Particle fracture, retention, and fluid flow in metal matrix composite friction joints

Friction joining of metal matrix composite (MMC)/MMC and MMC/AISI 304 stainless steel base materials is examined using a combination of experimental testing and numerical modeling. In particular, the fracture of reinforcing particles during the friction-joining operation is investigated. The particle diameter and interparticle spacing decrease and the area fraction of particles markedly increases in material immediately adjacent to the bondline. Smaller particles are observed in friction-welded joints produced using high friction pressures. The principal effect of the forging operation is in decreasing the interparticle spacing. There was excellent correspondence between predicted fluid flow in Al/Al joints and experimental test results examining the transfer of Al2O3 particles during the alloy 6061/alloy 6061 friction-joining operation. It is suggested that small-diameter particles formed due to fracture early in the friction-joining operation are retained al the bondline of MMC/MMC joints as a direct consequence of the flow of plasticized material and reinforcing particles in the contact zone. A combination of numerical modeling of fluid flow and direct experimental testing have confirmed that Al2O3 particles transfer from the stationary to the rotating boundary in MMC/MMC friction joints. Also, limit cycles embedded within the pow favor the retention of small-diameter fractured particles at the bondline of MMC/MMC joints. A quite different situation exists in dissimilar MMC/AISI 304 stainless steel joining. In dissimilar joints, a dynamically quiescent region is formed immediately adjacent to the stainless steel boundary. It is suggested that the absence of flow of plasticized material promotes retention of fractured alumina particles in dissimilar joints.