Effects of Cu-Coated SiC Content on Microstructure and Properties of Laser Cladding SiCp/Al–Si Composite Coatings

SiC particles (SiCp)-reinforced Al–Si matrix composite coatings were synthesized on 4032 aluminum alloy by laser cladding using powder mixtures of Al-20 wt.% Si alloy and electroless copper-plated SiC particles (SiCp-Cu). The effects of SiCp-Cu content on microstructure, phase composition, and microhardness of the SiCp/Al–Si laser cladding layer (LCL) were investigated systematically. The results showed that the microstructure of SiCp-Cu/Al–Si LCL was mainly composed of undissolved SiCp, lump-like primary Si, lump-like Al2Cu, plate-like Al4SiC4, and Al–Si–Cu ternary eutectic. In addition, the eutectic microstructure became finer with the increasing of SiCp-Cu content. The average microhardness of the LCL increased with the increasing of SiCp-Cu content. When SiCp-Cu content was 50 wt.%, the average microhardness of the LCL reached 508 HV0.05, which was about 3.5 times larger than that of the substrate. The LCL reinforced with a SiCp-Cu content of 30 wt.% exhibits the best wear resistance.


Introduction
Aluminum alloys are extensively applied in the automotive industry and aircraft and other fields due to their reduced density, light weight, and high specific values of stiffness and strength. Nevertheless, the use of aluminum alloys for a wider range of application is limited due to their low surface hardness and poor wear resistance [1][2][3][4]. Therefore, it is necessary to improve surface properties and mechanical properties and prolong the service life of the parts made from aluminum alloys [5][6][7]. In order to improve surface properties of aluminum alloys, various attempts have been made, such as electroplating [8], electroless plating [9], thermal spraying [10], anodizing [11], and microarc oxidation [12]. However, the disadvantages of pollution of the environment and weak adhesion between the substrate and the coating still exist for these methods mentioned above, resulting in the difficulty of meeting the requirements under severe conditions. Compared with conventional surface treatment methods, the laser cladding process has the advantages of low clad dilution, rapid heating and cooling, small heat-affected zone, and good adaptability of surface properties [13][14][15][16].
Considerable research studies have been carried out to examine laser treatment of aluminum alloys. Sun et al. [17] fabricated composite coatings of Al-Si alloy reinforced with SiC particles on AlSi12 substrate; the microstructure and microhardness of the coatings were investigated, and the results showed that the coatings had much higher microhardness than that of the substrate, and the coatings were divided into two sublayers; the upper layer was composed of Al-Si eutectic, acicular primary Si, α-Al dendrites, and a little SiC p , while the bottom layer consisted of α-Al dendrites, Al-Si eutectic, and a large amount of SiC p . The oxidation effects during the laser treatment of aluminum coated with SiC p /Al composite coating was studied by Hegge et al. [18], and the authors found that inert gas stream is not always enough to sufficiently prohibit contact between the air and the melt. Anandkumar et al. [19] studied the influence of the laser cladding process on the microstructure and abrasive wear resistance of SiC p /Al-Si composite coating; they observed that the microstructure and properties of the SiC p /Al-Si laser cladding layer (LCL) depends strongly on the processing parameters, especially power density and interaction time. The influence of addition of alloy elements on microstructure and microhardness of the SiC p /Al-Si LCL was investigated by Riquelme et al. [13]. The result indicated that the addition of Si or Ti particles to the composite coating is an effective method to avoid the formation of Al 4 C 3 . However, there is no research on the effect of electroless copper plating on SiC p on the properties of SiC p /Al-Si LCL.
In the process of laser cladding, SiC p tends to react with molten aluminum, leading to the formation of Al 4 C 3 and Al 4 SiC 4 and so on during solidification depending on temperature [20]. Between 667 • C and 1347 • C, reaction (1) takes place and produces Al 4 C 3 . When the temperature exceeds 1347 • C, reaction (2) takes place. When the temperature reaches 1927 • C, Al 8 SiC 7 will be formed [21].
