**1. Introduction**

The electrical machine is considered a key part in electric drives, which account for approximately 50% to 70% of electricity usage in the EU and the United States [1]. Its applications include, but are not limited to, compressors, HVAC systems, power tools, generators, electric and hybrid vehicles, elevators, and MAGLEV trains. In the last decade, there have been consistent efforts from both the US Department of Energy and the EU to advance the design and development of future generations of electrical machines that positively impact the environment and reduce greenhouse gas emissions [1,2]. The next generation electrical machines include designs with high efficiency and power density; however, another important aspect is their environmentally friendly construction, including aspects such as minimal material waste and recyclability.

Additive manufacturing (AM), also known as 3D printing, is an emerging manufacturing technology that can potentially enable and facilitate development toward the next generation electrical machines. AM provides key advantages over traditional manufacturing methods. AM can reduce material waste and scrap parts associated with many traditional manufacturing processes. The 3D printing process, in general, recycles unused raw materials such as powder and wire filament [3], potentially achieving full use of the raw material. Recycling and reusing the raw material are critical to reduce cost, especially for high-cost raw materials such as permanent magnets. Also, recent technological

**Citation:** Pham, T.; Kwon, P.; Foster, S. Additive Manufacturing and Topology Optimization of Magnetic Materials for Electrical Machines—A Review. *Energies* **2021**, *14*, 283. https://doi. org/10.3390/en14020283

Received: 20 December 2020 Accepted: 4 January 2021 Published: 6 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

advancements in AM allow the use of a wide range of materials, including copper [4,5], ceramics, and magnetic materials [6]. These materials are key for manufacturing electrical machine components.

One of the most important advantages is that AM requires minimal tooling and additional processing techniques to fabricate complex topologies. For very complex shapes, AM technologies may provide the most economical and expedited means for fabrication of small quantities. Topology optimization (TO) has been used in many application areas to identify novel designs that reduce weight without compromising mechanical integrity. TO determines the optimal way of distributing a single or multiple materials in a defined design space. Complex designs often result from TO. Application of TO is seen in the design of the rotor core of a switched reluctance machine [7]. TO is also used to find the optimal distribution of the permanent magne<sup>t</sup> and the iron rotor core in permanent magne<sup>t</sup> machines [8,9].

One drawback of TO designs is the manufacturability of the optimized solutions. This has hindered the adoption of TO toward electrical machine design. However, recent applications of AM toward electrical machine components, especially in ferromagnetic materials and permanent magnet, have revitalized the adoption of topology optimization. As AM can manufacture almost any complex topology, it has become clear that TO and AM have high levels of synergy and can be used in parallel to facilitate the development of next generation electrical machines.

There is limited literature on the integration between AM and TO for magnetic components in electrical machines. In [10], a combined magnetic-structural TO is applied for the design of a rotor core of a surface mount permanent magne<sup>t</sup> machine. The optimized rotor core is then 3D printed using high silicon steel. In [11], permanent magnets with multiple magne<sup>t</sup> grades are proposed for a surface mounted machine to reduce manufacturing cost without penalizing machine performance. Though AM was proposed for fabrication, TO was not applied; however, it could be used to identify optimal distribution of magne<sup>t</sup> grades.

There are works discussing the current state of additive manufacturing for electrical machines and their components, including magnetic materials and windings [12]. In [13], applications of AM technologies are broadly discussed for components of electrical machines, including iron cores, windings and insulation systems, magnets, and heat management/exchanger systems. For each component, ref. [13] provides a broad view around the performance of the 3D printed components and where they are compared to traditionally manufactured components. In [14], the advantages of AM technologies are discussed toward the construction and assembly side of the electrical machines.

In this paper, applications of additive manufacturing and topology optimization toward magnetic components for electrical machines are reviewed. Fundamental concepts regarding AM, especially for magnetic materials, are mentioned first to set the stage for discussing the integration of topology optimization in the later part of the paper. Also featured in more detail is the current state of the art in integration of additive manufacturing and topology optimization, especially toward iron cores and permanent magnets in electrical machines. These case studies highlight the novel integration between these emerging technologies and show their potential in future design of electrical machines.

