Laser Cladding of Al 102 Powder on Al 4047 with Direct Energy Deposition

Abstract

Large-scale serial production industries such as automotive and aerospace have focused on reducing weight to improve fuel economy, and many parts are manufactured from various aluminum alloys. Due to the ease of recycling of aluminum alloys, research on remanufacturing has not been very active. On the other hand, laser cladding on aluminum alloys is a surface modification and repair process that deposits a thin layer on a substrate using a laser beam whose output can be easily controlled. This makes it suitable for remanufacturing processes where thin layers of damaged parts can be easily repaired, and helps to save energy. In this study, laser cladding was performed using aluminum alloy powder (Al-102) containing Si to improve the surface hardness of Al 4047 used as automotive engine parts and repair damaged parts. Several experimental studies have been conducted regarding the effect on laser power and powder flow rate. In addition, the improvement effect through the hardness analysis of the cladded layer and the change of the microstructure through the cross-section analysis of the clad part are discussed. Finally, the experimental conditions analyzed in this study suggest a suitability for the actual remanufacturing process through multi-pass cladding through overlapping.

1. Introduction

Large scale serial production industries such as automotive and aerospace have focused on weight reduction to improve fuel economy, and many parts are manufactured from various aluminum alloys. In particular, the market size of aluminum alloys for automobiles is expected to continue to rise to about 12% per year from 2022 to 2030 [1], and the demand for remanufacturing parts is also expected to rise dramatically. Due to the ease of recycling of aluminum alloys, research on remanufacturing has not been very active. However, laser cladding on aluminum alloy is a surface modification and repair process that deposits a thin layer of 0.05–2 mm on the surface of a base material. Such precise control over a material makes this technique well-suited for remanufacturing processes aimed at repairing major surface-related damages, such as cracks, corrosion, and wear. Parts produced through remanufacturing not only incur lower production costs and energy expenditure than newly manufactured parts, but also offer environmental benefits, such as reduced emissions [2]. In particular, with laser cladding remanufacturing, a more precise and smaller heat-affected zone (HAZ) can be achieved compared to welding, plasma spraying, and flame spraying [2]. More importantly, laser cladding can be used to deposit heterogeneous materials that can improve surface properties, including corrosion, wear, heat resistance, and lubrication, thereby improving the performance of parts. Consequently, further research of this technique is needed to fully explore its capabilities and applications.

Laser cladding can be performed using various forms of input material, such as powder or wire, and different feeding methods, such as coaxial or extra-axial injection, in various combinations depending on the target material. Each process combination has its own advantages and disadvantages; therefore, the process selection must be made with careful consideration and understanding of the characteristics of the materials and components used.

Previous research on laser cladding has mainly focused on nickel-, chromium-, and titanium-based alloy coatings for steel and superalloy materials, whereas research focusing on aluminum, a lightweight metal widely used in the industry, is scarce.

Dinda et al. [3] applied the Direct Metal Deposition (DMD) process to deposit Al 4047 alloy powder on Al 7475 substrate. The samples were fabricated in a cross-hatched pattern using a doughnut-shaped 2 mm diameter CO₂ laser beam. The microstructural evolution was analyzed, showing that the microstructure is columnar dendritic just above the bead boundary and gradually changed to equiaxed dendrites to the center of layer. They proposed that there were not significant hardness changes from the area of bead boundary to center of the layer.

Meinert and Bergan [4] used the laser cladding technique to deposit 4000 series aluminum alloy powders on 6061 and 7075 alloy base materials with 3.0 kW Nd:YAG laser and f16 focusing optics. They also applied two different types of laser cladding technics such as scanning and stringer passes. The harness of the cladded layer is increased as a function of time. Subsequent tensile test results of the remanufactured specimen showed that it had 61% and 64% more strength compared to the 6061-T6 and 7075-T651 aluminum alloys, respectively.