The hardness of Al 4 SiC 4 is as high as 1200 HV and its brittleness is low. In addition, it is chemically inert in humid environments, so Al 4 SiC 4 is a favorable reinforcement phase [22,23]. In the process of preparing SiC p /Al-Si LCL, the Al 4 SiC 4 phase is desired. The poor wettability between Al and SiC p will adversely reduce the reaction rate between Al and SiC p during the laser cladding process and the bonding strength between Al matrix and SiC p .
In this work, the wettability between SiC p and Al alloy is expected to be improved by electroless copper plating on the surface of SiC p . SiC p /Al-Si coatings have been deposited on 4032 aluminum alloy by the laser cladding process. The effects of the SiC p-Cu content on microstructure and properties of the LCL have been investigated for the first time. The purpose of this paper is to provide a technical way to improve surface properties of aluminum alloys.

Substrate and Cladding Material
The 4032 aluminum alloy was used as substrate for laser cladding with a dimension of 50 mm × 18 mm × 4 mm. The surface of the substrate was ground with abrasive paper and cleaned with alcohol before laser cladding.
The cladding material was a mechanical mixture of AlSi20 aluminum alloy and SiC p (including SiC p or SiC p-Cu ) powders. The AlSi20 aluminum alloy powder used had a particle size of 50-100 µm and the SiC p had a particle size of 10-20 µm. The SiC p and AlSi20 powders were mixed in different compositions as shown in Table 1. The mixed powders were placed onto the surface of aluminum alloy with gum water as binders and dried at 80 • C for 6 h. The thickness of the precoated layer was approximately 0.5 mm.

Electroless Plating of SiC Particles
Before electroless plating, surface treatment was carried out on the SiC p . According to previous studies, pretreatment can be conducted by the traditional three-step method (coarsening, sensitization, and activation) [24,25]. After pretreatment mentioned above, electroless plating was conducted in a copper electroless bath. The composition and operating conditions of pretreatment solutions and electroless copper plating bath are displayed in Table 2. Lastly, the Cu-coated SiC p was washed with deionized water three times and dried under room temperature. Figure 1 shows the SEM images of SiC p and SiC p-Cu .

Laser Cladding Experiment
Laser cladding was performed by using the CY-WL600G type Nd-YAG pulsed laser with a wavelength of 1.06 μm. Based on the systematic experiments done previously, the laser cladding process parameters used in this study were 800 W for laser beam power, 4 mm/s for laser scan

Laser Cladding Experiment
Laser cladding was performed by using the CY-WL600G type Nd-YAG pulsed laser with a wavelength of 1.06 µm. Based on the systematic experiments done previously, the laser cladding process parameters used in this study were 800 W for laser beam power, 4 mm/s for laser scan speed, and 0.2 mm for laser beam diameter. There was a 50% overlap between two adjacent laser tracks. The thickness of LCLs obtained was about 0.45 mm. After laser processing, samples were cut for cross-section and polished with abrasive paper.
The microstructures of the LCL were analyzed by 10XB-PC optical microscope (OM) from Shanghai optical instrument factory and QUATA 250 FEG series field emission scanning electron microscope (SEM) from FEI. Semiquantitative analysis of element distribution was carried out by energy dispersive spectrometer (EDS), which was equipped with SEM. The LCL phase was tested by a DX2700 diffraction analysis system (XRD) from Shanghai Precision Instruments with Cu Kα radiation and XRD patterns were taken at 2θ angles from 15 • to 85 • at a scanning rate of 4 • /min. Also, a BUEHLER5104 microhardness tester from German Buehler was used to obtain Vickers microhardness profiles along samples cross-section up to 750 µm using a load of 50 g for 10 s. For each sample, the microhardness measurements were repeated at five locations at the center and edges of the samples. The given average values of microhardness were average values taking from all measurement points on LCLs. Wear experiments were carried out using a PRN01-04882A pin-on-disk-type tribometer from Swiss CSM Company under dry-sliding conditions. The diameter of pin samples was 3 mm. The ring of the wear couple was made of diamond. The wear conditions were given as 1.4 MPa, 0.4 m·s −1 sliding speed, and 250 m sliding distance. Wear was characterized using the mass loss of the samples and the observation of the wear scars.