### **2. Additive Manufacturing of Soft Magnetic Materials**

Soft magnetic materials are characterized with low intrinsic coercivity, typically below 1000 A/m, and can be easily magnetized or demagnetized [15]. As the iron cores are responsible for the guidance and improvement of the main flux created by the continuously moving magnetic field, there are some criteria in the selection of soft magnetic materials during the design phase. The following characteristics are considered to be key for the iron cores: magnetic saturation *Js*, intrinsic coercivity *Hc*, relative permeability *μ<sup>r</sup>*, hysteresis loss density *ph*, dynamic loss *pe*, and yield strength [16]. For electrical machines, the iron cores are traditionally made of either steel laminations or soft magnetic composites (SMCs). Steel laminations are typically formed from iron alloyed with silicon, nickel, cobalt, and other additives. To form the desired stator and rotor geometry, steel laminations are usually punched, either with mechanical or laser cutting technique. They are then stacked, welded, or bolted together to form the iron cores. It is well recognized that degradation of the magnetic properties can occur with mechanically handling techniques [17]. Thus, the magnetic properties of the stacked iron core may be very different from the properties of the mother coil. Another concern with stacked iron cores is waste of materials associated with the traditional manufacturing process. For segmented or complete laminated cores, the amount of steel waste due to cutting and punching of laminations can range between 50% to 80% [18]. This low use ratio signals a waste in producing iron cores. Additionally, forming the iron cores for machine topologies that require a 3D flux path can become a challenge with laminations. Thus, laminations are typically seen in iron cores where the preferred flux paths are parallel with the in-plane lamination directions.

In contrast to stacked iron cores, SMCs are selected for machine topologies where easy flux flow in three directions is preferred [19]. These machine topologies can include axial flux machines, tubular linear machines, or claw pole machines. Since iron cores made from SMCs are produced by compacting and molding iron particles into desired shapes, it may require less mechanical handling and post-processing steps. Thus, mechanical processing steps such as stamping and welding may not be required. Another advantage of SMCs in comparison with laminated steel is that it has lower eddy current loss at high excitation frequencies. At excitation frequency of 1000 Hz and above, the eddy current loss of SMC core is much lower compared to laminated steel core. This benefits SMCs for electrical machine designs where high speed operation is a requirement. There are, however, notable challenges regarding application of SMCs. They are subjected to high hysteresis loss, high intrinsic coercivity, low relative permeability, and low yield strength. Typical yield strength value of SMCs is below 20 MPa, while for lamination steel the typical value is around 350 MPa. For high speed electrical machines, rotor cores made from SMCs can be subjected to high von Mises stress, which is undesirable.

The recent proliferation in studies regarding AM for soft magnetic materials aim at providing alternative materials for fabricating the iron cores. In general, these studies try to exploit key features of AM, at the same time improving the performance of the printed soft magnetic materials that can be potentially used in the cores. Some of the early work in application of AM for fabricating iron cores focus on demonstration of AM as an easy mean of manufacturing complex geometries, Figure 1. Multiple demonstrations of additive manufacturing for a rotor core of a synchronous reluctance motor is shown in [20,21]. Here, the fabrication of the prototype rotor cores is achieved with two 3D printing techniques, fused deposition modeling (FDM) and selective laser melting (SLM) without any use of molding or tooling. In [22], the surface of the rotor core of a switched reluctance machine that resembles the structure of a honeycomb is analyzed. The use of the honeycomb structure was shown numerically to improve both the torque ripple and the leakage flux associated with the machine.

In electrical machines, the three commonly seen soft magnetic materials are iron-cobalt (FeCo), iron-nickel (FeNi), and iron-silicon (FeSi) alloys. As previous studies highlighted AM capability in fast prototyping of complex iron core geometries, other research in AM of soft magnetic materials focus on laying foundations for printing these iron alloys. These foundations include magnetic, mechanical, and microstructural characterization of the printed iron alloys, as well as their relationships to the printing parameters.

**Figure 1.** Additive manufactured rotor cores. These structures were fabricated without molding and tooling. (**a**) Printed rotor core for a synchronous reluctance machine [21]; (**b**) Printed rotor core for a switched reluctance machine [22].

### *2.1. Iron-Cobalt (FeCo)*

One of the most attractive properties of FeCo is that it has the highest magnetic saturation compared to other soft magnetic iron alloys, with *Js* value settling around 2.4 T. However, the conventional production of FeCo iron cores is subjected to the high material cost of cobalt, the low workability of the iron-cobalt alloy, and the additional requirement of heat treatment of the stacked iron core. This limits the use of FeCo iron cores, especially for cost-sensitive applications. As a result, reducing the challenges associated with the production of FeCo iron cores via additive manufacturing is of high interest among AM research groups. Efforts in printing FeCo iron cores have been shown via the applications of 3D screen printing and laser engineered net shaping technologies (LENS). It is reported in [6] that FeCo fabricated with 3D screen printing achieves magnetic induction comparable with commercial FeCo alloy with 15–20% cobalt content. Higher magnetic induction and saturation can be achieved if the porosity level in 3D screen-printed FeCo cores is reduced. Further comparison between screen-printed FeCo and commercial laminated FeCo shows that printed cores have higher iron loss, making its magnetic performance less appealing.

