF, along with detailed information on their compositions and associated process parameters. This valuable resource serves as a reference point for researchers and manufacturers, offering insights into a wide array of Alnico materials that can be effectively produced using these advanced AM methods.

A study conducted by Fuhui et al. successfully generated Alnico-5 samples through LPBF with a heated stage at 160 °C, achieving material density in excess of the 98% threshold [55,56]. However, despite these noteworthy density enhancements, the samples displayed surface defects, notably porosity and cracks in LPBF-processed Alnico-5 [56,57]. A similar scenario was observed when Alnico-8 was processed via LPBF with a heated stage at 800 °C, with evident porosity [34,59]. In contrast, Shakirov et al. demonstrated that meticulous optimization of process parameters, specifically employing settings at 190 W and 700 mm/s, resulted in Alnico-5 samples with reduced porosity and cracks [57]. Remarkably, Alnico-8 samples crafted using the L-DED technique exhibited a defect-free profile with a geometrical density of 7.25 g/cc [4]. These findings emphasize the pivotal role of fine tuning of process parameters, producing defect-free Alnico components. In LPBF-fabricated Alnico 5 with a heated stage of 160 °C, although increasing the laser energy density initially enhanced the compressive strength, followed by reduction after 65 J/mm³ laser input energy, it led to hardness peaking at 100 J/mm³, revealing a complex interplay between input energy density and mechanical properties [56].

Alnico alloys fabricated using the laser-directed energy deposition (L-DED) process have demonstrated good density. However, samples produced with Laser Powder Bed Fusion (LPBF) initially exhibited cracks, which were mitigated through the optimization of process parameters. All additively fabricated samples exhibited an equiaxed microstructure. Additive manufacturing holds promise for further enhancements and processing of Alnico materials.

2.2. Microstructure of Additively Manufactured Alnico

The type of microstructure and morphology and the size of grains within Alnico alloys affect the magnetic properties of Alnico. Alnico-8H (28Fe-7.5Al-15Ni-38Co-8Ti-3Cu) and Alnico-8H with reduced Co composition (32Fe-7.5Al-17Ni-33Co-8Ti-3Cu) printed using the L-DED process with a laser power of 160 W and 3000 steps/s and 100 W and 1600 steps/s, respectively, have shown the evolution of an equiaxed microstructure throughout the samples in Figure 6 [4].

Similarly, when the same Alnico-8H (with reduced Co) was processed using electron beam melting (EBM), an equiaxed microstructure emerged, accompanied by the presence of the gamma phase along the grain boundaries [4]. Meanwhile, L-DED-processed samples of Alnico-8H and Alnico-8H (reduced Co content) did not show any gamma phase [4]. This gamma phase, formed due to the slower cooling rate associated with EBM, poses challenges to the magnetic properties of Alnico alloys [4]. It disrupts the magnetic domain alignment, which in turn can negatively affect critical properties such as coercivity and remanence. This disparity underscores the critical role of processing methods in shaping the microstructural characteristics and subsequently influencing the magnetic performance of Alnico materials.

As mentioned earlier, in Alnico alloys, columnar grain morphology with a <100> crystallographic texture is particularly desirable for superior magnetic characteristics [23,30]. In additive manufacturing, the crystallographic texture and grain morphology of the produced materials are profoundly affected by process parameters. The process parameters, such as platform temperature, laser power, scan speed, layer thickness, and cooling rate, have a direct influence on the thermokinetics of the process [42,48]. These process parameters determine the solidification rate and melt pool size and shape and temperature distribution within the melt pool [47]. As a result, process parameters play a crucial role in dictating the crystallographic orientation of the grains and the overall microstructure within the AM-processed material. Thus, through proper optimization of these process parameters, one can obtain the desired microstructure and properties of the final component. By understanding and manipulating these influences, AM techniques can be used to tailor the crystallographic texture and grain morphology, leading to materials with desired mechanical and magnetic properties for specific applications.

2.3. Effects of Grain Size and Morphological and Crystallographic Texture on Magnetic Properties

In the realm of magnetic materials, the microstructural features significantly affect the magnetic properties. Understanding how various aspects of the microstructure influence magnetic properties is essential. Several factors, including grain size, grain morphology, and crystallographic texture, have a significant impact on the magnetic properties of magnets. The subsequent sections will delve into a detailed discussion of their individual effects.

According to Herzer’s findings, in the case of soft magnetic alloys like Fe6.5Si and 50NiFe, coercivity experiences a decrease as the grain size surpasses 100 µm; in contrast, for amorphous alloys and nanocrystalline Fe-Cu-Nb-Si-B alloys, coercivity is lower when the grain size is less than 15 nm and increases as the grain size grows up to 40 nm [61,62]. In permanent magnets such as NdFeB, it has been observed that coercivity decreases with an increase in grain size [63]. This phenomenon can be attributed to the nucleation of reversed domains occurring at surface defects; in cases where the grain size is small, the probability of surface defects is reduced, thereby increasing coercivity [63]. Thus, high coercivity (>1.89 kOe) can be obtained in the case of Alnico when the grain size is reduced. Herzer’s plot, which represents the effect of grain size on coercivity, is presented in Figure 7 [61,62,64]. Controlling the grain size can be achieved by managing the cooling rate [65,66]. In additive manufacturing, the cooling rate is controllable through adjustments in process parameters, the effects of which will be explored in the subsequent sections. The dependence of grain size on cooling rate and thermokinetic factors like temperature gradient (G) and solidification rate (R) is expressed as follows [67]:

$$d=a(\Delta t^n)={(G·R)}^n$$

(2)

where

d—grain size (μm)

$\Delta$t—cooling rate (K/s)

G—temperature gradient (K/m)

R—solidification rate (m/s)

G · R—cooling rate (K/s)

a, n—fitting factors

Figure 7. Effect of grain size on coercivity [61,62,64,68].

