Crack-Free Copper Alloy Coating on Aluminum Alloy Fabricated by Laser Cladding

Abstract

Crack-free Cu alloy coating has been fabricated on Al alloy substrate with the existence of a Ag buffer layer. The Cu alloy coating had 12 at.% Al and 45 at.% Ag, which contributed to the formation of Cu solid solution and the eutectic phase (transformation temperature 780 °C). The eutectic phase was characterized as finer Cu solid solution and finer Ag solid solution. The Ag buffer layer had the main contents of Ag₂Al and Ag solid solution, and it not only hindered the formation of brittle intermetallic compounds (IMCs)but also reduced the thermal stress as its intermediate coefficient of thermal expansion (CTE). Furthermore, the plastic deformation of Ag solid solution in the Ag buffer layer and Cu solid solution in Cu alloy coating also relieved the thermal stress which was generated during the cladding process. All these three aspects inhibited crack generation. And the hardness of the Cu alloy coating increased to approximately 275 HV due to the strengthening effect of Al solid solution, grain boundary within the finer eutectic phase, and nano twin in the Cu solid solution of the eutectic phase.

1. Introduction

Aluminum alloy is widely used in lightweight equipment and power transmission due to its low density, high strength, good conductivity, excellent machinability, etc. However, the low hardness and poor wear resistance make it easy to wear on the contact surface. Recently, reinforced layers such as Fe-based [1], Ni-based [2], and Co-based [3] coatings were reported to have been fabricated on Al alloy to improve the physical properties, such as tensile strength, corrosion resistance, high-temperature resistance, contact resistance, etc. [4,5], among which Cu alloy has the characteristics of high conductivity, excellent ductility, and good wear resistance [6], which makes it one of the most ideal coatings for Al alloy.

However, great attention [7,8,9,10,11] has always been paid on the cracking tendency and inhibition of the laser-cladded layer due to inner stress, materials difference, brittle components, structure defects, etc. Specifically, in the Al-Cu system, the brittle Al-Cu IMC [12] always leads to crack generation, which severely affects the performance of coatings and even substrates. Therefore, there are not too many reports about Cu alloy coating on Al alloy surface with a large area [7].

Gradient coating [13] is considered an effective way to reduce or even eliminate the cracking tendency of laser-cladded coating [14]. It is a method used to clad a buffer layer with intermediate properties between the surface coating and substrate in order to generate a gradual transition zone, prevent the element diffusion, inhibit IMCs generation, and relieve the inner stress gradient [15].

There are many ways to preparing coatings, such as sintering, plasma spraying, thermal spraying (thermal barrier coating) [16], laser cladding, etc., among which laser cladding [17,18] is a well-known process for additive manufacturing, parts repair, surface modification, and lots of other applications. In this paper, a laser cladding process was chosen, and pure Ag powder was employed to prepare the buffer layer between Al alloy substrate and Cu alloy coating. The element distribution, phase type, and microstructure of the Ag buffer layer and Cu alloy coating were carefully characterized. The hardness of the Cu alloy coating was measured as well. And finally, the crack inhibition mechanism and strengthening mechanism were systematically discussed in detail. The Ag buffer layer was proved to have not only good transition and bonding performance but also an excellent strengthening effect for the Cu surface coating.