FeCo parts printed with LENS technology, ref. [23] show good potential, with achieved magnetic saturation settling around 2.2 to 2.3 T, within 10% in comparison to commercial FeCo alloyed with vanadium. When as-built FeCo core is heat-treated, its maximum relative permeability increases approximately three-fold, while its intrinsic coercivity drops by two thirds, as shown in Figure 2. Here, the annealing process leads to the development of a bimodal grain size characteristic, where coarse grains with average grain size around 200 to 600 μm are surrounded by finer grains (around 2 μm in size). By tuning the printing parameters or mixing additives into the iron alloy starting powder, it is possible to 3D print FeCo cores with even further attractive properties [24,25]. These explorations in AM of FeCo alloys show promise in overcoming the workability issues associated with the conventional mechanical processing of FeCo cores, while achieving DC magnetic characteristics close to commercial products.

### *2.2. Iron-Nickel (FeNi)*

Iron-nickel alloys, in comparison to iron-cobalt alloys, have a much higher maximum relative permeability, more than 100,000, while saturates at a much lower *Js* value, usually between 0.7 and 1.6 T, depending on the nickel wt.% content. Two laser-based AM processes, SLM and LENS, are typically seen in 3D printing of iron-nickel alloys. Reported results on fabricated Fe−30%Ni and Fe−80%Ni showed grea<sup>t</sup> potential in achieving magnetic saturation *Ms* comparable to commercial FeNi at the same nickel wt.% [26]. Analysis on the relationship between magnetic characteristics and 3D printing parameters found that for printed FeNi alloys, magnetic saturation is significantly influenced by the laser

power and the laser scan speed [27,28]. These printing parameters directly impact the grain size and the density of the fabricated parts, which in turn impacts the magnetic saturation. Optimization of the laser parameters, however, is required for FeNi with different percentages of Ni content to improve the magnetic saturation *Js* value. As shown in [29], for Fe−30%Ni, SLM printed iron alloys show an increase of more than 20% in magnetic saturation when laser speed is increased, while for Fe−80%Ni, the magnetic saturation is slightly decreased as the laser scan speed increases [28]. Other printing parameters such as laser scan width or the number of scan passes have been found to have a low impact on magnetic saturation *Js*, thus optimization of these parameters may not be necessary [30].

**Figure 2.** Comparison between quasi-static hysteresis loops of as-built FeCo and annealed FeCo. Figure is adapted from [23].

One of the major issues with the FeNi processed with either SLM or LENS is the high intrinsic coercivity *Hc*. The measured coercivities of the fabricated FeNi alloys range between 80 A/m to 3000 A/m [31,32], which are much higher than typical values of intrinsic coercivity found in commercial FeNi alloys. High intrinsic coercivity indicates a high hysteresis loss associated with printed FeNi alloys. Additionally, high intrinsic coercivity can have a negative impact toward maximum relative permeability, which is one of the main features of iron-nickel electrical steel. Reduction of intrinsic coercivity in printed iron-nickel is thus important. Analysis has shown that reduction in intrinsic coercivity can be achieved by reducing the porosity level as well as microstructural defects in the printed parts. This can be done by optimization of the laser power and laser scan speed as these parameters have direct influence on the cooling rate and exposure time of the molten pool. These, in turn, impact the defects, porosity, and density levels of printed parts [33]. Alternatively, the coercivity may also be reduced by blending FeNi alloys with additives such as vanadium or molybdenum as shown in [27].

### *2.3. Iron-Silicon (FeSi)*

Considering performance per cost, iron-silicon electrical steel variants have high magnetic saturation, high maximum relative permeability, low intrinsic coercivity, low hysteresis loss, and low eddy current loss up to hundreds of Hz in excitation frequency. Variants of iron-silicon electrical steel are thus found in most iron cores used in electrical machines [16]. In pursuit of additively manufactured iron cores, most research and development activities for 3D printed ferromagnetic materials also focus on iron-silicon.

Similar to the 3D printing of iron-cobalt and iron-nickel, SLM is the most employed AM process for iron-silicon. In [34], SLM is proposed as an alternative method to produce iron-silicon with silicon content at 6.9%wt., which is brittle and challenging to produce with via conventional manufacturing method. Here, the investigation of the SLM printing parameters on the magnetic properties shows that there is a non-linear relationship between laser energy input and the relative permeability, intrinsic coercivity, and the total loss density of the printed iron-silicon. It is thus important to optimize the printing process to obtain optimal magnetic performance of printed iron-silicon. The nature of the SLM method, however, introduces defects and residual stresses on the microstructures of the printed parts, which hinders the magnetic properties of SLM iron-silicon. Compared to commercial iron-silicon lamination steel, the maximum relative permeability of asbuilt iron-silicon from the SLM process is lower [34,35]. Applying heat treatment to the as-built parts can help remove residual stresses and significantly improve the relative permeability as well as other magnetic properties of SLM iron-silicon [36]. In [37], the annealing process is shown to improve the maximum relative permeability of as-built parts, from an approximate value of 2000 to more than 24,000, which is on par with high performance iron-silicon steel laminations. Other magnetic properties, including total iron loss density, intrinsic coercivity, and saturation are also positively impacted via the annealing process.