Figure 7. Effect of grain size on coercivity [61,62,64,68].

In Charre’s study, it was observed that aligning columnar grains along the <100> direction in BCC alloys like Alnico resulted in improved magnetic properties such as coercivity, remanence, and BH_max when compared with cast Alnico-8 given in

Table 5. Magnetic properties of Alnico-8 for an equiaxed and columnar microstructure [69].

Alnico-8 Alloy	Intrinsic Coercivity (kOe)	Remanence (Tesla)	BHmax (MGOe)
Columnar grains with <100> crystallographic orientation	1.63	1.08	9.75
Equiaxed microstructure	1.55	0.910	5.5

, which typically exhibits an equiaxed microstructure [69]. In the case of MnAl permanent magnets, coercivity is notably higher for ultra-fine columnar grains when compared with single crystal grains; it is observed that coercivity increased from 0.16 kOe in a single crystal to 1.47 kOe in the case of ultra-fine columnar grains as the width of the columnar grains decreases [70]. For NdFeB, it has been found that coercivity is improved for an equiaxed microstructure from 13.8 kOe to 17.34 kOe for a microstructure consisting of columnar and equiaxed grains, whereas remanence is increased from 0.68 T for an equiaxed microstructure to 0.72 T for an equiaxed+columnar microstructure [71]. Figure 8 shows the effect of grain morphology and size on coercivity. In the case of NdFeB, coercivity and remanence demonstrate enhancement when the columnar grains exhibit a shorter length [71,72]. Therefore, it is imperative to achieve ultra-fine (sub-microns) short columnar grains that align along the <100> crystallographic texture in the case of BCC alloys like Alnico to optimize magnetic performance. Hence, it is essential to develop a comprehensive understanding of the impact of grain size, morphology, and crystallographic texture in the context of Alnico, as this knowledge can lead to further enhancements in magnetic properties.

2.4. Additive Manufacturing Process-Driven Morphological and Crystallographic Textures of Grain in Alnico

Thermokinetic parameters such as temperature gradient (G) and solidification rate (R) and cooling rate (G × R) associated with any fusion process play significant roles in dictating the type of grain morphology evolved in Alnico alloys. Specifically, when aiming for the formation of a columnar microstructure to optimize magnetic properties such as coercivity (H_c), remanence (B_r), and BH_max, the G/R that represents the grain morphology becomes a critical factor, and it is termed as grain morphology factor. This parameter influences the growth rate and morphological evolution of grains within the material, ultimately impacting its magnetic performance. Achieving the desired columnar microstructure in Alnico alloys relies on the meticulous control of the G/R ratio, offering a pathway to tailoring these materials for superior magnetic characteristics. A high G/R, which denotes a high temperature gradient (G) combined with a low solidification rate (R), fosters the formation of a columnar microstructure [73]. Meanwhile, a low G/R with a low temperature gradient (G) and a rapid solidification rate (R) leads to the development of an equiaxed microstructure [73]. Figure 9 presents the G vs. R plot, showing the effect of G/R on the grain morphology in CoCrFeMnNi high-entropy alloy [74]. Apart from G/R, nucleation density also has an effect on the type of microstructure that is obtained. The high density of nucleation sites (>${10}^{12}$/${\mathrm{m}}^3$) leads to the development of an equiaxed microstructure [75]. G × R, which is a cooling rate, will drive the size of the resultant grains as coarse or fine grains [65,66]. Figure 10 presents the G vs. R, depicting the effect of G × R (cooling rate) on the grain size in Ni45 alloy [65]. A high cooling rate likely generates finer grains, whereas low cooling is likely to yield coarser grains [65,66]. The types of microstructures that can be obtained for various combinations of G and R are schematically presented in Figure 11.

The solidification range of a material is one of the critical factors along with the kinetics of the solidification (thermal gradient, solidification growth velocity, cooling rate), local composition, etc., which exerts a substantial influence on the type of microstructure that ultimately develops [75,76]. The solidification ranges of various Alnico alloys are given in Table 2 (obtained from the CALPHAD database). The solidification range, solidification kinetics, and local composition together play a pivotal role in determining whether a material will form equiaxed or columnar grains during the solidification process. Understanding and controlling all these thermokinetics and composition-related parameters are therefore essential for tailoring microstructures to achieve specific material characteristics and performance attributes. Although the combined effects of all these parameters are very complex, several researchers have attempted to break down such complexity by approaching it in the most simplistic manner by separating the effect of each of these parameters [57,66,73,75,76,77,78]. Majority of these efforts have emphasized the effect of solidification range on grain morphologies. When the solidification range is high, there is a greater amount of undercooling provided during solidification, which leads to the high nucleation density; thus, alloys with a high solidification range lead to the development of an equiaxed microstructure [76]. The Alnico-8H alloy with a high solidification range of 283 °C (obtained from the CALPHAD database) is likely to develop an equiaxed microstructure due to the high nucleation density in the AM samples. This explains why both Alnico-8H and Alnico-8H (with reduced Co content), when fabricated using the L-DED process, exhibited a consistent equiaxed microstructure across the entirety of the sample. Meanwhile, Alnico-5, whose solidification range is around 24 °C (obtained from the CALPHAD database), developed an elongated grain morphology when processed through LPBF [57]. The large thermal gradient and localized melt pool characteristics of the LPBF process frequently result in a substantial anisotropic, columnar grain morphology [47,79].