Cottam et al. [5] produced a crack- and pore-free specimen by cladding 7075 aluminum alloy powder onto a 7075 alloy material using a 2.5 kW Nd:YAG laser with 3 mm Gaussian shape laser spot. They also reported an immediate transition from an equiaxed grain structure in the clad layer to an elongated grain structure in the base material. Additionally, the cladding layer exhibited a slightly lower hardness than the base material because of the heat treatment differences.

Caiazzo et al. [6] used a direct metal deposition process to deposit 2024 aluminum powder on a 2024 aluminum plate using a 4.0 kW Yb:YAG disk laser and a coaxial feed nozzle tilted at 4 degrees. The authors reported that a columnar microstructure was observed in the fusion zone, and about 80% hardness of the base material was observed in the fusion zone due to thermal effects.

Grohol et al. [7] experimentally investigated the laser cladding of a 6061 powder alloy onto a 6061-T6511 material using a high-power diode laser and established a process map for an optimal output and powder flow rate combination. They reported that the clad height is increased with powder feed rate and the clad width and depth are increased with laser power. In addition, the hardness of the clad layer was found to be slightly lower than that of the T6 heat-treated material because of the coarse and sparse β″ precipitates. On the other hand, the heat-affected zone had high hardness due to the fine and abundant β″ precipitates.

Song et al. [8] investigated the effect of aluminum alloy 7075 powder on 2024 specimens by laser cladding using a dual ellipsoidal source on residual stress and fatigue life. It showed the best crack resistance at 1.4 kW laser power and 10 mm/s scanning speed, and it was reported that about three times the fatigue life effect could be secured in the case of 50 mm laser cladding specimens. It was reported that the fatigue life was improved by increasing the compressive residual stress on the specimen surface through laser cladding treatment. It was explained that the higher the laser power and the slower the scan rate, the higher the compressive residual stress.

Cheng et al. [9] investigated the laser cladding of dissimilar aluminum alloys using 5052 aluminum alloy plates with 1.2 mm 5356 aluminum alloy filler wire in water. The laser heat source used was a 5 kW diode laser, and excellent cladding quality was realized at a laser output of 2.8 kW, a moving speed of 17.5 mm/s, and a wire feed speed of 90 cm/min. It was confirmed that the height of the cladding area and the angle of the cladding area increased as a result of the underwater laser cladding, but the width of the cladding area and the height of the molten area decreased. The microstructures of the cladding and fusion zones consisted of columnar and equiaxed dendrites, the same as those of the airborne cladding, but smaller in size. In the underwater cladding, the effect of increasing the hardness value due to the increase in magnesium content and the atomization of the microstructure was confirmed.

Overall, although some research has been done on the laser cladding characteristics of aluminum alloys, it is very limited due to material constraints such as high reflectivity and high thermal diffusivity. Moreover, manufacturing studies corroborating experimental results are rarely found in the literature. Therefore, this study aimed to investigate the process conditions for aluminum laser cladding that can result in a cladding layer with a higher hardness than the base material. Furthermore, the characteristics of the cladding layer were analyzed to confirm its superiority. The study employed 4047 aluminum alloy as the base material, which is widely used in the welding industry and applied to automobile parts, such as crankcases.

2. Experimental Setup and Materials

The equipment used in this study was EOSINT M250 (EOS GmbH, Munich, Germany), which supports a laser output of up to 1 KW and utilizes the direct energy deposition (DED) process with a resolution of 0.01 mm. A fiber laser (IPG Photonics, Oxford, MA, USA) was installed in the equipment and was used in this study to produce a defocused Gaussian beam with a wavelength of 1064 nm and a focal diameter of 1.2 mm. Powder was supplied together with carrier gas (Ar at 3 L/min), whereas coaxial gas (Ar at 9 L/min) and shield gas (Ar at 5 L/min) were simultaneously sprayed from the coaxial nozzle. For the process gas, argon is chosen because it is cheaper, although helium may give better results for porosity.

