摩擦攪拌銲接與製程(Friction Stir Welding/Processing, FSW/FSP)長期存在攪拌棒頭部之磨耗問題。本研究利用轉移式電漿電弧(Plasma Transfer Arc, PTA)熔解法將耐磨耗之Stellite 1 鈷基超合金被覆在具有優良抗熱裂特性且價格低廉之SKD61模具鋼攪拌棒頭部，期望研發出具有高磨耗阻抗且符合經濟效益之Stellite 1 鈷基超合金攪拌棒(SKD61- ST1)。本研究在相同實驗參數下將SKD61- ST 1攪拌棒與現行材SKH51高速鋼系列之攪拌棒(回火材SKH51-T與退火材SKH51-A)分別進行5083鋁合金摩擦攪拌製程，除了對其磨耗量作一比較，更將攪拌棒之磨耗部位作解析，探討攪拌棒之磨耗機制與影響攪拌棒磨耗行為之因素。
研究結果顯示Stellite 1被覆層為一含M7C3及M23C6 型碳化物的凝固組織，SKD61基材對Stellite 1被覆層之稀釋效應極低，被覆組織之基地相為γ-Co。在鋁合金FSP中，SKD61-ST1具有高於SKH51-A及SKH51-T高速鋼攪拌棒之磨耗阻抗。此外，藉由SEM及EPMA針對攪拌棒頭部之磨耗現象作進一步解析，SKD61-ST1、SKH51-A及SKH51-T攪拌棒在FSP中均產生滑動磨耗，來自棒頭之材料碎片與來自鋁合金之碎片混合，於攪拌棒之凸梢與肩部表面形成轉移層，推測轉移層之脫落最終造成攪拌棒之質量損失。由Thin Film XRD之分析結果可知，轉移層內部分材料碎片產生類似機械合金化之反應，於SKD61-ST1之轉移層內生成Al9Co2，SKH51-A及SKH51-T之轉移層內則生成Al5Fe2與AlFe。
根據實驗結果，硬度與材料本質為本研究中影響攪拌棒磨耗行為之主要因素:高硬度之SKH51-T，其轉移層厚度、轉移層包覆棒頭之覆蓋率以及磨耗量均較低硬度之SKH51 -A為低。SKD61-ST1雖然棒頭Stellite 1鈷基超合金組織之硬度低於SKH51-T，但可能因Co-Al之金屬親合性與adhesion energy 低於Fe -Al，產生較薄且覆蓋率較低之轉移層，最後導致較低之磨耗量。

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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