The equation below provides the numerical relationship among various thermokinetic and physical parameters affecting the grain morphology during the solidification process. Thus, the effect of G/R and N_o on the volume fraction of equiaxed grains is expressed as follows [75,79,80]:

$${\varphi }_{AM}=1-exp(\frac{-4·\pi ·N_o}{3·{[\sqrt{\frac{{nG}^n}{(R·a)·(n+1)}}]}^3})$$

(3)

where a and n are material constants, $N_o$ is the nucleation density, and G is the temperature gradient (K/m). Equation (2) can be used when the solidification rate is high and the temperature gradient is of the order of ${10}^6$ K/m [79]. When the volume fraction of equiaxed grains is greater than 49%, it will have an equiaxed microstructure, and when the volume fraction of equiaxed grains is less than 0.66%, it is a columnar microstructure [77]. Meanwhile, in between 0.66% and 49%, it will be a mixed grain morphology consisting of equiaxed + columnar grains. There is a critical value of G/R above or below which the microstructure changes from columnar to equiaxed or equiaxed to columnar [78]. As G/R increases above a critical value k, the microstructure changes from equiaxed to columnar. The equation below gives the effects of various parameters on the critical value [75,78]:

$$\frac{G^n}{R}>k$$

(4)

where k is a critical factor above or below which it will be a columnar or equiaxed microstructure, as follows:

$$k=a{(\frac{8.6·\Delta T_o·N_o^{1/3}}{n+1})}^n$$

(5)

where a and n are material constants, $N_o$ is the nucleation density, and $\Delta$$T_o$ is the solidification range.

Especially, in the realm of additive manufacturing, the ability to shape grain morphology was greatly influenced by aspects other than inherent material properties such as solidification range. Such aspects included but were not limited to the thermokinetics of the solidification (thermal gradient, solidification growth velocity, cooling rate), which in turn are influenced by synthesis/fabrication/manufacturing process parameters. Hence, a notable avenue for exerting control over grain morphology lies in the precise manipulation of process parameters. During AM processing, by strategically manipulating parameters like laser fluence, laser scanning speed, and precursor material layer thickness, the growth, morphology, and arrangement of grains within the material can be influenced. This deliberate parameter tuning can facilitate the tailoring of grain morphologies to align with specific performance criteria and application requirements. An increased laser scan speed reduces laser residence time, thereby leading to a higher temperature gradient G value [77]. Meanwhile, reduced laser fluence leads to an increase in temperature gradient G [77]. Thus, by lowering the laser fluence and increasing the laser scan speed, it is possible to achieve a thermal gradient for the generation of the columnar grain morphology. In line with this, Alnico-8 alloy fabrication using the L-DED process, with a constant melt pool size achieved by power variation from 50 W to 200 W, revealed a distinct columnar-like grain morphology in its central region [54]. This phenomenon can be attributed to the reduction in laser power to 50 W to maintain the melt pool size constant, a measure that significantly increases the temperature gradient within the material. On the contrary, when the substrate temperature is kept at a higher level, it results in a reduction in the temperature gradient, thereby fostering the formation of an equiaxed microstructure [81]. Thus, the maintenance of a higher substrate temperature serves as a pivotal factor in promoting a more uniform and equiaxed grain morphology within the material. Accordingly, the Alnico-8 alloy, processed using LPBF with a heated stage, exhibited a consistent equiaxed grain morphology throughout the entirety of the sample (Figure 12) [34]. In summary, through proper optimization of AM process parameters, it is possible to tailor the microstructure of Alnico for desired columnar grains, which is otherwise an expensive process in the case of conventional processes like casting. Thus, AM emerges as a versatile tool not only for manufacturing complex geometries but also for engineering microstructural features, further enhancing the adaptability and versatility of the technology in diverse industries.

The intricacies of crystallographic texture and morphology are profoundly influenced by the thermokinetics of the process itself and the selected laser scanning strategy [53,82]. Furthermore, the chosen laser scanning strategy, which dictates how the material is deposited and fused, plays a crucial role in determining the orientation and size of grains within the final product. The thermokinetics, encompassing the temperature profiles, thermal gradients, cooling rates, and heat distribution, directly influence the growth and alignment of the crystallographic texture of grains. Understanding and optimizing these factors are paramount in harnessing the full potential of AM. Consequently, the mastery of the creation of the crystallographic texture and morphological orientation of grains during AM holds the key to unlocking unprecedented levels of performance and versatility in this cutting-edge manufacturing technique. In the realm of Alnico alloys, achieving a columnar grain morphology characterized by a <100> crystallographic texture stands as a fundamental pursuit to enhance their magnetic properties. This specific texture preference plays a pivotal role in optimization key parameters such as coercivity, remanence, and BH_max. Generally, in the context of a powder-bed-fusion-type AM of metals, the solidification process within the melt pool is characterized by the growth of columnar cells. These cells exhibit a tendency to elongate parallel to the direction of heat flow, which, in this scenario, runs nearly perpendicular to the tangential direction of the melt pool boundary [50,51,52]. Thus, the crystallographic texture developed depends on process parameters like laser power, laser scan speed, laser beam diameter, and precursor material layer thickness, which together influence the melt pool size and shape [53,83].

When working with Ti-15Mo-5Zr-3Al and introducing variations in the process parameters during AM printing, distinct crystallographic textures were observed [83]. However, it is important to note that, in addition to these parameters, the laser beam scanning strategy employed plays an important role in shaping the type of crystallographic texture that evolves. Whether the scanning is performed exclusively in the unidirectional or bidirectional profoundly affects the morphological orientation and crystallographic texture of grains of the material being AM-processed [53]. This control over the crystallographic texture and morphological orientation through a scanning strategy adds a layer of complexity to the AM process, offering engineers and researchers a powerful tool to fine-tune material properties for specific applications. During the AM processing of Ti-15Mo-5Zr-3Al, exclusively unidirectional scanning resulted in the generation of <001> and <110> crystallographic textures along both the laser scanning direction and the build direction orthogonal to the laser scanning direction, respectively [53,82,84,85,86]. However, when the bidirectional laser scanning was conducted, the resulting crystallographic texture predominantly aligned with <001> [53,82,84,85,86]. Therefore, through the meticulous optimization of process parameters and the strategic choice of a scanning strategy, it is possible to achieve the desired crystallographic texture within a material. This deliberate control over the microstructure provides a means to tailor enhanced material properties.

