Study On Microstructure And Mechanical Properties Of Rheo-squeeze Casting A390 Aluminum Alloy

Hypereutectic Al-Si alloys are widely used in various types of wear-resistant parts due to their excellent wear properties,high elastic modulus,low thermal expansion,and excellent high temperature resistance.However,due to the coarse primary silicon phase in its alloy microstructure,the matrix of the microstructure is severely cracked and the mechanical properties are suppressed.This severely limits the application scope and potential of hypereutectic Al-Si alloys.Therefore,researching on the refinement of primary silicon particles in the hypereutectic Al-Si alloy structure has important theoretical significance and practical application value.This subject takes A390 alloy as the research object,and uses optical microscope,scanning electron microscope,X-ray diffractometer,hardness and tensile test to study the influence of P-RE composite modification parameters on the microstructure and properties of castings,and confirms that P and RE has the effect of regulating the microstructure of A390 alloy and optimized the composition of P-RE composite modifier.Based on this,the effects of process parameters on the microstructure and mechanical properties of liquid squeeze casting and rheological squeeze casting of A390 alloy were systematically studied,and the mechanism of the impact of squeeze pressure,pouring temperature,and drum rotation speed on the microstructure and properties of the castings was clarified.Then optimized rheological squeeze casting parameters were obtained.After that,the heat treatment process of rheological squeeze casting of A390 alloy was researched,and an optimized heat treatment scheme was provided for its practical application.Research results:In the process of P-RE composite modification of A390 alloy,the addition of P mainly regulates the primary silicon particles,so that the primary silicon particles are significantly refined and rounded.The addition of RE mainly regulates the eutectic silicon phase,so that the eutectic silicon phase is more dispersed and finely distributed in the matrix.The best P-RE composite modifier is 0.035wt.%P+0.2wt.%RE.The silicon phase in the tissue treated by this modifier is evenly distributed,and the average diameter of the primary silicon reaches 23?m,which is 34.5%of that of unmodified microstructure.In liquid squeeze casting,the microstructure and properties of the castings under all parameters are significantly improved compared with gravity castings.With the increase of squeeze pressure,the?-Al matrix structure is continuously refined,which is mainly because the pressure increases the liquidus temperature of the alloy,which increases the subcooling degree,improves the nucleation rate of the melt,and the heat transfer efficiency from the melt to the outside world is accelerated;it is worth pointing out the primary silicon phase precipitation temperature is too high,therefore the pressure change has no obvious effect on the primary silicon particles morphology.As the pouring temperature increases,the average diameter of primary silicon particles in the microstructure decreases first and then increases.When the casting temperature makes the melt temperature in the mold slightly higher than the liquidus,the average diameter of primary silicon particles is the smallest,because the squeeze pressure can be effectively affect the nucleation and growth of primary silicon particles.The optimized liquid squeeze casting process parameters are:the pouring temperature is 760°C,and the extrusion pressure is 100MPa.In this process,the average diameter of primary silicon particles in the microstructure of A390 alloy squeeze casting is 26?m,and it is evenly distributed in the dense?-Al matrix.The ultimate tensile strength is 226MPa and the elongation is 0.66%,which are improved by 37.0%and 120%compared with gravity castings.During the preparation of semi-solid slurry,with the increase of the rotation speed of the drum,the size of the primary silicon particles in the A390 alloy microstructure decreases first and then increases.This is because faster rotation speed can provide better cooling speed and stirring effect to optimize the structure,but when the speed is too fast,the strong cooling and stirring will deteriorate the tissue and even bring defects.With the increase of the pouring temperature,the size of the primary silicon particles in the semi-solid slurry microstructure decreases first and then increases,and?-Al dendrites always exist.In the microstructure of rheological squeeze casting,the same changes have occurred in primary silicon particles,but the?-Al dendrites decreased significantly.And the primary silicon particles is obviously round and fine compared with the liquid squeeze casting structure.When the pouring temperature makes the outlet slurry temperature slightly lower than the liquidus temperature,the microstructure is optimal.The roller stirring acts on the nucleation of the primary silicon and a small degree of subcooling is provided to suppress the growth of the primary silicon particles.The optimized rheological squeeze casting process parameters are:pouring temperature is 780°C,and drum speed is 90r/min.In this process,the silicon phase in the rheological squeeze casting structure of A390 alloy is finely distributed,?-Al has no obvious dendrites,the average diameter of primary silicon is18?m,and the ultimate tensile strength and elongation are 250MPa and0.60%,respectively.Compared with liquid squeeze casting,the tensile strength is increased by 10.7%,and the elongation was not significantly increased.After the optimized T6 heat treatment process(500?8h+175?4h)for the rheological extrusion castings of A390 alloy,the primary silicon particles in the microstructure is obviously rounded,the Al₂Cu phase is effectively dissolved in the matrix,and the mechanical properties of the casting are significantly improved.The tensile strength reaches 404MPa and elongation reache 5.95%,which is 64.0%and 5.0 times higher than the as-cast condition,respectively.The inferior phase primary silicon particles in A390 alloy are the earliest precipitated phase in the microstructure,and the precipitation temperature is high.Therefore,when improving the molding technology of A390 alloy,it is necessary to pay attention to whether the technical means effectively affect the nucleation or growth of primary silicon.It is the key to improve the microstructure and properties of A390 alloy that it can play an appropriate role in the primary silicon nucleation and growth.