| As the representative of light alloy,aluminum alloy has been more and more popularity because of the promotion of lightweight. Die casting is the most widely-used liquid molding method, and it is often used to produce aluminum-alloy large complicated thin-wall castings. However, the performance of the commonly-used Al-Si die casting alloy are not good enough. So developing aluminum alloy with high strength and ductility which meets the requirements of die casting is a effective way to lose weight of aluminum die casting.Cu is a common element for Al to solid solution alloying. Different content of Cu(4%-6%) is added to A356. Experimental die casting, metallographic observation, tensile properties, fracture SEM are applied to study on the effects of Cu content on the as-cast microstructures, T6 heat treatment microstructures, tensile properties and fracture behavior when the content of Cu is high.Conclusions are drawn:①The cast-ability and filling-ability of A356 have a little decrease when the content of Cu is more than 5%. But the decrease doesn’t have much influence on the formation of the casting under the high pressure of die casting.② Strip Si is the main phase of the die casting A356 microstructure. When Cu is added to A356, Al2 Cu begins to appear. Al2 Cu distributes around Si and the grain boundary. Si change into agglomerate skeleton-like from strip with the rise of Cu content. Strip Si may be break into agglomerate skeleton because of the increasing Al2 Cu. There is strengthening phase W(AlxMg5Si4Cu4)besides Al2 Cu.③ The tensile strength of the die-casting A356 is 206 MPa, and the ductility reaches to 8.89%. The strength of A356 is greatly improved when Cu is added. When Cu is 5.5%,the tensile strength is highest, which is 299 MPa, and the ductility is 6.52%. However the ductility decreases if the content of Cu is too high. When the content is 6%, the tensile strength decreases to 270 MPa, and the ductility is only 3.77%.④ The alloy aging-hardness reaches the maximum(74.5HRE) after 3 hours 530℃solid solution and 3 hours 160℃ aging. Hardness then fluctuate in a small scope which is below the peak. Strip Si basically complete spheroidization in 2 hours under the 530℃ solid solution. When the solution time prolongs, Si is more uniformly distributed. And Al2 Cu gradually dissolves into the matrix. Bubbles appears to the die casting samples’ surface after 530℃×3h solid solution. Bubbles show an uneven distribution and different sizes.⑤ Strength of die casting with different Cu content increase after T6 heat treatment and the ductility is all above 5%. When Cu is 4.5%, the properties is best, the tensile strength reaches to 337 MPa, and the ductility is 6.23%.⑥ Macro fracture model of die casting is brittle fracture. The micro fracture model is quasi-cleavage fracture. There is no stomata visible to naked eye, but shrinkage porosity can be observed under the SEM. After T6 heat treatment, the macro fracture model is still brittle fracture. However, the micro fracture model turns to quasi-cleavage and ductile morphology mixture fracture. Stomata can be observerd without amplification, and the shrinkage porosity size enlarges under the microscope. |
