Abstract
The surface overlaying treatment is aimed to enhance surface properties of materials such as microhardness, wear-resistance, corrosion -resistance and etc. In this study, Stellite 1 cobalt-based alloy was overlaid on SKD61 steel by plasma transfer arc (PTA) process to form SKD61-ST1 friction stir tool for friction stir welding and process (FSW/FSP). The solidification structure of overlayer was explored. Furthermore, the wear resistance of SKD61-ST1, annealed SKH51 (SKH51-A) and tempered SKH51 (SKH51-T) high speed steel friction stir tools was evaluated by FSP test in which 5083 aluminum alloy was processed. The wear characteristics of SKD61-ST1, SKH51-A and SKH51-T friction stir tools were compared and studied as well. Results indicated that Stellite 1 overlayer of SKD61-ST1 is hypereutectic structure with large amount of M7C3 and M23C6 carbides under 90A overlaying current. The phase of matrix is -Co. Applying SKD61-ST1, SKH51-A and SKH51-T to FSP tests, sliding wear occurs and weight loss results revealed that the wear resistance of SKD61-ST1 is the highest. In addition, according to SEM and EPMA analysis, a transfer layer containing fragments from both sliding parts was formed on the interface between 5083 aluminum alloy and friction stir tools after FSP. It was proposed that the decohesion of transfer layer results in the weight loss of friction stir tools. Analyzing the transfer layers one step further by Thin Film XRD, Al9Co2 was found in the case of SKD61-ST1; however, Al5Fe2 and AlFe occur in the cases of SKH51-T and SKH51-A. The formation of these compounds is similar to the process of mechanical alloying in a ball mill. Thickness and coverage of transfer layers on friction stir tools were measured by SEM. Since SKH51-A is softer than SKH51-T, both thickness and coverage of transfer layer on SKH51-A were greater than that of SKH51-T. Nevertheless, thickness and coverage of transfer layer on SKD61-ST1 is thinnest and lowest among the three cases although SKD61-ST1 is softer than SKH51-T. It is proposed that hardness is not the only factor to influence transfer layer and wear characteristics of friction stir tools. Compatibilities and adhesion energies between Al-Co and Al-Fe also play significant roles in this study.
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