Effect Of T6 Heat Treatment On The Microstructure And Mechanical Properties Of Ti@(Al-Si-Ti)_P/A356Al Composites Fabricated Via Powder Thixoforming

In this paper,to overcome the strength-ductility trade-off of particulate-reinforced metal matrix composites,a core-shell structured particle-reinforced A356 aluminum matrix composite（Ti@（Al-Si-Ti）_P/A356Al）was prepared by powder-thixoforming technique,and the composite was treated by T6 to further improve the comprehensive mechanical properties.The solid solution treatment and subsequent ageing treatment were carried out to investigate the effect of heat treatment on the microstructure and mechanical properties of the composite.The achieved main results include:（1）The Ti@（Al-Si-Ti）_P/A356Al composite not only has improved hardness,tensile strength and yield strength of the A356 matrix aluminum alloy（increased by11.9%,17.7%and 28.5%,respectively）but also possesses a comparably plasticity（10.4%）the A356 matrix aluminum alloy（12.4%）.After solid solution treatment,the primaryα-Al particles coarsen,the solubility of Mg in the primary particles increases,and that of Si element reaches the saturability（1.81 wt.%）.The eutectic Si phases spheroidize besides coarsening,which improves the intergranular microstructure and increases the bond strength between the primary particles,and thus,deforms more homogeneously during the tensile test.The reactions between Ti and Al matrix continuously operate,and Ti@（Al-Si-Ti）_P with dense microstructure and suitable thickness of the intermetallic shell layer are achieved.Compared with the as-fabricated composites,the elongation of the solutionized composite increases significantly（48.1%）,while the strength and hardness only slightly increase（3.3%and 4.0%,respectively）.（2）During the aging treatment at 170℃,the Ti@（Al-Si-Ti）p does not change significantly.The composite shows a double-aging peak.The precipitates change from the GP regions to theβ"phases during aging for 1-7h,both the strength and hardness increase,and the first aging peak achieves at 8h due to the formation of coherentβ"phases.During the period of 9-48h,theβ"phases gradually become intoβ’andβphases,and the equilibrium phases of Mg₂Si are formed after 48h.Simultaneously Al₉Si phases increase,and they completely are coherent with the matrix,have relatively small sizes,and thus,have a higher strengthening effect than theβ"phases,the second aging peak thereby appears at 24h,and the peak is higher than that at 8h.The tensile strength,yield strength,and hardness of the composite were 318.0 MPa,247.6 MPa,and 105.3 HV,respectively,increased by 29.1%,75.1%,and 40.6%compared with those of the as-fabricated composite respectively,maintaining good plasticity（elongation of 8.2%）.Compared with the A356 matrix alloy,the composite at the peak age has a higher elongation,implying the Ti@（Al-Si-Ti）_P still has a good toughening role for the T6-treated composite.（3）For the composites aged at different temperatures for 8h,the precipitated phases at 150℃mainly include Pre-β"and Si phases,and a small amount of GP zone andβ"phases,the composite is in an under-aging state.At 190℃,the precipitates are mainly composed ofβ’and Si phases,and a small amount ofβ"and Al9Si phases,and the composite is in an over-aging state.With the rise of temperature,the reaction between Ti and Al matrix is improved,and the composite experience the three stages in turn of under-aging,peak-aging,and over-aging stages,and its strength and hardness first increase and then decrease,and achieves a peak at 170℃.