The material used was Al4047, an aluminum alloy widely used in the welding and automobile industries, as the base specimen material. The specimen was prepared with measurements of 50 mm wide by 50 mm long and 10 mm thick. The surface to be cladded was wire-brushed to offer a relatively constant surface finish and increase laser absorption. It was laminated with Al 102 powder (Praxair Surface Technologies, Wiggensbach, Germany) for the purpose of producing a cladding with a greater hardness than the base material. The diameter range of the powder used was 50–150 μm, and its melting point was 660 °C. In a previous study on the effect of the diameter of the powder used on the quality of the cladding, it was found that the smaller the diameter of the powder, the higher the meltability by laser, so that a better quality cladding layer could be made [10]. Therefore, a commercially available power was chosen with uniform powder distribution and small diameter.

Table 1. Chemical composition of the considered materials (in % mass).

	Si	Fe	Cu	Zn	Mn	Mg	Al
Al 4047	12.0	0.4	~0.1	~0.1	0.3	~0.1	Bal.

and

Table 2. Chemical composition of the powder (in % mass).

	Composition—Spec.	Composition—Actual
Aluminum	>75	88
Silicon	5–20	12

present the chemical composition of the materials used.

3. Experimental Conditions

The process conditions that have a major influence on the results of laser cladding are laser power (W), scanning speed (m/min), and powder feed rate (g/min). Among them, the scanning speed is often limited due to process conditions. In this study, it was fixed at 850 m/min, which is the effective speed provided by the equipment. The experiment was then planned for a laser power range of 0.6–0.8 kW and a powder supply speed range of 1.0–2.0 g/min. For the first set of experiment, the experiments were conducted at three levels for each condition and duplicated three times. The cladding starts 5 mm from the beginning and 5 mm from the end of the base material, creating a single 40 mm long path. The second set of experiments with a multi-path to provide insight into the effect of wide area repairing were also conducted at three levels for each condition but did not duplicate. The gap between the lines was set to 1.0 mm, and the 40 mm length lines overlapped six times. Each path starts 5 mm from the beginning of the base material. An outline of the cladding line arrangement and the actual experimental results is shown in Figure 1, and the conditions used in the experiment are summarized in

Table 3. Experimental parameters (scanning speed is fixed at 850 m/min).

	Exp #	Power (W)	Feed Rate (g/min)	Dilution Rate (%)
	1–3	600	1.0	71–83
	4–6	700	1.0	76–81
	7–9	800	1.0	58–69
	10–12	600	1.5	52–62
1 path	13–15	700	1.5	65–71
	16–18	800	1.5	37–72
	19–21	600	2.0	54–60
	22–24	700	2.0	58–65
	25–27	800	2.0	55–59
	28	600	1.0
	29	700	1.0
	30	800	1.0
Multi-	31	600	1.5
paths	32	700	1.5	n.a.
	33	800	1.5
	34	600	2.0
	35	700	2.0
	36	800	2.0

. The conditions used in the experiment were limited to those in which actual effective deposition could occur, which were selected based on a number of previous experiments.

4. Results

Figure 2 shows the results of the cladding experiment. After the experiment, the surface was cleaned with a metal brush to remove inaccurately bonded powders and to provide better visibility. A visual inspection of the surface was performed to confirm the cladding layer formation and creak- and pore-free surface. The progress in cladding formation is shown from the top to the bottom of Figure 1, and it was confirmed that the deposition was successful under all experimental conditions. Therefore, these results successfully demonstrated that cladding is possible in all process parameter windows, corresponding to the considered experimental condition, and the experimental results are shown in Figure 2. The area shown in Figure 2 is the endpoint of each experiment.