Phase Analysis
The XRD patterns of SiC p-Cu /Al-Si coatings with different SiC p-Cu content are shown in Figure 2. As can be seen, in addition to Al and Si phases, great amounts of SiC p , Al 2 Cu, Al 4 Cu 9, Al 4 SiC 4 , and Al 4 C 3 were found in the LCLs. The SiC p was the additive and Al 2 Cu, Al 4 Cu 9 , Al 4 SiC 4 , and Al 4 C 3 were in situ formed novel phases. Furthermore, with increasing SiC p-Cu content, more and more Al 2 Cu, Al 4 Cu 9 , Al 4 SiC 4 , and Al 4 C 3 compounds formed within the LCL and their diffraction peaks became obvious. Table 3 describes the variation trend of 2θ values and intensities of Al peaks in LCLs with different SiC p-Cu content. At the same time, the 2θ values of the standard diffraction peak of Al are also listed. It can be seen that when the SiC p-Cu content is less than 50%, the 2θ values of Al diffraction peaks in the LCLs increase with increasing SiC p-Cu content, and all of them are larger than the standard 2θ values. When the SiC p-Cu content is 50 wt.%, the 2θ value of Al diffraction peak is smaller than the standard 2θ value. According to Bragg's law [26], 2d sin θ = nλ (n = 1, 2, 3, . . . ), the larger 2θ values indicate the smaller interplanar spacing of the corresponding crystal planes. It implies that the lattice deformation of the aluminum was caused by the high cooling rate and the huge residual stress during the laser cladding process.  Table 3 describes the variation trend of 2θ values and intensities of Al peaks in LCLs with different SiCp-Cu content. At the same time, the 2θ values of the standard diffraction peak of Al are also listed. It can be seen that when the SiCp-Cu content is less than 50%, the 2θ values of Al diffraction peaks in the LCLs increase with increasing SiCp-Cu content, and all of them are larger than the standard 2θ values. When the SiCp-Cu content is 50 wt.%, the 2θ value of Al diffraction peak is smaller than the standard 2θ value. According to Bragg's law [26], 2d sin θ = nλ (n = 1, 2, 3, ...), the larger 2θ values indicate the smaller interplanar spacing of the corresponding crystal planes. It implies that the lattice deformation of the aluminum was caused by the high cooling rate and the huge residual stress during the laser cladding process. As can be seen from Table 3, the intensity of XRD diffraction peaks of Al decreases with the increasing of SiCp-Cu content in cladding materials. Meanwhile, the half high width of the XRD diffraction peaks (FWHM) of Al increases. On the basis of the Scherrer formula [26], (where K is the Scherrer constant, D is the average thickness of the grain perpendicular to the direction of the crystal plane, B is the half high width of the diffraction peak of the measured sample, θ represents the diffraction angle, and λ is the X-ray wavelength), the increase of FWHM of 20 40 60 80  As can be seen from Table 3, the intensity of XRD diffraction peaks of Al decreases with the increasing of SiC p-Cu content in cladding materials. Meanwhile, the half high width of the XRD diffraction peaks (FWHM) of Al increases. On the basis of the Scherrer formula [26], D = Kλ B cos θ (where K is the Scherrer constant, D is the average thickness of the grain perpendicular to the direction of the crystal plane, B is the half high width of the diffraction peak of the measured sample, θ represents the diffraction angle, and λ is the X-ray wavelength), the increase of FWHM of Al indicates that the grain size of Al matrix decreased. It indicates that the crystal structure of the SiC p-Cu /Al-Si composite coating produced by laser cladding process was significantly refined. When the content of SiC p-Cu was 40 wt.%, the FWHM of Al was the largest, which means that the grain refinement was the most significant. Figure 3 shows the SEM images of the cross-section of the LCLs with different SiC p-Cu content. SiC p-Cu with different sizes and shapes within the LCLs is observed. The SEM micrograph of the LCL without SiC p-Cu is shown in Figure 3a; it can be seen from the figure that the grain size is coarser than that in the coating reinforced with SiC p-Cu (Figure 3b-f). It can be seen from Figure 3a that there are cracks in the coating, and the existence of the cracks will have a negative impact on the properties of the coating. As seen in Figure 3b-f, the eutectic microstructures of the laser cladding layer become finer with increasing SiC p-Cu content due to the LCLs absorbing rapid heating and cooling during the laser cladding process and possess fast solidification [27]. In addition, these solidification rates increase with the increasing of thermal conductivity and the SiC p-Cu content, and as a result, thermal conductivity of SiC p (259 Wm −1 K −1 ) [28] and copper (401 Wm −1 K −1 ) is higher than that of aluminum (237 Wm −1 K −1 ) [29].