Magnetic properties are better when the permanent magnets have columnar or fine-grained structures, which can be achieved through the control of the grain morphology factor (G/R) or cooling rate (G × R) in the additive manufacturing process. These factors can be influenced to some extent by adjusting process parameters such as laser power and scan speed. Additionally, magnetic properties benefit from a specific crystallographic texture, which can be controlled through the selection of scan strategies during fabrication.

3. Post-Additive Manufacturing Thermal Treatments and Magnetic Properties of Alnico

Components that are fabricated using AM, casting, and sintering are required to be subjected to an essential post-process multistep heat treatment involving solutionization, (magnetic) annealing, and tempering to enhance the magnetic properties, as schematically presented for Alnico in Figure 13. The first step in such heat treatment involves solutionization, where the samples are heated to a specific temperature to dissolve the unwanted phases evolved during the above-listed fabrication processes to homogenize the alloy composition. After solutionization, samples are subjected to annealing or magnetic annealing (MA), followed by the final treatment of tempering. These post-fabrication/processing heat treatments, collectively known as full heat treatment (FHT), have a significant impact on both the magnetic properties and microstructure of magnetic materials, including Alnico magnets. Their effects are discussed in detail below:

3.1. Solutionization

The solutionization heat treatment process involves heating the Alnico samples to 1250 °C and maintaining them at this temperature for about 30 min under vacuum conditions, followed by rapid quenching in an oil bath [26]. Samples were subjected to solutionization to attain homogenization, as at this high temperature (1250 °C), Alnico exists as a single-phase bcc $\alpha$ structure [1,5]. Furthermore, as Alnico is composed of six elements (Table 2), the segregation of any of these elements can deteriorate its magnetic properties. Rapid quenching from 1250 °C prevents the formation of the fcc $\gamma$ phase, which is detrimental to the magnetic properties of Alnico. As the $\gamma$ phase is stable in the temperature range of 850–1175 °C, by rapid quenching, its formation is avoided [87]. Thus, this controlled heat treatment process plays a critical role in optimizing the magnetic properties and the microstructure of Alnico. Inherently, the LPBF process is associated with high cooling rates of the order of ${10}^5$ to ${10}^7$ K/s [88]; thus, the evolution of the fcc $\gamma$ phase can be avoided in the LPBF process, thereby minimizing or eliminating the need for the solutionization process and providing a possibility of directly going for an annealing treatment.

3.2. Magnetic Annealing

After the samples were subjected to solutionization, samples are subsequently annealed in the temperature range of 800–840 °C in the magnetic field. Annealing in the presence of a magnetic field leads to anisotropic magnetic properties, whereas annealing in the absence of a magnetic field leads to isotropic magnetic properties in Alnico magnets. Figure 14a,b show the intrinsic coercivity and remanence of Alnico-8H annealing performed without a magnetic field and with a magnetic field [5]. The research demonstrated that magnetic annealing is highly effective in enhancing the magnetic properties of Alnico magnets when conducted at a temperature close to the onset of spinodal decomposition and below the curie temperature. Figure 14 shows the effect of annealing temperature on coercivity and remanence [5,89]. During the magnetic annealing of Alnico, the BCC $\alpha$ phase spinodally decomposes into ${\alpha }_1$ and ${\alpha }_2$ phases. Spinodal decomposition is a mechanism by which a single thermodynamic phase spontaneously separates into two phases without the nucleation.

Table 6. The phases observed in Alnico samples fabricated through various processes for both the as-processed and post-heat treatment conditions [1,4,34,54,55,56,57,58,59,91].

Processing	As Processed (Microstructure)	Solutionization (Microstructure)	Magnetic Annealing (at 830 °C) (Microstructure)
Conventional process (Sintering/Casting)	$\alpha$ phase + $\gamma$ phase	$\alpha$ phase	${\alpha }_1$ phase + ${\alpha }_2$ phase
Additive manufacturing (LPBF/L-DED)	$\alpha$ phase	$\alpha$ phase	${\alpha }_1$ phase + ${\alpha }_2$ phase
LPBF with heated stage	${\alpha }_1$ phase + ${\alpha }_2$ phase + ${\alpha }_{\gamma }$	-	-

displays the phases identified in Alnico across various fabricated samples, indicating both the as-processed and heat-treated conditions. Alnico initially exhibits an $\alpha$ microstructure, which transforms into ${\alpha }_1$ and ${\alpha }_2$ phases after magnetic annealing (annealing). Specifically, in the Alnico samples processed using additive manufacturing with a heated stage, the as-processed state reveals the presence of ${\alpha }_1$ and ${\alpha }_2$ microstructures [34]. The phase diagram of a pseudo-binary alloy such as Alnico-8, composed of ${\alpha }_1$ and ${\alpha }_2$ phases with a miscibility gap, is schematically presented in Figure 15. In alloys that exhibit a miscibility gap, the formation of a two-phase microstructure can be achieved through either nucleation and growth or spinodal decomposition [90]. Nucleation and growth take place when a small amount of undercooling is provided, whereas spinodal decomposition occurs when a larger amount of undercooling is provided, and it involves a homogeneous transformation throughout the alloy via continuous growth of initial small compositional fluctuations [90]. When the Alnico sample is annealed at 840 °C or a lower temperature in the presence of a magnetic field, the two-phase microstructure is formed by a spinodal decomposition process. Meanwhile, when it is annealed in the magnetic field at a temperature greater than 840 °C, the two-phase microstructure is formed by the nucleation and growth process [5]. When a magnetic field is applied, the coherent spinodal line shifts upwards, as shown in Figure 15 [5]. The mechanism of transformation changes from nucleation and growth to spinodal decomposition in the presence of a magnetic field due the elongation of a ferromagnetic phase (${\alpha }_1$) along the field direction, which changes the strain energy [5]. Thus, the phase/microstructure evolved during magnetic annealing is very sensitive to the temperature of annealing and the magnetic field applied.

