Fabrication, Microstructure And Mechanical Properties Of In Situ Aluminum Matrix Composites By Friction Stir Processing

Friction stir processing (FSP) is a new solid state processing technique for plastic processing and fabrication of metallic materials, and has found several applications in fabrication of ultra-fine grained materials, microstructural modification and in situ synthesis of intermetallic compounds and metal matrix composites. In the present study, in situ aluminum matrix composites (AMCs) were fabricated via vacuum hot pressing (VHP) and subsequent FSP in Al-Ti and Al-TiO2systems. The reasons why in situ reaction took place during FSP and the formation mechanism of in situ reinforcing particles during FSP were studied. The effects of processing parameters on the microstructures and properties of in situ AMCs were also investigated.For in situ composites based on the Al-Ti system, the formation mechanism of Al3Ti and the effects of VHP and FSP parameters on the resultant microstructure and mechanical properties were investigated. Al3Ti formed due to reactive diffusion between Al and Ti during VHP, and the number of Al3Ti increased with increasing the temperature and holding time of the VHP. FSP not only induced the Al-Ti reaction, but also resulted in a significant refinement of Al3Ti, thereby creating a homogeneous distribution of Al3Ti particles in the Al matrix. These microstructural changes led to a significant improvement in the tensile properties of the in situ Al3Ti/Al composite. However, the change trends of the tensile properties of the FSP samples were dependent on the extent of the Al-Ti reaction during VHP. For the samples in which the Al-Ti reaction did not take place or take place partly during VHP, the tensile strength increased with the increase of tool rotation rate, while for the sample in which the Al-Ti reaction took place completely, the tensile strength decreased with the increase of tool rotation rate.The kinetics of the Al-Ti reaction during isothermal annealing and FSP was studied. The Al-Ti reaction during isothermal annealing and FSP can be described by an empirical relationship:VAl3Ti=ktn. The reaction was diffusion-controlled at525and550℃, and interfacial-reaction-controlled at575-650℃. The Al-Ti reaction during FSP at rotation rates of1000and2000rpm was interfacial-reaction-controlled. The reaction rate constant k during FSP was substantially enhanced compared to that during annealing, which can be attributed to the decrease in activation energy caused by severe deformation during FSP. The effective temperature was proposed to show the contribution of the mechanically activated effect of FSP, and was calculated to be 710and716℃for FSP with rotation rates of1000and2000rpm.The effects of Cu or Mg addition on the microstructure and mechanical properties of in situ composites based on the Al-Ti system were studied. Cu addition in the Al-Ti system enhanced the Al-Ti reaction and resulted in the formation of more Al3Ti particles due to the presence of a small amount of liquid phase during FSP. Al2Cu experienced dissolution and precipitation during FSP, and part of Cu was kept in the Al matrix as solute after FSP. Mg addition in the Al-Ti system led to the formations of some Ti2Mg3Al18particles with fine twin lamellas during FSP, resulting in an increase in the total volume fraction of reinforcing particles. Cu and Mg addition increased the strength of the in situ composites substantially due to introduction of more strengthening modes or more reinforcing particles, however the elongation decreased dramatically.Large-area bulk in situ AMCs based on the Al-Ti and Al-Ti-Cu systems were fabricated by overlapped FSP with an overlapping ratio of50%. The extent of Al-Ti reaction during overlapped FSP was similar with that in in situ4pass FSP. The hardness distribution in the cross section of overlapped FSP Al-Ti sample was relatively homogeneous, and the tensile properties were also similar with those of the Al-Ti sample subjected to in situ4pass FSP. The hardness of overlapped FSP Al-Ti-Cu samples increased with the increase of the distance from the first pass, and the tensile strength was lower than that of in situ4pass FSP Al-Ti-Cu sample. This was caused by the increase and coarsening of Al2CU in the matrix close to the first pass due to more thermal cycles experienced. For overlapped FSP Al-Ti-Cu sample, the hardness distribution became homogeneous and tensile properties increased after solid solution treatment.The reactive mechanism of Al-TiO2reaction and the strengthening mechanism of in situ composites based on the Al-TiO2system were investigated. The occurrence of the Al-TiO2reaction during FSP can be attributed to the enhanced solid diffusion and mechanical activation effect caused by the severe deformation of FSP. The formation mechanisms of Al2O3and Al3Ti during FSP are determined to be a deformation-assisted interfacial reaction and deformation-assisted solution-precipitation. The strengthening mechanisms of the FSP in situ composites included load transferring, grain refinement and Orowan strengthening, among which Orowan strengthening contributed the most to the yield strength of the composites.The effects of VHP and FSP parameters on the microstructure and mechanical properties of in situ composites based on the Al-TiO2system were studied. The Al-TiO2reaction hardly took place during VHP at500℃with a holding time of5min. And the Al-TiO2reaction took place partly during VHP at550℃with a holding time of50min. All the TiO2reacted with Al to form coarse and polygonal Al3Ti particles and Al2O3particles with a size of150nm during VHP at630℃with a holding time of240min. For samples in which the Al-TiO2reaction took place hardly or partly, subsequent FSP induced the Al-TiO2reaction, resulting in the formation of Al3Ti and Al2O3with a size of about80nm. The tensile properties of the FSP in situ composites increased substantially compared with those of FSP pure Al. For sample in which the Al-TiO2reaction took place completely during VHP, subsequent FSP blunted the polygonal Al3Ti particles and distributed the Al2O3homogeneously in the matrix. However, the tensile strength of the FSP sample in which the Al-TiO2reaction took place completely during VHP was lower than that of FSP samples in which Al-TiO2reaction took place hardly or partly during VHP.For FSP sample in which the Al-TiO2reaction did not take place during VHP, the volume fraction of reinforcing particles increased with decreasing the traverse speed, however, the matrix grain size was not influenced by the traverse speed of FSP. Thus the strength of FSP samples increased as the traverse speed of FSP decreased. When the traverse speed was25mm/min, increasing tool rotation rate from1000to2000rpm caused little influence on the microstructure and mechanical properties of the in situ composites. Additional2pass FSP in water did not change the size and distribution of reinforcing particles, but refined the matrix grains, resulting in the increase in the tensile strength and the decrease in the elongation of the in situ composite.