Microstructural Analysis
LCL without SiCp-Cu is shown in Figure 3a; it can be seen from the figure that the grain size is coarser than that in the coating reinforced with SiCp-Cu (Figure 3b-f). It can be seen from Figure 3a that there are cracks in the coating, and the existence of the cracks will have a negative impact on the properties of the coating. As seen in Figure 3b-f, the eutectic microstructures of the laser cladding layer become finer with increasing SiCp-Cu content due to the LCLs absorbing rapid heating and cooling during the laser cladding process and possess fast solidification [27]. In addition, these solidification rates increase with the increasing of thermal conductivity and the SiCp-Cu content, and as a result, thermal conductivity of SiCp (259 Wm −1 K −1 ) [28] and copper (401 Wm −1 K −1 ) is higher than that of aluminum (237 Wm −1 K −1 ) [29].   Figures 3c and 4, respectively. The SiC p remained almost unmelted and was still in an irregular polygonal shape as is shown in Figure 4. Conversely, the most SiC p-Cu was oval-shaped and had a smaller size than that of the SiC p-Cu originally used. It is indicated that the wettability of aluminum melt and SiC p can be improved by electroless copper plating process. SiC p-Cu is more likely to react with molten aluminum during the laser cladding process. It can be seen from Figure 3 that some SiC p were also in an irregular polygonal shape. This is because not all SiC p were coated entirely with copper during electroless plating as shown in Figure 1. and had a smaller size than that of the SiCp-Cu originally used. It is indicated that the wettability of aluminum melt and SiCp can be improved by electroless copper plating process. SiCp-Cu is more likely to react with molten aluminum during the laser cladding process. It can be seen from Figure 3 that some SiCp were also in an irregular polygonal shape. This is because not all SiCp were coated entirely with copper during electroless plating as shown in Figure 1. The microstructure of the LCL with SiCp-Cu content of 50 wt.% at a higher magnification is shown in Figure 5. It is clear that the microstructure of the LCL was mainly composed of undissolved SiCp and dark gray lump-like crystals which were distributed on the ternary eutectic of Al-Si-Cu. The Si content of AlSi20 alloy powders used in cladding materials is 20 wt.%. At the same time, SiCp-Cu reacts with molten aluminum and forms Al4SiC4 and Si during the laser cladding process. The Si is dissolved in AlSi20, increasing its Si percentage, so that the content of Si in the molten aluminum could exceed the eutectic point. On the basis of the Al-Si binary phase diagram, the hypereutectic Al-Si matrix microstructure consisted of Al-Si eutectic and acicular primary Si crystals [17].
After electroless copper plating of SiCp, the weight gain percentage of SiCp is close to 100%, so the cladding material with SiCp-Cu content of 50 wt.% is comprised of 50 wt.% of Al-Si powder, 25 wt.% of SiCp, and 25 wt.% of Cu. The Al-Cu-Si ternary eutectic alloy is mainly composed of primary crystal Si, Al + Al2Cu binary eutectic, and Al + Si + Al2Cu ternary eutectic composition due to the high cooling rate (in the range of 10 3 -10 8 K/s during the laser cladding process) [30][31][32].