Effect of Annealing-Derived Phases on Magnetic Properties

The phases ${\alpha }_1$ and ${\alpha }_2$ evolved in Alnico during annealing possess B2 and $L{2}_1$ crystal structures, respectively, as shown in Figure 16 [3]. The ${\alpha }_1$ phase is rich in Fe and Co, whereas the ${\alpha }_2$ phase is rich in Ni, Al, and Ti [29]. The ${\alpha }_1$ phase is ferromagnetic in nature, whereas the ${\alpha }_2$ phase is paramagnetic in nature [13]. The TEM microstructure of LPBF-printed Alnico-8 with a heated stage containing the ${\alpha }_1$ and ${\alpha }_2$ phases is presented in Figure 17 [5]. Magnetic properties in Alnico are induced due to the shape anisotropy, and the ferromagnetic ${\alpha }_1$ phase is elongated in the applied magnetic field direction [92,93,94]. The ${\alpha }_1$ phase is present in the matrix of the ${\alpha }_2$ phase.

The coercivity of Alnico is typically significantly influenced by the presence or absence of Cu bridges between the ${\alpha }_1$ phases [29]. Generally, Cu- or Ni-rich bridges are formed between the two ${\alpha }_1$ phases. The concentration of Cu is greater than that of Ni (Cu > Ni) in the Cu bridge, and the concentration of Ni is greater than that in Cu (Ni > Cu) in the Ni bridge [29,95]. The composition of Alnico affects the formation of bridges between the ${\alpha }_1$ phases [29]. In a study on an Alnico alloy by Zhou et al., with the increase in magnetic annealing temperature, the magnetic properties continued to improve up to a temperature of 840 °C, and with further increase in magnetic annealing temperature beyond 840 °C, the coercivity reduced due to the disruption of the well-defined mosaic pattern [5]. Accordingly, Figure 18 shows the effect of magnetic annealing temperature on spinodally decomposed phases. In another study on an Alnico alloy by Zhou et al., it was observed that although the magnetic annealing performed at 840 °C under 1 tesla(T) magnetic field increased coercivity with the increase in magnetic annealing time from 30 s to 10 min, the increase in magnetic annealing time did not change the volume fraction of the ${\alpha }_1$ phase; the microstructural variation with magnetic annealing time is shown in Figure 18 [5,26]. However, the size and shape of the ${\alpha }_1$ phase tend to impact the magnetic properties of Alnico, as elongated and small-diameter (high aspect ratio > 20) ${\alpha }_1$ phase grains provide improved magnetic properties [2]. Considering this aspect, in commercial Alnico alloys, the aspect ratio of the ${\alpha }_1$ phase is maintained in the range of 0.6–10 nm [3,7]. The magnetic properties both ${\alpha }_1$ and ${\alpha }_2$ should be pure; that is, ${\alpha }_1$ should be free from Al, Ni, and Ti and ${\alpha }_2$ should be free from Fe and Co [2].

Furthermore, the magnetic characteristics exhibited by Alnico-8 processed through the L-DED method were on par with those of the conventionally sintered samples [54]. Meanwhile, in the case of Alnico-8H, its intrinsic coercivity closely resembles that of sintered and cast Alnico-8H [4]. Considering its ability to preserve magnetic properties comparable with those achieved through traditional sintering and casting methods, L-DED emerges as a highly suitable technique for the processing of Alnico-8 and Alnico-8H materials. Figure 19 depicts the B–H loop of laser-directed energy-deposited Alnico-8 in comparison with the Alnico-8 processed by casting. The B–H loop of L-DED-processed Alnico-8 closely resembles that of Alnico-8 processed through casting. This endorsement stems from the capacity of L-DED to maintain intrinsic coercivity levels, crucial for magnetic applications involving intricate magnetic components, while offering the advantages of additive manufacturing, such as design flexibility (tailored geometries) and reduced material wastage with enhanced manufacturing efficiency. In fact, Alnico-8, printed using the LPBF technique with a 800 °C heated stage, provided a coercivity of approximately 0.6 kOe even without the need for a post-process heat treatment [34]. This coercivity value remains well within the typical range exhibited by commercial Alnico alloys (Table 2), highlighting the promise of LPBF as a viable manufacturing method for achieving magnetic properties comparable with established industry standards. Hence, AM holds the potential to eliminate the necessity for a post-processing heat treatment. This advancement has the capacity to not only streamline production processes but also enhance the magnetic properties of the resulting materials, promising a new frontier in Alnico magnetic material fabrication.

Table 7. Table summarizing the post- processing parameters and magnetic properties of AM-processed Alnico [4,34,54,55,56,57,58,59,60].

