Pulse laser processing of a SiC/Al-alloy metal matrix composite
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Lawrence F. Allard High Temperature Materials Laboratory, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 21 June 1990; accepted 5 December 1990) The microstructural changes and the tensile behavior of laser processed A356-A1 alloy matrix composites reinforced with 10 and 20 vol. % SiC particulates are characterized. The autogenous bead-on-plate welds were made using a pulsed CO2 laser operating at a peak power level of 3.2 kW. The pulse on-time was constant at 20 ms and the off-time was varied from 20 to 2 ms (duty cycles of 50-91%). The microstructure of the laser melted region was investigated by optical, scanning, and transmission electron microscopy, and x-ray microchemical analysis techniques. The extent of microstructural changes varied directly with duty cycle, i.e., being a maximum for the longest (91%) duty cycles. Pulsed laser processing produced partial to complete dissolution of SiC particles and sometimes resulted in the formation of aluminum carbide. The associated rapid cooling also produced a fine distribution of nonequilibrium complex precipitates. In addition, the laser energy modified the SiC surface both physically and chemically. The results of tensile tests indicated that the modified SiC and the distribution of fine nonequilibrium precipitates enhance the mechanical properties of the laser processed composites. Optimum changes in microstructure and mechanical properties were obtained in the composites processed with intermediate (67 and 74%) duty cycles; therefore pulsed processing appears to be a strong candidate for successful joining of these MMCs.
I. INTRODUCTION
Discontinuously reinforced metal matrix composites (MMCs) represent a group of materials that combine the strength and hardness of the reinforcing phase with the ductility and toughness of the matrix. The resulting high stiffness-to-weight ratio of these composites makes them attractive as primary stiffening members in weight-critical air-frame and aerospace structures. The incorporation of discontinuous reinforcement/dispersoids (such as whiskers or particulates of SiC, B4C, B, W, etc.) in the metal (such as Al, Cu, etc.) matrix conserves the isotropic nature and permits the MMCs to be shaped using traditional methods such as forging, rolling, extrusion, etc. However, the ultimate implementation of these MMCs will depend on how easily and reliably they can be fabricated and joined to other similar composites and standard structural metals and alloys. The possibility of welding/joining, cutting, etc., of MMCs is thus an important consideration in the fabrication of high-strength, lightweight, and economical prototype structures. Unfortunately, composite welding/joining remains largely problematical, primarily because both the inherent thermal effects on reinforc514 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 3, Mar 1991 Downloaded: 23 Mar 2015
ing material and matrix-reinforced material compatibility are not fully understood. Conventional welding/joi
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