Study On Segregation And Hot Cracking Of WE43A Magnesium Alloy During Solidification

WE43A magnesium alloy is widely used in the aerospace field,due to its good strength at room temperature and high temperature creep resistance.This alloy can be used to produce complex thin-walled castings by sand casting.But it is prone to segregation and hot cracking in the actual production,which leads to low qualified rate of casting and high cost.In this work,the numerical simulation technology is used to study the macrosegregation of WE43A magnesium alloy during solidification process and the method of eliminating microsegregation is sought.Further,the influencing factors and mechanism of hot cracking of WE43A magnesium alloy are explored,which provides a theoretical basis for solving the crack problem of WE43A magnesium alloy castings.Initially,the three-transfers mathematical prediction model of macrosegregation of WE43A magnesium alloy was developed based on the volume average element technology.The metallurgical transmission behavior of WE43A magnesium alloy during solidification process was numerically calculated from the macroscopic scale.The results show that the solidification speed was fast during sequential solidification.However,due to the large temperature gradient,there was an asymmetrical flow pattern in the solidification process,and the flow velocity was large,which led to a higher degree of macrosegregation of the WE43A magnesium alloy.During the equilibrium solidification,the solidification rate was slower and the temperature gradient was smaller.There was a symmetric convection mode in the solidification process,and the flow velocity was smaller,which led to a less macrosegregation degree of WE43A magnesium alloy.The three rare earth elements of Y,Nd and Gd have the same macrosegregation tendency,and the Nd element is main macrosegregation element of WE43A magnesium alloy.The macrosegregation numerical model of coupled solid-phase movement was developed based on a two-phase flow.It was found that under the conditions of this study,the equiaxed crystal movement was not the driving force of macrosegregation in the solidification process of castings.Secondly,the influence of two-stage homogenization heat treatment on microstructure and properties of WE43A magnesium alloy was investigated.The results show that the as-cast WE43A magnesium alloy is mainly composed of?-Mg matrix,grid-type Mg₄₁Nd₅ phase and granule-type Mg₂₄Y₅ phase.WE43A magnesium alloy was treated by the one-stage homogenization heat treatment at520?for 16 h.The Mg₄₁Nd₅ and Mg₂₄Y₅ phases in the alloy can be completely dissolved into the?-Mg matrix.Compared with the as-cast condition,the grain size of WE43A magnesium alloy treated by the one-stage homogenization for 16h was changed from 73?m to 125?m.The yield strength was increased from 89MPa to 104 MPa,and the ultimate tensile strength was increased from 164 MPa to 196 MPa.The elongation was increased from 6.5%to 14.3%.After the WE43A magnesium alloy was two-stage homogenized by 520?×8 h+510?×12 h,the Mg₄₁Nd₅ and Mg₂₄Y₅ phases were completely dissolved into the?-Mg matrix.Compared with the one-stage homogenization treatment,the grain size was decreased from 125?m to 101?m.The yield strength,ultimate tensile strength and elongation were increased from 104 MPa,196 Mpa,and 14.3%to 119 MPa,231 Mpa,and 16.3%,respectively.The second phase of WE43A magnesium alloy can be completely dissolved by two-stage homogenization heat treatment,and the degree of grain growth can be reduced,so that the plasticity of WE43A magnesium alloy can be improved as well as the strength.Finally,the influence of sand-mould strength,variable section size of the hot cracking specimen and the pouring temperature on hot cracking of WE43A magnesium alloy during solidification was studied by casting the costrained rod.The results show that the resin content in the sand mould was 1.0%,sand mould had a low strength of 0.120 MPa.The floating sand of the sand-mould surface was caught in the sample during the filling process of the alloy,forming a slag inclusion defect and resulting in complete fracture of the constrained rod.When the resin content increased to 1.5%,the sand-mould strength increased to 0.175MPa,and the constrained rod had no hot cracking.When the resin content is2.0%,the sand-mould strength reached the maximum value of 0.275 MPa,and the constrained rod had no hot cracking.With the increase of resin content again,when the resin content reached 2.5%,the strength of sand mould decrease to0.165 MPa,and there is no hot cracking in the constrained rod.When the final resin content increased to 3.0%,the sand-mould strength dropped to 0.150 MPa.When the resin content was too high,a large amount of gas was generated during the solidification process of the alloy,and the casting formed a porosity defect.The constrained rod was hot cracked.It can be seen that under the condition of resin sand casting,the high strength of sand mould will not cause hot cracking of WE43A magnesium alloy,and the change of resin content will lead to slag inclusion or porosity defects in the alloy,which will lead to hot cracking.It can be known by adjusting the variable section size of the constrained rod in the hot cracking test:when the constrained rod changed from 20 mm to 35 mm in cross-section,there was no hot cracking.When the constrained rod changed from15 mm to 35 mm in cross-section,the constrained rod was hot cracked.When the constrained rod changed from 10 mm to 35 mm in cross-section,the constrained rod was complete fracture.The hot cracking tendency of constrained rod sample of the WE43A magnesium alloy increased with the increase of the variable cross-sectional dimension ratio.The position where the cross-sectional area of the casting varied greatly,the degree of obstruction of solidification shrinkage was higher,and the solidification shrinkage force was larger,which easily caused hot cracking of the casting during solidification.By changing the pouring temperature of WE43A magnesium alloy,it can be found that when the pouring temperature was 720?and 750?,the constrained rod had no hot cracking.When the pouring temperature was raised to 810?,the constrained rod completely fractured.With the increase of pouring temperature,the solidification speed of the constrained rod of WE43A magnesium alloy was weakened,the grain size of the alloy was larger,and the intergranular liquid film is thicker,which reduced the intergranular bonding force of alloy and increased the hot cracking tendency of the constrained rod.Therefore,if the resin content in the resin sand mould is controlled to 1.5%².5%,the sand strength is 0.16 MP⁰.275MPa,the ratio of variable section of casting wall thickness is greater than 4:7,and the pouring temperature is lower than 810?,the hot cracking defects of WE43A magnesium alloy can be prevented.