Another interesting characteristic of the SLM process is that it introduces grain elongation in the build direction of the printed parts. As a result, iron-silicon fabricated using SLM can have high levels of magnetic anisotropy [38]. Additionally, higher laser energy input can even change the crystallographic texture of the printed iron-silicon, leading to the formation of Goss texture also known as cube-on-edge texture, which is seen in grain-oriented electrical steel [39]. This suggests that SLM can be potentially used as an alternative approach in producing grain-oriented iron-silicon, which in turns can be used for applications such as transformers or large electrical machines.

To avoid the effect of residual stress caused by the local melting due to the laser energy source as in SLM, other AM techniques have also been explored. In [6], FeSi sample is prepared using 3D screen printing and then compared with commercially available FeSi lamination steel. In this AM process where the powder is held together via binder, the printed part is heat-treated uniformly upon printing completion. The magnetic induction and relative permeability at low magnetic field strength are comparable to commercial FeSi steel. However, the magnetic saturation of screen-printed iron-silicon is lower than commercial lamination equivalent, owing to the low density and high porosity level of printed parts.

Binder jet printing (BJP) is another AM technique that does not use laser as an energy source. In [40], the BJP process is used to prepare iron-silicon samples with superior relative permeability in comparison to commercial soft magnetic composites (SMCs). Maximum relative permeability of BJP iron-silicon can be improved to more than 10,000 under postprocessing heat treatment [41]. As a laser is not used to melt the powder particles together, there is no grain elongation associated with the BJP process. Thus, an advantage of the BJP process is that it can produce iron-silicon with low level of magnetic anisotropy. This is shown in [42], where the three-dimensional magnetic characterization confirms that BJP iron-silicon can achieve a low level of magnetic anisotropy, similar to SMCs. As a result, iron cores prepared with BJP process can be suitable for applications where easy flux flow in three dimensions is preferred. Hysteresis loss of BJP iron-silicon can be further reduced with the addition of boron into the starting powder [43,44]. This can be explained by the increase of 40% in average grain size of BJP iron-silicon with the addition of boron.

#### *2.4. Performance of Additively Manufactured Soft Magnetic Materials*

There are promising results at this early stage of AM for soft magnetic materials for electrical machines. AM allows freedom in design of magnetic cores, which can increase the performance of electrical machines. Additionally, magnetic properties of 3D printed materials, especially iron-silicon, are improving and reaching the levels of many commercial electrical steel laminations as well as SMCs, see Figure 3. Maximum relative permeability of printed iron-silicon is high and comparable to iron-silicon steel lamination, especially when the printed sample is heat-treated, Table 1. As the AM iron-silicon samples undergo post-processing heat treatment steps, their grain size can significantly improve, which in turn leads to higher magnetic induction and permeability.

**Figure 3.** Comparison between additively manufactured and commercial electrical steel. (**a**) Comparison between reported coercivity of 3D printed electrical steels, commercial electrical steels, and SMCs; (**b**) Comparison between reported magnetic induction at 4 kA/m, *B*40, of 3D printed electrical steels, commercial electrical steels, and SMCs.


**Table 1.** Comparison of grain size and maximum relative permeability between as-built and heattreated printed soft magnetic materials [34,37,40,43,45].

Application of AM for soft magnetic materials can also provide the capability to manipulate the crystallography of fabricated parts on demand. As the crystal structure of the soft magnetic material changes to the desired requirements, the magnetic properties of the soft magnetic materials would also change. In [46], gas atomized silicon powder coated with nickel layer is used for printing soft magnetic cuboid sample. The SLM fabricated sample shows changes in the crystallographic structure, switching from bcc structure, which is prevalent for iron-silicon, to fcc structure, which is typically seen for iron-nickel alloy, Figure 4. Tuning the printing parameters along with customized feedstock powder can thus offer unique opportunities in achieving customized magnetic properties.

(**a**) Illustration of nickel layer coating on iron-silicon, gas atomized powder.

**Figure 4.** Illustration of impacts of using nickel iron silicon coated powder on the crystallographic structure of printed samples. Figures are adapted from [46].