EDS analysis was performed to examine the exact composition of LCL with SiCp-Cu content of 50 wt.%. Results of EDS analysis conducted on points a-d in Figure 6 are then summarized in Table The microstructure of the LCL with SiC p-Cu content of 50 wt.% at a higher magnification is shown in Figure 5. It is clear that the microstructure of the LCL was mainly composed of undissolved SiC p and dark gray lump-like crystals which were distributed on the ternary eutectic of Al-Si-Cu. aluminum melt and SiCp can be improved by electroless copper plating process. SiCp-Cu is more likely to react with molten aluminum during the laser cladding process. It can be seen from Figure 3 that some SiCp were also in an irregular polygonal shape. This is because not all SiCp were coated entirely with copper during electroless plating as shown in Figure 1. The microstructure of the LCL with SiCp-Cu content of 50 wt.% at a higher magnification is shown in Figure 5. It is clear that the microstructure of the LCL was mainly composed of undissolved SiCp and dark gray lump-like crystals which were distributed on the ternary eutectic of Al-Si-Cu. The Si content of AlSi20 alloy powders used in cladding materials is 20 wt.%. At the same time, SiCp-Cu reacts with molten aluminum and forms Al4SiC4 and Si during the laser cladding process. The Si is dissolved in AlSi20, increasing its Si percentage, so that the content of Si in the molten aluminum could exceed the eutectic point. On the basis of the Al-Si binary phase diagram, the hypereutectic Al-Si matrix microstructure consisted of Al-Si eutectic and acicular primary Si crystals [17].
After electroless copper plating of SiCp, the weight gain percentage of SiCp is close to 100%, so the cladding material with SiCp-Cu content of 50 wt.% is comprised of 50 wt.% of Al-Si powder, 25 wt.% of SiCp, and 25 wt.% of Cu. The Al-Cu-Si ternary eutectic alloy is mainly composed of primary crystal Si, Al + Al2Cu binary eutectic, and Al + Si + Al2Cu ternary eutectic composition due to the high cooling rate (in the range of 10 3 -10 8 K/s during the laser cladding process) [30][31][32].
EDS analysis was performed to examine the exact composition of LCL with SiCp-Cu content of 50 wt.%. Results of EDS analysis conducted on points a-d in Figure 6 are then summarized in Table The Si content of AlSi20 alloy powders used in cladding materials is 20 wt.%. At the same time, SiC p-Cu reacts with molten aluminum and forms Al 4 SiC 4 and Si during the laser cladding process. The Si is dissolved in AlSi20, increasing its Si percentage, so that the content of Si in the molten aluminum could exceed the eutectic point. On the basis of the Al-Si binary phase diagram, the hypereutectic Al-Si matrix microstructure consisted of Al-Si eutectic and acicular primary Si crystals [17].
After electroless copper plating of SiC p , the weight gain percentage of SiC p is close to 100%, so the cladding material with SiC p-Cu content of 50 wt.% is comprised of 50 wt.% of Al-Si powder, 25 wt.% of SiC p , and 25 wt.% of Cu. The Al-Cu-Si ternary eutectic alloy is mainly composed of primary crystal Si, Al + Al 2 Cu binary eutectic, and Al + Si + Al 2 Cu ternary eutectic composition due to the high cooling rate (in the range of 10 3 -10 8 K/s during the laser cladding process) [30][31][32].
