Research On Hot Extrusion Behavior Of Casting Aluminum Matrix Composite

Light weight, high strength and high modulus in situ aluminum composite pipe have a broad prospect of application in aerospace, national defense industry fields. The main purpose of the paper is to produce high strength and high modulus of aluminum composite pipes through backward extrusion. In order to get high modulus, we choose large volume fraction TiB₂ particle reinforcement. Large volume fraction particle reinforcement have bad flow ability and interfacial bonding, so we choose casting eutectic aluminum composite as matrix material which have good flow ability. However, casting aluminum composites have bad plasticity. We choose the method of backward extrusion to improve plasticity and gain the composite pipes.This paper combines the thermal simulation and actual experiments to research flow stress behaviors of composite materials at high temperature. The contents include: discussing flow stress behaviors of composites at high temperature and how temperature and strain rate influence flow stress behaviors; establishing the thermal deformation constitutive equation of composites; drawing power dissipation maps and processing maps according to dynamic material model; doing backward extrusion experiments to prove results of processing maps and making performance comparisons of composites under different conditions and related analysis.Firstly, the paper studied flow stress behaviors of TiB₂/Al-Si-Mg-Cu composite at high temperature using Gleeble3500. The results showed that TiB₂/Al-Si-Mg-Cu composite has steady-state flow characteristics during hot deformation. The flow stress increased with the strain rate and decreased with the temperature. Meanwhile, the paper figured out the constitutive equation according to the true stress—true strain curves of composites.According to the data of hot simulation, the paper draws the power dissipation maps and processing maps. The power dissipation maps showed that power dissipation efficiencies of TiB₂/Al-Si-Mg-Cu composite increased with temperature and decreased with strain rate. That was helpful to dynamic recrystallization. From the processing maps, it would be found there were two instable regions. One was the region with high strain rate and low temperature which was mainly due to matrix crack and particle debonding and crack. The other was the region with high temperature and middle strain rate which could be due to wedge crack and particle debonding and crack. The two regions should be avoided in the hot deformation. The optimum working region was the region with high temperature and low strain rate, for example, 500℃,10-3 s^-1. The region was stable and the power dissipation efficiency was highest. According to the processing map, it was found that 500℃,10-3 s^-1 was the optimum working region after backward extrusion. The composite had smooth surface and no cracks. But under the condition of 300℃,1 s^-1, there were macro cracks in the surface.At last, the paper tested the performances of composites under different conditions. The result of microstructure hardness showed: original composite < composite after backward extrusion < composite after backward extrusion and T6 heat treatment. The results of tensile tests showed that after extrusion, the tensile strength, modulus and elongation increased very much, by 53.8%, 16.9% and 191.4% separately. Whether the composite was heat treated, it could be found that the tensile strength increased, the modulus had no change and the elongation decreased. The best composite was the one after backward extrusion and heat treatment.