Microstructural evolution in laser and electron beam welds on SiC-reinforced A356 aluminum.

摘要

A comparative study of laser beam welding (LBW) and electron beam welding (EBW) of cast A356/SiC/15p aluminum metal matrix composite (Al-MMC) has been completed. Laser and electron beam welds have been made at identical powers, rapid travel speeds (up to 200 ipm) and focusing conditions to allow direct comparison of the effects of the welding process characteristics with the resulting microstructures. Microstructures of the welds as well as the base material were characterized using optical microscopy (OM) with image analysis, scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) with EDS with the goal of understanding the microstructural evolution of the welds. Microstructural evidence suggests that the SiC particles dissolve into a homogeneous solution at high temperatures during LBW, and Al{dollar}sb4{dollar}C{dollar}sb3{dollar} precipitates from the solution during cooling. Conversely, for EBW, dissolution and displacement of the SiC is limited, and relatively little Al{dollar}sb4{dollar}C{dollar}sb3{dollar} is formed. Variations in microstructures of the EB and LB welds made using identical powers, travel speeds and focusing conditions are attributed to the differences in the thermal cycles experienced with the two welding processes. In turn, the disparities in thermal cycles are rationalized in terms of: (a) the differences in the physics of beam/substrate energy transfer for the two processes and (b) pressure-temperature relations (EBW is performed in vacuum while LBW is performed near atmospheric pressure). Results of the microstructural analysis are combined with arguments based on physical and thermodynamic properties of the Al alloy and SiC to develop qualitative models for the microstructural evolution in the EB welds and the different regions of the LB welds on the Al-MMC.