Alloy	Composition (wt%)	AM Process	Solutionization (°C)	Magnetic Annealing (°C)	Tempering (°C)	Hcj (kOe)	BHmax (MGOe)	Notes
Alnico-8	33Fe-7Al- 13Ni-38Co- 7Ti-3Cu	L-DED	1250 30 min	840 10 min, 1T	650–5 h 580–5 h	1.83	6	L-DED-processed samples have shown Hc comparable with the sintered AlNiCo-8H and BHmax higher than the sintered Alnico-8H.
Alnico-8	30Fe-7Al- 14Ni-38.5Co- 7.5Ti-3Cu	L-DED	1250 30 min	840 9 min,1T	670–1.5 h 650, 620, 580, 550, 520, at each temperature for 4 h	2.03	4.8	Samples printed on stainless steel substrate has shown high coercivity and BHmax when compared with samples printed on AlNiCo-9 substrate.
Alnico-8H (Co lean)	32Fe-7.5Al- 17Ni-33Co- 8Ti-3Cu	EBM/PBF	1210 10 min	830–870 1–8 min, 1 T	650–5 h 580–5 h	1.6	-	When compared with L-DED samples, EBM/PBF samples have lower magnetic properties.
Alnico-8H	28Fe-7.5Al- 15Ni-38Co- 8Ti-3.8Cu	L-DED	1210 10 min	830–870 1–8 min,1 T	650–5 h 580–5 h	2.1	4.9	The magnetic properties of printed samples are similar to those of sintered and cast Alnico magnets.
Alnico-8H (Co lean)	32Fe-7.5Al- 17Ni-33Co- 8Ti-3Cu	L-DED	1210 10 min	830–870 1–8 min, 1 T	650–5 h 580–5 h	2.1	4.2	The magnetic properties of printed samples are similar to those of sintered and higher than the cast Alnico magnets.
Alnico-5	50.4Fe-24Co- 13Ni-7Al-3Cu	LPBF	From 1290 °C cooling at a rate of 14.5/min	20 min cooling in electromagnet to 400 °C	610–3 h 590–8 h 570–15 h 550–20 h	0.49	4.5	Hc, Br and BHmax increased with  increasing laser power and scan speed.
Alnico-8	29Fe-38.5Co- 7.5Al-7.5Ti- 14.5Ni-3Cu	LPBF (Heated Stage)	-	-	-	0.6	-	The magnetic properties of printed samples are similar to those of sintered Alnico magnets.
Alnico-5	48Fe-24Co- 14Al-12Ni-2Cu	LPBF	1250 15 min	-	800–10  min 650–3 h 600–8 h 550–10 h	0.48	1.46	BHmax is increased with the increase in laser power and scan speed.

provides a comprehensive summary of the magnetic properties exhibited by several additively manufactured Alnico alloys. Alongside these magnetic characteristics, the table also provides detailed insights into the post-processing parameters applied, including solutionization and magnetic annealing parameters, which have a significant impact on the final magnetic properties of these materials.

3.3. Tempering

In processing of Alnico alloys, tempering holds significant importance as a crucial step for further improving the magnetic properties, especially coercivity. Tempering, which is also known as low-temperature draw, is performed in the temperature range of 650–550 °C. Multistage tempering has shown to produce better magnetic properties when compared with single-step tempering, as shown in Figure 14d [96]. Generally, a low-temperature multistage draw is performed at 650 °C for 4 h, 600 °C for 6 h, and finally for about 1 h at 550 °C [1]. The complete heat treatment process including the tempering phase is shown in Figure 13.

Alnico magnets fabricated using additive manufacturing techniques exhibit magnetic properties comparable with conventionally processed Alnico. The magnetic properties of Alnico can be further improved by controlling the aspect ratio and purity of the ${\alpha }_1$ phase. Those produced with an LPBF-heated stage demonstrate magnetic properties within the range of commercially available Alnico, even without a post-heat treatment. This suggests that additive manufacturing holds promise for potentially eliminating the need for post-heat treatment processes.

4. Potential Additive Manufacturing Techniques for the Fabrication of Alnico

Besides casting, sintering is another conventional technique used for manufacturing Alnico alloys. Considering Alnico’s suitability for sintering, it opens the potential for utilizing alternative AM methods like binder jet printing and extrusion-based processes in the production of Alnico alloys.

4.1. Binder Jet Printing and Extrusion-Based Additive Manufacturing

In the binder jet printing (BJP) process, the printhead selectively deposits a liquid binding agent onto a powder bed, thereby fusing the powder particles together to create a solid object [97]. The process is repeated layer by layer, until the entire desired object is completed. Following binder jet printing, the part produced is subjected to curing, followed by sintering [98]. Paranthaman et al. successfully produced near-net-shape NdFeB permanent magnets using the binder jetting technique; the intrinsic coercivity of the fabricated NdFeB part is 9 kOe, which is the same as that of the original NdFeB powder [99]. Additionally, a Fe-Si soft magnetic alloy was created using binder jet printing, with a remarkable density of 99% following the sintering process [100]. Hence, binder jet printing holds potential as a practical method for producing near-net-shape Alnico components.

Extrusion-based additive manufacturing refers to a fabrication process in which material is deposited layer by layer through an extrusion nozzle to build a three-dimensional object [101]. Ling Li and colleagues successfully manufactured NdFeB permanent magnets by extruding a mixture of NdFeB and Nylon pellets through a nozzle at an operating temperature of approximately 270 °C; the resulting part exhibited a density of approximately 92% [102]. The coercivity and remanence of NdFeB fabricated using extrusion are 8.7 kOe and 0.51 T, respectively [102]. Figure 20 shows the B–H loop of NdFeB produced by extrusion-based additive manufacturing in comparison with the spark plasma-sintered NdFeB. Therefore, extrusion-based additive manufacturing presents itself as another promising technique for the production of Alnico permanent magnets. In any case, components manufactured through binder jetting or extrusion must undergo post-processing steps such as sintering or solutionization and magnetic annealing. Especially, solutionizing and then magnetic annealing are essential steps for permanent magnetic materials manufactured using all AM processes. These procedures are essential for further enhancing the magnetic properties of permanent magnetic materials, including Alnico.