EDS analysis was performed to examine the exact composition of LCL with SiC p-Cu content of 50 wt.%. Results of EDS analysis conducted on points a-d in Figure 6 are then summarized in Table 4. Combining with the XRD phase analysis (Figure 2), it was reasonable to consider that the white lump-like crystals (point b) were Al 2 Cu, the black plate-like crystals (point c) were Al 4 SiC 4 , and the light gray region (point d) was Al + Al 2 Cu binary eutectic. The dark gray lump-like crystals (point a) in Figure 6 may be Si. Figure 7a shows the SEM image of the laser cladding layer. Figure 7b-d show the elemental maps of the LCL corresponding to the distribution of Al, Si, and C, respectively. According to that, the dark gray lump-like crystals (point a) in Figure 6 and acicular crystals can be recognized as Si phase. In summary, the microstructure of SiC p-Cu /Al-Si LCL mainly comprises undissolved SiC p , lump-like primary Si, lump-like Al 2 Cu, plate-like Al 4 SiC 4, and Al-Si-Cu ternary eutectic. a) in Figure 6 may be Si. Figure 7a shows the SEM image of the laser cladding layer. Figure 7b-d show the elemental maps of the LCL corresponding to the distribution of Al, Si, and C, respectively. According to that, the dark gray lump-like crystals (point a) in Figure 6 and acicular crystals can be recognized as Si phase. In summary, the microstructure of SiCp-Cu/Al-Si LCL mainly comprises undissolved SiCp, lump-like primary Si, lump-like Al2Cu, plate-like Al4SiC4, and Al-Si-Cu ternary eutectic.  Table 4. EDS analysis of laser cladding layer with a SiCp-Cu content of 50 wt.% (corresponding to a, b, c, d points in Figure 6).   Table 4. EDS analysis of laser cladding layer with a SiC p-Cu content of 50 wt.% (corresponding to a, b, c, d points in Figure 6).    Figure 8a. The curved edges of the interface between the coating and the substrate caused by the laser beam center had a higher temperature than that of the edge region. After etching the surface of the LCL using the Keller's reagent, the OM images obtained are shown in Figure 8b. It can be seen that there are many orientated growth dendrites between the substrate and the coating, and the growth direction is   Figure 8a. The curved edges of the interface between the coating and the substrate caused by the laser beam center had a higher temperature than that of the edge region. After etching the surface of the LCL using the Keller's reagent, the OM images obtained are shown in Figure 8b. It can be seen that there are many orientated growth dendrites between the substrate and the coating, and the growth direction is substantially perpendicular to the substrate. Moreover, the LCLs show good metallurgical bonding to the substrate due to the orientated growth dendrites which are intergrown with the substrate.  Figure 8 shows the interface between the LCL and the substrate. (The samples were cut perpendicular to the LCL direction and polished. The images were taken in the center of the LCL region.) OM images of the coating before etching are presented in Figure 8a. The curved edges of the interface between the coating and the substrate caused by the laser beam center had a higher temperature than that of the edge region. After etching the surface of the LCL using the Keller's reagent, the OM images obtained are shown in Figure 8b. It can be seen that there are many orientated growth dendrites between the substrate and the coating, and the growth direction is substantially perpendicular to the substrate. Moreover, the LCLs show good metallurgical bonding to the substrate due to the orientated growth dendrites which are intergrown with the substrate.   Figure 9 shows the relationship between microhardness of LCLs measured on the cross-section and the SiC p-Cu content in cladding materials. It can be seen that the average microhardness increases with the increasing SiC p-Cu content. It is noteworthy that the average microhardness of the LCL with SiC p-Cu content of 50 wt.% (508 HV 0.05 ) is about 3.5 times higher than in the 4032 aluminum alloy substrate (145 HV 0.05 ).  Figure 9 shows the relationship between microhardness of LCLs measured on the cross-section and the SiCp-Cu content in cladding materials. It can be seen that the average microhardness increases with the increasing SiCp-Cu content. It is noteworthy that the average microhardness of the LCL with SiCp-Cu content of 50 wt.% (508 HV0.05) is about 3.5 times higher than in the 4032 aluminum alloy substrate (145 HV0.05).   The variation of microhardness along depth direction of the LCLs is shown in Figure 10. As can be seen, the microhardness of LCL reinforced with SiC p-Cu ranges from 190 HV 0.05 to 250 HV 0.05 , and the average value is 210 HV 0.05 . The microhardness of the other LCL reinforced with SiC p is between 170 and 209 HV 0.05 and the average value is 192 HV 0.05 . This indicates that electroless copper plating on SiC p can improve the microhardness of SiC p /Al-Si composite coating.