4.1. Layer Size and Cross-Sectional Analysis

To analyze the results of the laser cladding experiment, the cross-section of the fabricated specimen was analyzed. For this, the cross-section was first cut at 30 mm after laser irradiation, which is 3/4 of the one path, and we believe the laser and powder feeder are stable. Samples were prepared by sanding to 2000 grit with sandpaper and then polishing with 3 μm diamond paste. After polishing, samples were etched to improve visibility for the fusion zone and heat-affected zone. It was then quantitatively analyzed by measuring the height and width of the cladding and qualitatively analyzed for internal pores. The height is defined as the sum of the penetration depth of the clad thickness above the substrate surface and the melt thickness below the substrate surface. The width is defined as the width of clads. Defects such as surface cracks and undercuts could not be found through visual inspection.

Figure 3 shows the cross-sectional images of the laser power under the condition of a 1.5 g/min powder flow rate. The images confirmed that a clad layer with a good bonding area was formed under each condition.

Based on the example cross-sectional image shown in Figure 4, the height and width of the cladding were found to vary with laser power at a powder flow rate of 1.5 g/min. Under each condition, although the formation of clad layers with good bonding was confirmed, some pores were present. However, with an increase in the power of the laser, more flow occurs on the surface, and it was confirmed that a part of the molten portion protrudes. The penetration depths were increased with the laser power increased, as Grohol et al. [7] reported.

Some previous studies mention the dilution rates of various materials, such as Ni and iron-based powders in steel, [11,12] composite materials in zirconium, [13] NiCrAlY powder in Inconel 738, [14] and Al 6061 powder in Al 6061-T6. [7] However, the rate values are highly application-dependent. Zhu et al. [15] summarized that when the dilution rate is small, the mechanical behavior is diminished due to the weakening of the metallic bond. This can cause the clad layer delamination. On the other hand, when the dilution rate is excessive, it can cause a larger area of HAZ, bending, and cracks on the base material. For this reason, it is necessarily needed for a suitable dilution rate. The dilution rate, D, (%) is defined as melt thickness below the substrate surface (h₂) divided by the sum of the penetration depth of the clad thickness above the substrate surface (h₁) and the melt thickness below the substrate surface (h₂), shown in Equation (1) [7]:

D [%] = h₂/(h₁ + h₂)

(1)

The results were summarized in Table 3 and show that the dilution rate for the experiments were at the range of 37–83%. The dilution rate increased with the laser power and decreased when the powder feed is increased. In the remanufacturing process, the laser cladding is used to repair damaged parts, and mechanical machining is often performed as a post-process, it is considered advantageous to select a relatively high dilution rate within a range that does not affect the characteristics or shape of the part.

Figure 4 summarizes the results of the length variation obtained by analyzing the cross sections with respect to the laser power. The results showed the trends that the width and height of the clads increased with an increase in the laser power and powder flow rate. This tendency was also demonstrated in a previous study, [16] in which it was reported that bonding of the cladding layer occurs when a sufficient laser power is attained. Furthermore, the higher the laser power, the wider the molten pool; likewise, the higher the powder supply, the greater the height of the stacked part.

However, in this experiment, the height of the cladding layer did not increase significantly, whereas the height of the total fusion zone tended to do so. In most conditions, the width of the clad layer was in the range of 800–1100 μm, and the height was also distributed in the range of 300–500 μm. This indicates that the transfer speed used in the experimental conditions was not optimally proper; thus, sufficient time was not afforded for the powder layer to be layered and melted. However, as the depth of the fusion zone increases, the bonding strength with the raw material also increases. Therefore, depending on the target of the coating layer, an increase in the bonding strength can be achieved by increasing the depth of the fusion zone. On the other hand, the higher clad layer can be achieved by decreasing or balancing the penetration depth of base material.

These results further confirm that determining an appropriate energy for deposition is possible within the process window, considering the conditions used in this experiment. Moreover, the various conditions can be selectively set depending on the application.

4.2. Microstructural Analysis

The analysis of the microstructure in the case of a laser power of 800 W and a powder supply of 1.5 g/min from the experiments conducted is shown in Figure 5.

