Laser fabrication of Mo-TiC MMC on AA6061 aluminum alloy surface

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Abstract

A Nd-YAG laser was used to clad Mo/TiC onto the surface of Aluminum Alloy AA6061. Experimental results indicated that the Mo/TiC clad layer has good metallurgical bonding with the substrate. TiC particulates are uniformly dispersed in the clad surface. The optimised clad layer with a pre-mixed paste composition of 30%Mo70%TiC exhibited the best wear resistance during a pin-on-disc abrasive wear test. The microhardness of the clad layer is 5–10 times higher than the as-received Al alloy. The microstructures, chemical compositions and the worn surface of the clad layers were analysed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD).

Introduction

Recent laser surface engineering research has shown increased–interest in laser cladding of ceramic-metal composite coatings on a range of alloys for the purpose of improving the wear resistance property [1], [2], [3], [4], [5]. Various ceramic or ceramic–metal systems on steel, titanium alloys and aluminum alloys have been studied. TiC/Stellite 6 [4], (WC+W2C)/Co-Cr-C and (WC+W2C)/Ni-B-Si [5], in situ (Ti+C)/Ni [6] and Ni-Cr3C2/(Ni+Cr) [7] were clad on steels and Cr3C2/Ti [8], TiC [9], TiN [9], SiC [9] and Ti/VC [10], Ti/ MoSi2 [10] were clad on titanium alloys. Various ceramic or ceramic–metal systems on Al alloys have also been studied, such as Si [11], Al2O3/TiO2 [12], ZrO2/Y2O3 [12], TiC [13], WC [14], Mo/WC [15], SiC [16] and SiC/Si3N4 [17]. Among the alloys that have been investigated, aluminum alloys and titanium alloys have drawn a lot of interests because of the potential significant improvement in tribological properties that these coatings can offer.

Our previous study [15] on laser cladding of Mo/WC metal matrix composite (MMC) on Al alloy showed that the Al-Mo matrix plays an important role in binding the WC particles in the MMC layer. This also confirmed the results of Almeida [18] and McMahon [19] that Mo and Al can form a hard matrix. However, because of the density of tungsten carbide is approximately 6 times higher than that of Aluminum Alloy (Table 1), the convection or Maragoni flow of the molten Al was not strong enough in achieving a homogeneous distribution of WC particles in the clad layer. Most of the undissolved WC particles in the clad layer were found to be concentrated at the bottom of the clad layer. Similar phenomenon of inhomogeneous distribution was also observed by other researchers. On laser remelting of WC and TiC coated Al alloy, A. Roósz [14] found that WC had higher tendency to sink to the bottom of the clad and TiC was agglomerated at the surface. Non-uniform distribution of undissolved particles in the clad layer was also observed in TiB2 in titanium alloy [20] and W in ferrous alloy [21] when there exists a large difference in density. On mixing TiC with Al-Si and Ni-based powders, Sallamand et al. [22] found that random distribution of undissolved TiC particles occurred in the clad layer, mainly attributed to the similar density of the phases concerned. They also concluded that no dissociation of TiC particles occurred because of the measured melt pool temperature of 1800–2000 °C is lower than the melting point of TiC [22].

The density of TiC is only slightly higher than that of the Al alloy (Table 1) and is much less than that of WC. Thus, in theory the distribution of TiC particles in the clad layer should be better than that of WC if the same Al alloy AA6061 as substrate and Mo as the matrix material were used. The paper discusses the results of cladding Mo/TiC on Al alloy AA6061 and the wear properties of this MMC cladding.

Section snippets

Material preparation and laser cladding process

Aluminum alloy AA6061-T651 samples were machined into rectangular blocks with 50×30×10 mm. Its nominal chemical composition in wt.% is 0.4–0.8 Si, 0.15 Mn, 0.25 Zn, 0.7 Fe, 0.15–0.4 Cu, 0.004–0.35 Cr, 0.8–1.2 Mg, 0.15 Ti and balance Al. The samples were degreased and sand-blasted prior to the cladding process for better adhesion and to remove any surface contaminants. The blocks were then rinsed with ethanol. The physical properties of Mo, TiC and Al are shown in Table 1. The average particle

For 100% TiC pre-placing powder

When the laser beam irradiates on the PVA bound TiC powder on the Al alloy, the PVA is vaporised immediately and heat energy is being absorbed by the TiC particles and transferred by conduction to the aluminum alloy surface. Due to the large difference between the melting point of TiC and Al, i.e. 3065 and 582 °C respectively, aluminum alloy is being melted preferentially and the molten metal is dragged upwards by capillary force to fill up the empty spaces between the TiC particles. If the

Conclusion

The fabrication of a Mo/TiC surface MMC on aluminum alloy substrate can be achieved using laser cladding of pre-pasted powders of Mo and TiC. The values of microhardness of the clad layers with different pre-mixed compositions of Mo/TiC are 5–10 times higher than that of the as-received Al alloy. Compared with the as-received Al alloy AA6061, the cumulative wear loss of the laser clad specimens is 20–28 times better. It was found that clad layers with a powder paste of 70% TiC with 30% Mo

Acknowledgements

The authors would like to acknowledge the support from the Research Committee of the Hong Kong Polytechnic University (Project No. GV 904).

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