4.2. Magnetic Field-Assisted Additive Manufacturing

Magnetic annealing is necessary in Alnico to transform it into a permanent magnet; the application of a magnetic field during the additive manufacturing process could enhance its magnetic properties. Magnetic field-assisted additive manufacturing has been carried out on various materials including non-magnetic alloys (such as AlSiMg and 316L stainless steel), soft magnetic alloys (Fe-Si-B), and permanent magnets (NdFeB + SmFeN). The effects of this process on the microstructure and magnetic properties are discussed below. In a study conducted by Dafan Du et al., they ingeniously integrated a magnetic field source into the LPBF process for printing AlSiMg, a non-magnetic alloy [105]. Their primary objective was to investigate the influence of the magnetic field on both the microstructure and mechanical properties of AlSi10Mg; the application of a 0.12 T magnetic field microstructure has shown an increase in the volume fraction of equiaxed grains, as shown in Figure 21, and there has been an improvement in both strength and ductility [105]. Filimonov’s research focused on the fabrication of Inconel superalloy using the L-DED process with the application of a 0.2 T magnetic field and has shown an increase in the average elongation by 4% [106]. As a result, research in the field of magnetic field-assisted LPBF/L-DED has extended its investigation to non-magnetic alloys, seeking to understand its impact on microstructure and mechanical properties. The schematic in Figure 22 shows the magnetic field-assisted AM of the LPBF and L-DED processes.

Sarkar et al. conducted a study involving the extrusion-based 3D printing of hybrid NdFeB and Sm-Fe-N bonded magnets, all under the influence of a magnetic field, and the extrusion process was carried out within a temperature range spanning from 180 °C to 300 °C [108]. The act of printing in the presence of a magnetic field gave rise to magnetic anisotropy, resulting in an increased coercivity when measured in the perpendicular direction given in

Table 8. Coercivity of NdFeB+SmFeN fabricated by the extrusion process with and without the magnetic field [108].

NdFeB+SmFeN	Coercivity (kOe)
Fabricated without magnetic field (isotropic sample)	12.2
Fabricated with magnetic field in perpendicular direction (anisotropic sample)	13.2

[108].

The application of magnetic field-assisted additive manufacturing has demonstrated its capacity to enhance the density of printed samples while also mitigating the occurrence of cracking—a particularly prevalent issue when dealing with Alnico alloys, with the application of a 17.91 mT magnetic field density, which has increased from 98.54% to 99.17% in LPBF-fabricated 316L SS as in Figure 23 [105,107,109,110]. Magnetic fields in additive manufacturing exert a notable influence on the dynamics of the melt pool and the subsequent solidification process [110]. For instance, in the case of AlSi10Mg, the microstructure shifts from an elongated to an equiaxed form when a magnetic field is applied, as in Figure 24 [110,111]. In Fe-Si-B soft magnetic material printed by laser patterning, which was printed on an Nd-Fe-B magnetic substrate, which produced a 0.4 T magnetic field, a notable reduction in grain size was observed, decreasing from 90 nm to 40 nm [112]. This decrease in grain size can be primarily attributed to an increase in the nucleation rate [112,113]. A steady-state nucleation rate (I) is given by Equation (6) [113], as follows:

$$I=I_0·e^{-\frac{Q}{RT}}·e^{-\frac{\Delta G_c}{RT}}$$

(6)

where

$I_0$—pre-exponential coefficient,

Q—activation energy

Figure 23. SEM microstructure of (a) AlSi10Mg without a magnetic field and with a 0.2 T magnetic field [110,111] and (b) density and microstructure of 316L SS without a magnetic field, with 17.91 mT and 0.39 mT magnetic fields [107,110].

Figure 23. SEM microstructure of (a) AlSi10Mg without a magnetic field and with a 0.2 T magnetic field [110,111] and (b) density and microstructure of 316L SS without a magnetic field, with 17.91 mT and 0.39 mT magnetic fields [107,110].

Additionally, $\Delta$$G_c$ is a free energy change required for the formation of a critical nucleus, which is given by the following expression [114]:

$$\Delta G_c=\frac{16\pi {\sigma }^3}{3[|\Delta G_v{|+\frac{H^2}{2({\mu }_2-{\mu }_1)}]}^2}$$

(7)

where

$\sigma$ is the interfacial energy between matrix and phase, $\Delta$$G_v$ is the volume free energy change, H is the external magnetic field, and ${\mu }_2$ and ${\mu }_1$ are permeabilities of matrix and phase.

The application of a magnetic field reduces the free energy change required to form critical nuclei, as represented by Equation (7) [114]. Consequently, the critical nucleation energy ($\Delta$$G_c$) is decreased upon the application of a magnetic field, leading to an increase in nucleation rate [112]. This, in turn, results in the reduction in grain size in the case of Fe-Si-B or the transformation from columnar to equiaxed grains, as observed in the case of Al-Si-Mg. This alteration underscores the importance of investigating the impact of a magnetic field on the microstructure of magnetic materials like Alnico during AM. Hence, when dealing with Alnico, a material for which solutionization and magnetic annealing are crucial is used to improve magnetic properties, employing a magnetic field during printing along with a heated stage likely to substantially diminish the need for solutionization and magnetic annealing, thereby ultimately resulting in substantial cost reductions in Alnico production.

Magnetic field-assisted additive manufacturing is another approach that can be potentially adopted to fabricate the material without or with minimal physical defects (porosity, cracks) and to achieve the desired microstructure for improved magnetic and mechanical properties. The feasibility of this approach is demonstrated on non-magnetic AlSi10Mg and 316L SS alloys.

