Effects Of Alloying Elements On Reactions During Partial Remelting Of Al-Ti Bulk Alloy Prepared By Cold Pressing Of Powder Mixture

To overcome the shortcomings of the preparation technologies and low plasticity of particle reinforced Al based composites,a new technology named powder thixoforming has been proposed by combining merits of powder metallurgy technology and thixoforming technology,a s well as a new Al matrix composite reinforced by Ti@compound core-shell structured particles.Previous studies found that the alloying elements had large effect on the thickness and compactness of the shells.Therefore,the effects of alloying elements?Si,Cu,Mg and Zn?on the formation of core-shell structured particles have been investigated,from both the thermodynamic and dynamics during partial remelting of Al-Ti bulk alloy prepared by cold pressing of powder mixture in this paper.The results indicated that the Si element can not only cause the phase transformation of the reacted shells,but also change the reaction rate.The addition of Cu,Mg or Zn does not cause any phase transformation and the Al₃Ti was the only reaction product.Mg element can greatly reduce the Al melt surface ene rgy,and thus promote the reaction with Ti powders.Zn element can not only reduce the Al melt surface energy,but also accelerate the liquid phase of Al melt due to its low smelting point,and thus enhance the reaction with Ti powders.Cu element has little impact on the reaction rate.The phase transformation in the system with Si element led to the system stability change,so the Gibbs free energy of the system will change with the increase of the content of Si element.However,the addition of the other three elements can facilitate the formation of stable Al₃Ti,so the resulting system becomes more and more stable.In the Al-5Ti-Si system heated for 10 min at 720 °C,the reaction product changes from the unique?Al,Si?₃Ti phase in turn to?Al,Si?₃Ti +?1 phases and ?1(Al₅Ti₇Si₁₂)phase as the Si addition increases from 2wt.% to 9wt.%.A compact ?1shell can be obtained when the 9wt.% Si is added.For the Al-5Ti-7Si,a more compact and thicker ?1 shell?about 7.6?m?is obtained after being heated for 30 min at 600 ° C.The volume dilatation?about 180%?of the transformation from Ti to ?1 occurred at 600 °C is less than that?about 255%?of the transformation from Ti to Al₃Ti occurred at 720 °C,so the ?1 shell is more compact and thicker.In addition,the hardness and elasticity modulus of the ?1 shell are higher.That is,the resulting core-shell structured particles not only have better strengthening role,but also good roughening role.