The microstructures were observed by using a scanning electron microscope (JSM-7800 F Prime, JEOL, Tokyo, Japan). The acceleration voltage was set to 15.0 kV. As was confirmed in a previous study, [3,4,5,6,7] an equiaxed microstructure was formed at the center of the clad, whereas the bonding area exhibited a columnar dendrite structure between the clad layer and the base material, owing to a rapid heat flow near the interface. In addition, the dendrite structures generally tend to grow perpendicular to the interface. In the center of the melting zone, the heat flow proceeds in all directions, resulting in an equiaxed structure being formed with a dense microstructure.

4.3. Hardness

For the analysis of hardness, measurements were made on six, three, and three points in 0.3 mm increments in the fusion zone (FZ), heat-affected zone (HAZ), and base metal area, respectively, and then, the average value was calculated for each point. The microhardness measurements were performed with an automated Vickers hardness tester (Mitutoyo HM-210, Kanagawa, Japan). The loading force and time were 25 g and 20 s, respectively. Figure 6 summarizes the results. The average hardness value is in the range of about 90–100 Hv in FZ and HAZ and the base material is about 72–78 Hv. Based on the microstructural analysis, the FZ has smaller and relatively uniform equiaxed structure. The HAZ may contain some of the columnar dendrite area and base material. Although the columnar dendrite is not notably low in hardness, it can be relatively soft due to the course silicon particles. The trend of the analysis was analyzed to assess the effect of laser power at the same powder flow rate. The analysis results confirmed that the variation for each part was not large and that similar hardness values could be derived under different valid process conditions. In general, aluminum alloys of the 4000 series are considered difficult to heat-treat. Even in the results of this experiment, a marked increase in the hardness of the heat-affected zone was not observed. The hardness was higher than that of the base material in the fusion zone, where mixing through melting occurred and the equiaxed microstructure was uniform, and in the heat-affected zone, where partial melting and recrystallization occurred. Because of the higher hardness and relatively uniform microstructure, this enables the advantage of relatively low friction or wear, depending on the environment, achieved through remanufacturing.

4.4. Multi-Path Experiments

From the multi-path experiments conducted at various conditions to check the feasibility of wide area cladding, all valid stacking results were derived, as shown in Figure 2 and Figure 7. The height tended to increase about 10–20% from the first path to third path due to overlapping, which is more melting made with elevated temperature at the laser heating area. The porosity is also increased because more paths can offer higher chance of trapped gas or moisture. The hardness measurements were made on nine points in 0.5 mm increments in the fusion zone of each path. The averaged hardness value is not different with single-path experiments. In addition, the interface between the fusion zone also changed due to remelting; however, it did not have a direct effect on bonding. Like single path microstructural analysis results, an equiaxed microstructure was formed at the center of each path, whereas the bonding area exhibited a columnar dendrite structure between the clad layer and the next clad layer. Therefore, the considered experimental conditions are not greatly affected by multi-pass cladding, and an effective cladding layer can be formed.

5. Conclusions

Laser cladding of Al 102 powder on Al 4047 base material was successfully completed through the direct energy deposition process, and suitable conditions could be selected according to the applicated based on the conditions used in this experiment.

The cladding joint was confirmed to increase in width and depth with an increase in the laser output. In addition, it was less affected by heat treatment due to the nature of the base material. The dilution rate was in the range of about 37–83%. The dilution rate increased with the laser power and decreased when the powder feed increased.

In terms of hardness, the hardness of the cladding layer was improved by approximately 10% compared to the base material, as intended, suggesting that an appropriate hardness can be selected according to the target material.

The rationality of various process conditions was confirmed through a single-path experiment, and based on this, a multi-path experiment was conducted with the single line overlapping six times. As a result of the experiment, it was confirmed that an effective cladding layer could be secured even in the overlapping experiment, thus demonstrating its applicability to larger area surfaces requiring restoration and repair.

Therefore, the results of this study have confirmed that it is possible to restore friction or wear parts using the laser cladding process for Al 4000 series alloys used in various industrial fields.