2. Materials and Methods

An Al alloy plate of 100 mm × 100 mm × 10 mm was used as the substrate (base material). Before use, the oxide layer was removed and then cleaned with acetone. Cu-based alloy powder (particle size 150 μm, provided by TIJO Metal Powder, Changsha, China) was selected as the surface coating material; pure Ag powder (particle size 50 μm, provided by TIJO Metal Powder, Changsha, China) was selected as the buffer layer material. The laser source IPG YLS-3000 has a maximum power of 3 kW. The laser cladding tool LAMLH-SV was obtained from Nanjing Huirui Optoelectronic Technology Co., Ltd., (Nanjing, China). Argon was used as both the powder feeding gas and shielding gas of the laser cladding process, with a flow rate of 9 L/min and 15 L/min, respectively. The laser cladding process with coaxial powder feeding was used to prepare both the buffer layer and surface coating, with a square laser spot of 1.5 mm × 1.5 mm, overlapping rate of 60%, and laser power of 1300 W for Cu-based alloy powder and 1100 W for Ag powder. After cladding, DPT-5 (dye penetrant, Shanghai Xinmeida, Shanghai, China) was used for penetration detection. The surfaces and cross sections of the Ag buffer layer and Cu surface coating were observed using Scanning Electronic Microscopy (SEM) (JEOL JSM-7900F, Tokyo, Japan). Transmission Electron Microscope (TEM) (JEOL JEM-2100plus, Tokyo, Japan) was used to generate the element map. X-Ray Diffraction (XRD) (Rigaku Smartlab 9 kW, Akishima Japan) and SAED (Selected Area Electron Diffraction) was carried out to characterize the microstructure and the composition. The microhardness tester (FM-ARS900, Future-Tech, Kawasaki, Japan) was used to measure the microhardness.

3. Results and Discussion

3.1. Surfaces and Cross Sections of Cu Alloy Coatings

The pictures of the Cu alloy coating surfaces with and without the Ag buffer layer are shown in Figure 1a–e, in which the cracks were distinctly observed on the Cu alloy coating without the Ag buffer layer, as shown in Figure 1b. On the contrary, the existing Ag buffer layer effectively inhibited the cracks, as shown in Figure 1e. The cross section of the Cu alloy coating without Ag buffer layer is shown in Figure 1c, in which the Al concentration of the Al-Cu transition layer and Cu alloy cladding layer were separately 74 at.% and 25 at.%. The high Al content within the interfacial Al-Cu layer contributed to the brittle IMC formation, which induced the crack initiation. Afterwards, the crack propagated to the Cu alloy coating due to the thermal stress caused by the heating and cooling process during laser cladding. The cross section of the Cu alloy coating with the Ag buffer layer (approximately 550 μm in thickness) is shown in Figure 1f, in which the Al concentration of the Ag buffer layer and Cu alloy coating was 24 at.% and 12 at.%, respectively. This indicates that the Ag buffer layer effectively hindered the diffusion of Al element from substrate to Cu alloy coating and hence reduced the formation of brittle Al-Cu IMCs.

Furthermore, the CTEs of Cu, Ag, and Al are 16.7, 19, and 23.6 (10⁻⁶·K⁻¹), respectively. The CTE of Ag lays between Cu and Al, which contributed to a smaller thermal stress mismatch of Cu/Ag and Ag/Al interfaces. Therefore, the thermal stress of Cu alloy coating was reduced.

Based on the above discussion, one of the main reasons to explain the cracking inhibiting mechanism could be concluded as the Ag buffer layer inhibiting the initiation of the crack in the Cu alloy coating by avoiding the generation of brittle Al-Cu IMCs and reducing the thermal stress.

3.2. Microstructure of Ag Buffer Layer

The microstructure of the prepared Ag buffer layer was characterized by TEM, and the images are shown in Figure 2a. The element maps of the indicated region in Figure 2a are shown in Figure 2b,c, as the direction indicated by the arrows in which an Al-rich region and Ag-rich region was observed. The XRD pattern of the Ag buffer layer shown in Figure 2d indicated that the main contents are the Ag₂Al and Ag solid solution. Therefore, the Ag-rich region and Al-rich region were Ag solid solution and Ag₂Al, respectively. As a buffer layer, this cladded Ag layer trapped the Al element by forming Ag₂Al and hence prevented the diffusion of Al into Cu alloy coating; this process is in favor of cracking inhibition (as described in Section 3.1). Specifically, the microstructure of the twin in the Ag solid solution is observed in Figure 2e, which was the result of the plastic deformation of FCC-type Ag solid solution. From the macro aspect, due to this plastic deformation, the Ag buffer layer reduced the thermal stress generated during the heating and cooling process of laser cladding, as mentioned in Section 3.1. This could be one of the main reasons for cracking inhibiting the mechanism.

