| In view of the disadvantages of traditional two-dimensional characterization methods that it is difficult to distinguish the internal pores types and spatial positions of die castings,through the application of Shanghai Synchrotron Radiation Facility(SSRF)and computer assist three-dimensional reconstruction technology,the morphology and distribution of porosity in AZ91 D magnesium alloy and AE44 magnesium alloy die castings produced with different high pressure die casting(HPDC)process parameters were characterized,and the interaction relationship between die casting process-microstructure-mechanical properties were studied,furthermore the die-casting process was optimized by feedback.Different from two-dimensional characterization,three-dimensional characterization can accurately distinguish four kinds of pores in die-casting magnesium alloy: gas pores,gas shrinkage pores,island-shrinkage and net-shrinkage.The study found that the morphological distribution of gas pores and gas shrinkage pores are mainly affected by the phenomenon of gas entrainment during the filling process;the morphology and distribution of shrinkage porosity is mainly affected by the Externally Solidified Crystals(ESCs),and the size and quantity of ESCs are mainly affected by the low slow shot speed of the molten metal in the die chamber(the growth time of ESCs)and the fast slow shot speed when the molten metal entered the mold cavity(the shear stress of the molten metal flow).There are defect bands in AZ91 D magnesium alloy castings,and the width of the defect bands is proportional to the size of the ESCs in the castings.The introduction of vacuum process had little effect on the width of the defect bands.Due to the large number of ESCs gathered and interconnected irregular pores in the defect bands,the cracks of AZ91 D magnesium alloy castings are prone to propagate in the defect bands during the tensile fracture process.In addition,the expansion of the crack in the central area of the sample was limited by the surrounding environment.The mechanical properties of AZ91 D magnesium alloy castings are negatively related to the width of the defect bands,but have nothing to do with the average volume fraction of porosity.In AE44 magnesium alloy castings,the size and number of ESCs gradually decrease from the center of the sample to the edge.This phenomenon is attributed to the combined effect of the chilling effect and the molten metal turbulence during the die-casting filling process.Both the fast slow shot speed process and the pressurization process can reduce the size and amount of ESCs.Among them,the fast slow shot speed process had a greater impact on the distribution and morphology of ESCs.There is no defect bands in AE44 magnesium alloy castings.The fast slow shot speed process can reduce the shrinkage porosity in the casting,and promote the morphology of porosity to change from shrinkage porosity to gas pores.The pressurization process can reduce the amount and volume of gas pores and shrinkage porosity in the casting,and make the porosity more concentrated to the core of the sample.Compared with the fast slow shot speed process,the pressurization process had a greater impact on the total volume of the porosity in the casting.Using Anycasting casting simulation software to simulate the molten metal filling process found that the molten metal first fills the middle of the rod-shaped sample,and finally fills the near gate position.This leads to differences in the morphology and distribution of ESCs and porosity at the middle of the rod-shaped sample and the near gate position of the castings under the same die-casting process... |
