Investigation On Welding Of SiC Particle Reinforced Aluminum Matrix Composites

Two welding methods on SiCp/Al matrix composites that are vacuum brazing technology and transient liquid phase bonding are studied in my dissertation. By investigating the weldability of 60% high volume percentage SiC particle reinforced aluminum matrix composites, I use BAl88SiMg as brazing filler metal and copper foil as interface layer to weld this kind of material in vacuum furnace. The effects of several procedure factors such as jointing materials, welding temperature, holding time and the thickness of copper foil to the performance of the joints are researched by utilizing these experimental equipments: digital camera, metallographic microscope, X-ray diffractometer, micro-scleromete, scanning electron microscope and tensile testing machine. Results showed as follows:(1) Both Vacuum Brazing Technology by copper foil as interface layer and transient liquid phase bonding by BAl88SiMg as brazing filler metal can weld this kind of material.(2) Welding SiCp/Al matrix composites by transient liquid phase bonding, welding temperature is unfavorable and exorbitant, and the pressure on the welding joints should not be too big, otherwise the junction between mother materials and the joints will produce more stress concentration and cracking. The cleanliness of the welding surface has large effect on welding quality, so welding surface must be sanded and ultrasonic cleaned, etc, in order to remove oxidation film before welding. As temperatures rising, reaction and diffusion area of the welding joints increases.(3)All welding seams are which were gotten by utilizing transient liquid phase bonding relatively small. With the extension of holding and diffusion time, the welding seams gradually disappeared and welding quality was improved. Effects which were produced by variation of Cu foil thickness on welding macro morphology is not obvious, but in the same process conditions, diffusion zones which were gotten by using 0.02mm thick Cu foils are much broader than those by using 0.01mm thick Cu foils.(4) I did not find that many of SiCp gathered and depleted in vacuum brazing seam, which are mentioned by many documents,but found a phenomenon that some SiCp had transited to brazing seam at high brazing temperature(up 573℃). As temperatures rise, brazing seam tapered off. At high brazing temperatures (up 598℃), vacuum brazing can get similar welding microstructure with transient liquid phase bonding. But at high brazing temperature(up 573℃), mother materials will produce lateral inflation and cracking, so the exterior quality will deteriorative.(5) In the joints which were gotten by transient liquid phase bonding have intermetallic compound AlCu3, and the joints which were gotten by using 0.02mm thick Cu foils have a little more AlCu3 than those by using 0.01mm thick Cu foils, while vacuum brazed joints with BAl88SiMg as brazing filler have Mg2Si and MgAl2O4. Whether by transient liquid phase or vacuum brazing,hardness of welding seams are not high, which show there are not hard brittle phases gathered largely in the welding seams. At the low brazing temperatures, hardness of vacuum brazed seams is very low but as brazing temperature rising, their numeric gradually increases. Hardness of welding seams which were gotten by transient liquid phase bonding is lower than one of brazed seams at high brazing temperatures, but higher than one of the brazed seams at low brazing temperatures.(6) With the extension of holding and diffusion time, shear strength of the joints which were gotten by transient liquid phase bonding gradually improves,while as the brazing temperatures increasing, their numeric gradually decline. With the extension of holding and diffusion time, shear strength of the joints which were gotten by using 0.02mm thick Cu foils is higher than those by using 0.01mm thick Cu foils. As the brazing temperatures increasing, shear strength of the vacuum brazed joints firstly increases, then declines. In all the welding procedures, compare to vacuum brazing, shear strength of the joints which were gotten by transient liquid phase is common higher and welding temperatures are lower so that mother materials can avoid lateral inflation and cracking, so transient liquid phase is a more appropriate welding method than vacuum brazing for SiCp/Al matrix composites.