1. Introduction
Laser cladding, one of the modern coating techniques which can strengthen and repair the surface of materials well, has extensive application prospects [
1,
2,
3,
4]. Of all the materials used for laser cladding, ceramic particle-reinforced metal matrix composite coatings (PRMMCCs) possess many desirable properties, such as high heat resistance, good corrosion resistance and wear resistance, because they have a high melting point, high hardness and good chemical stability. Thus, it is easy to produce a coating with particular new properties, and the surface properties of machine parts made of a wide variety of materials can be modified [
5,
6,
7]. Unfortunately, the surface of the coated components is usually rough and its dimensional accuracy is poor. It needs to be machined to obtain the desired dimensional accuracy and surface roughness. However, it is difficult to machine metal matrix composite coatings with ceramic particles because the properties of the ceramic particles and the base metal are quite different. Therefore, to achieve the desired surface finish and dimensional accuracy, grinding is the most appropriate method for finishing the coatings.
Many studies have been carried out to investigate the effects of processing parameters on the microstructures, mechanical properties, crack initiation and propagation behavior of laser cladding PRMMCCs [
8,
9,
10,
11]. However, for grinding coatings, most of the research has focused on the sprayed ceramic coatings [
12,
13,
14,
15,
16]. Characteristics such as grinding forces, surface integrity, surface roughness, the wear on the wheel, etc., have been investigated in grinding sprayed coatings. Liu et al. conducted a grinding experiment on thermally sprayed nanostructured coatings (n-WC/12Co and n-Al
2O
3/13TiO
2), using diamond grinding wheels under different grinding parameters. It was observed that the normal grinding force increased when the wheel had a larger grit size and the bond was harder. Both plastic flow and brittle fracture occurred in grinding these coatings. For the n-Al
2O
3/13TiO
2 coatings, there was an optimum cut depth to obtain minimum surface roughness [
12]. Kar et al. used single-layer electroplated diamond wheels to study the grinding performance of different plasma sprayed oxide ceramics. They revealed that at a low grinding speed, the removal of material was mainly achieved by microbrittle fracture [
13]. During precision grinding of these coatings, the density of subsurface damages and wheel wear increased. Huang and Liu reported that during high-speed grinding of sintered ceramics, their surface finish could be improved, the grinding ratio increased and the subsurface damages reduced [
17]. Kar et al. reported that in high-speed grinding of ceramic oxide coatings, grinding forces reduced obviously when the wheel speed increased. Much less damage and much lower surface residual stress were obtained [
18]. Rausch et al. reported that using a ceramic binder and resin binder diamond grinding wheel could obtain a better surface quality than using a metal-based and electroplated diamond grinding wheel, in grinding two different thermally sprayed coatings. A smaller grinding force but a greater surface roughness was obtained by using a diamond wheel than by using a cBN wheel [
19].
As for grinding laser cladding coatings, very few reports have been published so far. Lin et al. used an electroplated diamond wheel to grind an Fe–Mn–C alloy laser cladding coating by electrolytic grinding and a mechanical grinding method. The ground surface roughness of the coating met the actual working requirements [
20]. In the authors’ previous research on the grinding performance of laser cladding Cr
3C
2/Ni composite coatings, it was found that cracks formed in the coating when the mass fraction of Cr
3C
2 in the coating was higher than 20%.Higher surface roughness (Ra) of the ground coating was obtained when the size of the Cr
3C
2 particles was larger [
21].
There are two types (in situ and ex situ reinforced particles) of PRMMCCs, according to the adding methods of reinforcement. In situ PRMMCCs are found to have superior mechanical properties and their cracking susceptibility can be reduced [
22,
23]. Iron-based matrix reinforced with tungsten carbide (WC/Fe) composite coating has a wide practical application value because of its peculiar combination of high wear resistance and relatively low cost. Many researchers have focused on the laser cladding process, microstructure and mechanical properties of WC/Fe coatings [
23,
24,
25,
26,
27]. The results reveal that the WC particle formed in situ has a much smaller size and distributes more uniformly. As a result, the mechanical properties of the WC/Fe coating are improved. In contrast, ex situ WC/Fe coatings crack easily. Coatings with a higher hardness generally have an increased cracking sensitivity, particularly in coatings which have a high content of WC particles [
28,
29].
Usually, the grinding performance of a workpiece depends on its microstructures and mechanical properties. Moreover, the constituents of the composite coating should be considered in selecting the wheel type and the grinding parameters [
30]. Based on the above literature, it is known that the investigation of the grinding performance of laser cladding coatings is quite limited. Additionally, the difference of the grinding performance characteristics between the composite coatings fabricated by two different methods has not been well studied. In order to provide a theoretical basis for the grinding process for composite coatings by different methods of adding reinforcement, comparison of the microstructures and mechanical properties, as well as grinding performance, under different grinding conditions between in situ and ex situ composite coatings is essential. Thus, in this paper, crack-free ex situ WC/Fe composite coatings with 10% WC (weight percentage) and in situ WC/Fe composite coatings were laser cladded for the grinding performance study. Two types of wheels, made of an ordinary abrasive and a superabrasive, were used to perform the grinding tests.