Figure 24. EBSD of LPBF-fabricated AlSi10Mg (a) without a magnetic field and (b) with a magnetic field [110,111] and wire arc additively manufactured(WAAM)-fabricated Inconel 718 alloy (c) without a magnetic field and (d) with a magnetic field [110,115].

Figure 24. EBSD of LPBF-fabricated AlSi10Mg (a) without a magnetic field and (b) with a magnetic field [110,111] and wire arc additively manufactured(WAAM)-fabricated Inconel 718 alloy (c) without a magnetic field and (d) with a magnetic field [110,115].

Printing in the presence of a magnetic field has demonstrated the ability to reduce grain size by increasing the nucleation rate. Consequently, fabricating Alnico in a magnetic field environment could result in a reduction in grain size, thereby enhancing its magnetic properties. This process may also eliminate the need for magnetic annealing, which is typically required for Alnico when printed without a magnetic field. In the case of NdFeB + SmFeN, extrusion performed in the presence of a magnetic field led to an increase in coercivity. Similarly, in LPBF printing with a magnetic field, there was a reduction in porosity observed in AlSi10Mg and 316L stainless steel materials. These findings suggest that incorporating a magnetic field during additive manufacturing processes can have significant benefits, including reduction in grain size, enhanced magnetic properties, and reduced porosity in the final printed parts. Furthermore, it is important to note that while these beneficial aspects can be achieved in magnetic field-assisted additive manufacturing (MFAAM)-processed non-magnetic alloys (such as AlSi10Mg and 316L stainless steel), this approach has not been extensively explored in magnetic material systems other than soft magnetic materials like Fe-Si-B and permanent magnets like NdFeB+SmFeN. There is a need for further exploration of this technique for other permanent magnetic materials such as Alnico.

To date, only LPBF and L-DED have been utilized for Alnico fabrication. Exploring other AM techniques like binder jet printing and extrusion-based AM could offer valuable insights into Alnico’s magnetic properties. Additionally, magnetic field-assisted AM, proven effective for AlSiMg fabrication by reducing porosity and refining microstructure, holds promise for Alnico. Implementing magnetic field-assisted AM may further refine Alnico’s microstructure, potentially enhancing its magnetic properties, and it could eliminate the post-heat treatment.

5. Summary

The magnetic characteristics of Alnico produced through additive manufacturing are comparable with those of conventionally processed Alnico materials. It highlights a successful density achievement near 7.25 g/cc through parameter optimization, crucial for reducing defects like cracks and porosity. Analysis reveals an equiaxed microstructure, emphasizing the role of grain size and crystallographic texture on magnetic properties. The control of thermokinetic factors and process parameters allows for improved magnetic characteristics. This significant accomplishment highlights the promising capabilities of the AM process in fabricating Alnico permanent magnets.

Both L-DED- and EBM-processed Alnico-8H and Alnico-8H (with reduced Co) have shown an equiaxed microstructure. Alnico magnets benefit from an ultra-fine columnar microstructure characterized by a <100> crystallographic texture, leading to enhanced magnetic properties such as coercivity, remanence, and BH_max. In addition to alloy properties (solidification range), thermokinetic factors such as temperature gradient, solidification rate, and cooling rate play a crucial role in determining the resulting microstructure. Within the additive manufacturing process, it becomes feasible to govern the morphological orientation and crystallographic texture of grains through the meticulous optimization of process parameters such as laser power, laser scan speed, and scan strategy. Temperature gradient increases with the increase in laser scan speed and the decrease in laser power. By reducing the laser power and increasing the scan speed, the temperature gradient increases, eventually surpassing the critical value of $G^n$/R, thereby enabling the formation of a columnar grain morphology. In addition to laser power and laser scan speed, the crystallographic texture is also influenced by the chosen scan strategy. When scanning proceeds in the bidirectional direction pattern between the layers, a <100> crystallographic texture is developed, especially in materials like Ti-15Mo-5Zr-3Al, which possess a body-centered cubic (BCC) crystal structure. Similarly, for Alnico, another BCC metal, employing a bidirectional scanning pattern could allow for the attainment of a <100> crystallographic texture. Consequently, through the optimization of laser power, laser scan speed, and laser scan strategy, the realization of columnar grain morphology with a <100> crystallographic texture could be possible. Alnico magnets rely on specific microstructural characteristics, including a fine grain size and a preferred crystallographic texture, to showcase optimal magnetic properties. However, controlling and achieving these precise microstructural features through additive manufacturing (AM) processes poses notable challenges. This includes ensuring uniformity and consistency in grain size and texture across the fabricated Alnico magnets, which is crucial for their magnetic performance. Integrating a magnetic field into additive manufacturing (AM) during the printing poses a challenging task, and understanding its effects on the microstructure and magnetic properties of Alnico is essential to enhance its magnetic characteristics. Additionally, exploring other additive manufacturing processes for Alnico fabrication could lead to improved Alnico fabrication, but this area requires further investigation and presents its own set of challenges.

The exploration of alternative AM methods like binder jet printing and extrusion-cased additive manufacturing provides new avenues for Alnico advancement, promising enhanced magnets for industrial applications. Alnico is typically subjected to post-AM processing heat treatment steps, including solutionization, magnetic annealing, and tempering, as common practices to enhance its magnetic properties. LPBF-printed Alnico-8, utilizing a heated stage at 800 °C, has demonstrated magnetic properties that are on par with commercial Alnico alloys, indicating that there is no need for additional heat treatment steps. Further, the introduction of a magnetic field during the AM printing process with the heated stage established a string potential for the further enhancement of magnetic properties. This innovative approach holds the promise of eliminating the need for a post-process heat treatment, resulting in cost savings while simultaneously improving the magnetic properties of Alnico alloys.