Development Of High Hardness ZAC Zinc Alloy Used In Trial-Mould Dies For Auto Parts

In the stage of automobile R&D and trial production,the use of zinc alloy as the mold material is the most ideal choice.In this paper,combined with the needs of enterprises,the microstructure and properties of Zn-Al-Cu(ZAC)alloys with different compositions were studied by means of SEM,hardness testing and flow property testing,and a high-hardness ZAC alloy for molds was selected.For small ZAC zinc alloy molds,the effects of solidification rate and heat treatment process on its microstructure and properties were studied;for large zinc alloy molds,the solid-liquid fusion and remelting properties of ZAC alloys were studied.The following main conclusions are obtained:The main components(mass fraction)of the preferred zinc alloy for molds are: 14% Al,10% Cu,0.03% Mg,0.04% Ti,0.2% RE(La,Ce mixed rare earth).There are η,α,β,ε and θ phases in this ZAC alloy,and a large number of high-hardness Cu-rich phases(ε,θ)are the main reasons for the high hardness of the alloy;the hardness,tensile strength,elongation after fracture of the alloy at room temperature 140.8 HBW,398.33 MPa and 4.1% respectively,and the alloy has good flow properties.With the increase of the solidification rate,the microstructure of ZAC alloy is significantly refined,especially the Cu-containing second phase is more finely dispersed;the tensile strength and elongation of the alloy increased with the solidification rate,but the Brinell hardness did not change much.The suitable annealing process for small ZAC alloy mold is320～350℃×3 h.After annealing,fine-grained structure is formed,and the content of β phase and ε phase increases;the hardness of the die is up to156.68 HBW,the highest compressive strength is up to 978.53 MPa,and the elongation after fracture is over 4.0%.When the ZAC alloy mold body is prepared by the fused deposition method,the oxide layer is the main reason for hindering the solid-liquid fusion of the ZAC alloy.In order to achieve good solid-liquid fusion,it is necessary to properly preheat the surface of the solid phase.The suitable preheating temperature range is 330～350℃.Under this process condition,the strength of the solid-liquid fusion interface can reach more than 70%of the strength of the matrix,which can meet Requirements.Remelting leads to the increase of Fe element and the decrease of Al,Cu and RE content.The tensile strength and hardness of ZAC alloy first increased and then decreased with the increase of direct remelting times,but the elongation after fracture did not change much.If it is remelted directly for 3～4 times without feeding,the properties of the alloy can still meet the requirements for use;if it is remelted directly for more than 5times without feeding,the performance of the alloy cannot meet the standard.The stability of the alloy properties can be maintained by the supplementary remelting scheme.A suitable supplementary remelting scheme is: Al: x*14%*0.5%;Cu: x*10%*0.5%;RE: x*0.2 %*50%,where x refers to the total mass of the returned charge.60 figures,15 tables and 97 references are contained.