Microhardness
The variation of microhardness along depth direction of the LCLs is shown in Figure 10. As can be seen, the microhardness of LCL reinforced with SiCp-Cu ranges from 190 HV0.05 to 250 HV0.05, and the average value is 210 HV0.05. The microhardness of the other LCL reinforced with SiCp is between 170 and 209 HV0.05 and the average value is 192 HV0.05. This indicates that electroless copper plating on SiCp can improve the microhardness of SiCp/Al-Si composite coating. The two reasons for the high microhardness of the SiC p /Al-Si composite coating are as follows: Firstly, the increase in microhardness is mainly attributed to the dissolution of SiC p-Cu in the LCLs and the resulting increase in the numbers of Al 4 SiC 4 (1200 HV [30]) and Al-Cu intermetallic (microhardness of Al 2 Cu in the range of 400-600 HV 0.2 [33]) formed on resolidification. Secondly, the effects of laser rapid heating and cooling cause a finer and harder microstructure.

Wear Properties
The wear mass loss of all samples is presented in Figure 11. It can be seen that the wear mass loss showed a decrease as the content of SiC p-Cu increased from 0 to 30 wt.% and was less than that of the substrate. On the contrary, the LCLs with 40-50 wt.% of SiC p-Cu had more wear mass loss compared with the substrate. The wear mass loss of LCL with a SiC p content of 20 wt.% was less than that of the substrate, but more than that of the LCL with a SiC p-Cu content of 20 wt.%. Figure 12 shows the SEM images of the worn surface of all samples. In Figure 12a, it is clearly observed that the worn surface of the substrate was easily deformed plastically under stress and shows parallel grooves. The grooves on the worn surface of the LCLs became narrower and shallower as the content of SiC p-Cu increased from 0 to 30 wt.%, as shown in Figure 12b-e. The grooves on the worn surface of the LCL with 20 wt.% of SiC p were deeper and wider compared with those of the LCL with 20 wt.% of SiC p-Cu . Nevertheless, it can be seen from Figure 12f-g that the grooves became deeper as the content of SiC p-Cu increased from 40 to 50 wt.%. Therefore, the LCL with 30 wt.% of SiC p-Cu exhibits the best wear resistance, and electroless copper plating on SiC p can improve the wear resistance of SiC p /Al-Si composite coating. Improvement of wear resistance of the SiC p /Al-Si composite coating must be attributed to the presence of Al 4 SiC 4 and Al-Cu intermetallic and finer microstructure. When the content of SiC p-Cu reaches 40 wt.% or more, the content of AlSi20 decreases evidently, resulting in the difficulty of SiC p-Cu packed by Al-Si alloy. The SiC p-Cu were easily separated from the worn surface during the wear test and tended to plow the LCLs seriously. Consequently, the decrease of the wear resistance of the LCLs with high SiC p-Cu content is observed.

Conclusions
1) SiCp-reinforced aluminum matrix composite coatings with high microhardness can be successfully obtained on the surface of 4032 aluminum alloy by the laser cladding process. Electroless copper plating on SiCp can improve the properties of SiCp/Al-Si composite coating.
2) The microstructure of the SiCp-Cu/Al-Si laser cladding layer consisted of undissolved SiCp,

1)
SiC p -reinforced aluminum matrix composite coatings with high microhardness can be successfully obtained on the surface of 4032 aluminum alloy by the laser cladding process. Electroless copper plating on SiC p can improve the properties of SiC p /Al-Si composite coating.
2) The microstructure of the SiC p-Cu /Al-Si laser cladding layer consisted of undissolved SiC p , lump-like primary Si, lump-like Al 2 Cu, plate-like Al 4 SiC 4 , and Al-Si-Cu ternary eutectic. Meanwhile, the microstructure became finer with the increasing of SiC p-Cu content due to the fast solidification.
3) The microhardness of the laser cladding layer increased with the increasing of SiC p-Cu content.
It increased from 145 HV 0.05 to 508 HV 0.05 due to the presence of Al 4 SiC 4 and Al-Cu intermetallic and finer microstructure. 4) The wear resistance of the laser cladding layer increased with the increasing of SiC p-Cu content.
The LCL reinforced with a SiC p-Cu content of 30 wt.% exhibits the best wear resistance. When the SiC p-Cu content reached 40-50 wt.%, the wear resistance of the LCLs decreased due to the spalling of SiC p-Cu during the wear test.