2. Materials and Methods
Two kinds of WC/Fe composite coatings were laser cladded on steel substrates with 0.45% C (AISI 1045) and dimensions of 50 × 50 × 10 mm
3. Each substrate was abraded with an abrader to remove the rust. Then, it was smoothed with a WA120 grinding wheel and cleaned with ethanol before laser cladding. Commercial powders of Fe314 and Rockit701 were used to fabricate the coatings. Detailed compositions of the powders are presented in
Table 1 and
Table 2. The powders of Fe314 and WC (10% weight percentage, 38 μmin size) were mixed well. The thickness of the preplaced coating was set to be 0.8 mm. AXL-600AW YAG laser equipment was used. Based on our preliminary studies [
21,
27], several groups of parameters were explored. Finally, the laser parameters were selected to laser clad the crack-free WC/Fe coating specimens for the mechanical properties and grinding performance studies. For ex situ WC/Fe coating, an electric current of 120 A and a beam scanning speed of 3 mm/s were used. For the in situ WC/Fe coating, an electric current of 130 A and a beam scanning speed of 6 mm/s were used. In addition, for both coatings, a fluency of 30 Hz, pulse width of 3 ms, laser beam diameter of 1.5 mm, distance between tracks of 1 mm, with an overlap ratio of approximately 50%, were used.
Microhardness measurement was accomplished in a microhardness tester, under a load of 1.96 N and a retention time of 10 s. The transverse rupture strength (TRS) of the coating specimen was gained by the three-point bending test method. A WD-300K universal testing machine was used. The microstructure was examined by SEM.
Grinding performance studies were carried out on a precision grinder (M250). The samples’ size was 15 × 10 × 10 mm
3. Two types of grinding wheels made of alumina abrasive (referred to as WA120) and cubic boron nitride (referred to as 120B-cBN) were used under different grinding conditions. The specifications of the grinding wheels and the experimental parameters are listed in
Table 3 and
Table 4. Because the removal allowance on the laser coating surface was large, the grinding parameters were set with the expectation to quickly remove the machining allowance without producing serious processing defects. A single-point diamond dresser was used to true and dress the WA120 wheel. An SiC wheel was initially used to true the cBN wheel, and then, a stone with SiC abrasives was used. Both wheels were dressed before and after each experiment during the investigations.
The tangential and normal grinding forces were measured online using a Kistler 9257BA dynamometer. The results of the measured grinding forces were averages of several repetitive grinding tests. The surface roughness (Ra) of the ground coatings was measured by a surface profile meter (Mahr XR20) along the parallel and perpendicular direction to the grinding direction, five times, respectively. The value of Ra was the average of these measurements. the surface morphology of the ground coatings was examined by SEM.
4. Conclusions
The experimental results in this paper reveal that with the same test conditions, the grinding forces and force ratio of the in situ WC/Fe coating are greater than those of the ex situ WC/Fe coating because the hardness of the former is almost twice as high as the latter. Using the WA120 grinding wheel had a greater grinding force and smaller force ratio than using the 120B-cBN grinding wheel, due to the lower hardness and the smaller protrusion of Al2O3 abrasives. The value of Ra of the ex situ coating (WC/Fe) was obviously bigger than that of the in situ coating. On the other hand, using the 120B-cBN wheel gains a much higher value of Ra than using the WA120 wheel, when grinding the two coatings. The subsurface damages observed on the ground surface of the ex situ WC/Fe coating by the two wheels included grinding traces, debris adherence, pits, cracks, fractured areas and broken WC particles. However, the ground surface morphology of the in situ WC/Fe coating was significantly different to that of the ex situ WC/Fe coating. Moreover, the surface appearance of the in situ WC/Fe coatings ground by the two wheels was distinctly different. Even at a small cut depth, quite a number of cracks were seen on the coating surface when the WA120 wheel was used. No obvious crack was found, but deeper grinding marks and fractured areas appeared on the surface when the 120B-cBN wheel was used. Therefore, through the comprehensive assessment of this investigation, one can find that for grinding the ex situ WC/Fe coating, the WA120 wheel is more suitable than the 120B-cBN wheel, but for grinding the in situ WC/Fe coating, the 120B-cBN wheel is more appropriate than the WA120 wheel.
Regarding the experimental results in the present work, there were severe subsurface damages on the ground coatings. A deeper study of the formation mechanism of the subsurface damages is needed. In addition, further grinding performance research should be carried out to find an appropriate grinding process to eliminate or reduce the grinding defects.