Study On Microstructure And Properties Of High Strength And Toughness Al-Si Alloy By Squeeze Casting

Chinese automobile production and sales accounted for about a third of the world's share in 2018,ranking first in the world for 10 consecutive years.The lighter the weight of the car,the more energy will be saved,so the lightweight of the car is an important topic at present.Replacing steel with aluminium alloys is the core of automobile lightweight.It is of great significance to develop aluminium alloy with high strength and toughness.At present,there is little research on development and squeeze casting of high strength and toughness Al-Si al oys,and this paper will focus on research of it.A new type of Al-9Si-Cu-Zn-Mg?AS9 alloy for short?alloy with high strength and toughness has been developed in this paper.Ce/La mixed rare earth and Sr have been used to modify the structure of the alloy and squeeze casting has been adopted.The as-cast mechanical properties of the high strength and toughness squeezed Al-Si alloy with a diameter of 5cm are 297.6 MPa in tensile strength,235.47 MPa in yield strength and 7.27%in elongation.Based on AS9 alloy,the effects of different Ce/La rare earth addition and Sr addition on the mechanical properties and microstructure were studied.It is found that with the increase of RE contents,the secondary dendrite spacing of?-Al in AS9 alloy decreases to a certain extent and eutectic Si refines,but at the same time more long needle-like rare earth-rich phases appear,which will have a negative impact on the mechanical properties of the alloy.The mechanical properties of AS9 alloy are the best when the RE content is 0.05wt.%.It is found that the addition of Sr can change eutectic Si from lamellar to fine fibrous,increase the number of?-Al dendrites,and increase the tensile strength and elongation of the alloy by20.03MPa,0.17%.The effects of squeeze pressure on the properties and microstructure of as-cast AS9-0.05RE alloy were studied.It is found that with the increase of squeeze pressure,the density and porosity of AS9-0.05RE alloy continue to increase and decrease,and the defects in castings are eliminated to a great extent.At the same time,changing the squeeze pressure does not change the phase composition of the alloy.AS9-0.05RE alloy is mainly composed of?-Al phase and eutectic Si phase.With the increase of pressure from 0MPa to 200MPa,the average grain size of the alloy can be refined and volume fraction of second phase cand be reduced.Therefore,with the increase of squeeze pressure,the mechanical properties of AS9-0.05RE alloy will be improved.When the squeeze pressure increases from 0 MPa to 50MPa,the mechanical properties of AS9-0.05RE alloy increase the most.The tensile strength,yield strength and elongation increase by 24.66%,23.25%and 44.25%respectively.The effects of T6 heat treatment and T1 artificial aging heat treatment on the microstructure and mechanical properties of AS911?50 MPa?were studied.It was found that the tensile strength and elongation of AS91 alloy were 357.04 MPa and 6.80%respectively when the solution temperature and time of T6 heat treatment were set at 530?and 3 h,respectively,with both high strength and high toughness.With the increase of solution temperature,the necking,melting,spheroidization and homogenization of eutectic Si and the dissolution rate of strengthening phase Mg₂Si into?-Al in AS91 alloy become faster,which can effectively improve the mechanical properties of the alloy.However,the excessive temperature may lead to the coarsening of eutectic silicon.It will deteriorate the mechanical properties of the alloy.In the process of artificial aging T1,the supersaturated solid solution precipitates fine dispersed strengthening phases such as Mg₂Si and Al₂Cu,which can improve the strength of the alloy.When aging temperature and time of T1 are 155?and 8 h respectively,the alloy has high strength and good toughness,and the tensile strength and elongation are 333.40 MPa and 5.40%respectively.