T. Prater, T. Bloodworth, P. Fleming, D. Lammlein, A. Strauss, G. Cook, Vanderbilt Univertsity, Nashville, TN; T. J. Lienert, M. Bement, Los Alamos National Laboratory, Los Alamos, NM
Diamond coating of tools is used for a wide range of manufacturing applications. In this investigation, we will examine changes to Friction Stir Welding (FSW) samples when a diamond coated molybdenum (Mo) tool is used to perform welds on Al alloys and Al metal matrix composites (Al-MMCs). The tool has a pin and shoulder of Mo which is coated with nanodiamonds using a chemical vapor deposition (CVD) process. The use of the diamond tool may reduce the axial force encountered during the weld, thus opening up possibilities for high-speed welding and robotic applications. Additionally, a diamond-coated FSW tool will not wear and may prove advantageous from a thermal conductivity or surface morphology standpoint. A metallurgical analysis of the welds will be conducted as well as mechanical tests to compare the weld integrity of the diamond/Mo tool with that of control samples welded at identical parameters.
Summary: A central concern of Friction Stir Welding research is the production of strong welds that are free of defects. We propose to analyze the effect of a diamond coated pin tool on weld integrity and operating parameters in FSW of Aluminum and metal matrix composites. The tool has been coated using the Chemical Vapor Deposition (CVD) method employed in the diamond fabrication lab at Vanderbilt. This CVD coating method is advantangeous because it can be applied to a diverse range of substrates and confers properties of good thermal conductivity, electrical insulation, and high temperature tolerance to the material which is coated. The size and characteristics of the coating chamber itself, however, present a problem. FSW tools are traditionally made of heat-treated steel, but the intense temperatures of the deposition chamber exceed the melting point of steel. Molybdenum is a more suitable material for CVD; its high melting point enables it to withstand the harsh environment of the deposition chamber. The typical length of the FSW tool makes it too long to fit inside the CVD unit, which is conventionally used to coat circular wafers of .25”-1.5” height (thickness). Thus, the tool must be constructed in two parts: 1) the Molybdenum smooth pin and shoulder (1.25”), which will be inserted into the CVD chamber for coating, and 2) the 5.75” steel “body” with a cylindrical opening in its center so that the pin and shoulder can be press fit after coating. The diamond tool may also have an impact upon operating parameters, particularly axial force. If the implementation of the tool results in a significant reduction in axial force, it has the potential to enhance robotic FSW applications. In FSW of “traditional” materials, tool wear is not a problem. However, a diamond-coated tool may prove useful in the welding of metal matrix composites (MMCs), where tool wear is an issue due to the abrasive nature of the material to be welded. Aluminum MMCs are of central interest, since they have potential applications in the transportation and aerospace industries. The increased strength of the diamond-coated Mo tool may enhance the FSW welding of Al-MMCs and prevent the tool wear which is common in FSW of these materials using steel tools. Through micrograph analysis and metallurgical testing of the completed welds, we can determine the operating parameters (rotation speed, pin depth, travel speed) which produce an optimum weld.