3.3. Microstructure of Cu Alloy Layer

In order to further investigate the crack inhibiting mechanism, the STEM image and element maps of the Cu alloy layer (with Ag buffer layer) are shown in Figure 3a–d, in which the Cu-rich primary phase and Ag-rich eutectic phase were observed as the main constitution. The STEM image and element maps of the eutectic region are shown in Figure 3e–h, in which the Ag-rich region and (Cu, Al)-rich region were characterized. The SAED image of the Cu rich region indicated in Figure 3i is shown in Figure 3j. It can be observed that the primary Cu-rich region (separated by the dash line with eutectic phase region)was FCC-type Cu solid solution. The SAED image of the region indicated in Figure 3k is shown in Figure 3l. It was concluded that the Ag-rich region was FCC-type Ag solid solution. The (Cu, Al)-rich region was FCC-type Cu solid solution. The HRTEM image of the nano twin structure and twin boundaries (TBs) was identified as shown in Figure 3m,n. This indicated that, in the eutectic phase of the Cu alloy layer, twin transformation occurred in the Cu solid solution. As many studies have indicated [19], the twin transformation in the twin boundaries deformation could apparently release inner stress, which greatly helps in inhibiting the cracking tendency.

From the phase constitution of the Cu alloy layer, the phase transformation during the cooling process could be illustrated as follows: The primary Cu solid solution was precipitated from the liquid first, followed by the eutectic transformation of the residual liquid into the eutectic phase at approximately 780 °C. Plus, when it was cooling down, volume shrinkage occurred, and thermal stress was generated accordingly due to the volume shrinkage during this stage. However, with a few of the Al elements diffused into the Cu alloy layer, the stacking fault energy (SFE) of the Cu solid solution was reduced [20], which led to an easier twin transformation. This plastic deformation of nano twin could absorb the energy and hence release the tensile thermal stress. With lower inner stress within the Ag alloy layer and Cu alloy layer, the cracking tendency could easily be alleviated. This is one of the main reasons for explaining the cracking inhibiting mechanism.

3.4. Hardness of the Cu Alloy Coating with Ag Buffer Layer

In order to characterize the macro-performance of the prepared Cu alloy coating (with the existence of the Ag buffer layer), the hardness distribution was tested. As a contrast, the hardness of Al and Ag were tested as 60 HV and 50 HV, respectively.

The hardness distribution is shown in Figure 4, from which we can see that the Ag buffer layer achieved a higher hardness of 150 HV. This can be attributed to the generated Ag₂Al IMC, which is brittle yet hard. According to existing studies about strengthening the mechanism [21,22,23], the Cu alloy layer achieved a higher hardness of 275 HV, which should be attributed to the following three parts: the first was the Al element evenly distributed in primary Cu solid solution and eutectic phase as a strengthening phase; the second was the finer Ag solid solution and Cu solid solution in the eutectic phase, as characterized in Figure 3e,k, which provided more boundary, which in turn hindered the dislocation movement and strengthened the eutectic phase; and, last but not least, nano twin in Cu solid solution, as characterized in Figure 3m,n, also remarkably strengthened the eutectic phase. Moreover, the increase in the coating hardness could inhibit the crack initiation due to the higher strength of the inner microstructure. The hardness of the Al substrate, Ag buffer layer, and Cu layer exhibits a gradient trend, which reduces the hardness difference in the interfaces.

4. Conclusions

The laser cladding process was employed to fabricate the crack-free Cu-based alloy coating on Al alloy substrate by adding the Ag buffer layer. The surface Cu alloy coating was observed by STEM to determine the main constitution as Cu-rich primary phase and Ag-rich eutectic phase. The Ag buffer layer was characterized by XRD, in which the main contents are the Ag₂Al and Ag solid solution. The cracking tendency was obviously inhibited due to the Ag buffer layer not only preventing the diffusion of the Al element and avoiding the cracking source through depressing the generation of brittle IMCs, but also by reducing the tensile stress as its CTE was appropriately laid between Al and Cu, and the inner stress was also relieved via plastic deformation.

Moreover, the surface Cu alloy coating was remarkably improved and the hardness is 275 HV, which is three times higher than that of the Al alloy substrate.

It can be concluded that the laser-cladded Ag buffer layer was proven to be an ideal transition layer when preparing Cu alloy coating on an Al alloy substrate.