Study On Strengthening And Toughening Trentment Of AM60 Magnesium Alloy

The strengthening and toughening treatment of AM60 magnesium alloy were studied in this investigation. The AM60 magnesium alloy was modified by adding different contents of Nd and C₂Cl₆ to refine microstructure, and the effect of solution treatment and aging on the solidification structure was studied. The cooling rate can be determined on the sand-mould step samples, and the relationship between the cooling rate and the solidification structure is established. The microstructure and phases of AM60 magnesium alloy were analyzed by optical microscopy, scanning electron microscopy and X-ray diffraction, and the mechanical properties of the alloy at room temperature was tested. The following conclusions can be drawn from the investigation.The addition of Nd modification agent can refine theα-Mg dendrite of AM60 magnesium alloy. With increasing Nd modification agent, it can refine microstructure significantly and decreases the amount ofβ-phase and improves the dispersion as well. There is the best refining effect when the addition of Nd is 0.6wt%. The average grain size decreases from 485μm to 125μm, and the tensile strength and elongation of the alloy increase as well. However, if the addition of Nd is above 0.9wt%, the grains become coarse, and the mechanical properties of alloy decrease slightly.The 0.6～1.2wt% addition of C₂Cl₆ modification agent can refine theα-Mg dendrite of AM60 magnesium alloy obviously, obtain dispersed distribution of fineβ-phase and increases the mechanical properties. When the addition of C₂Cl₆ is 0.9wt%, the good combination of mechanical properties can be obtained. If the addition of C₂Cl₆ is above 1.2wt%, it decreases the refining effect, and the mechanical properties of alloy also decrease slightly.After solution treatment at 415℃holding for 12 hr for AM60 magnesium alloy without modification and with rare earth Nd and C₂Cl₆ modification,β-Mg₁₇Al₁₂ eutectic phase is almost dissolved into the matrix and forms theα-Mg supersaturated solid solution. Followed by aging at 200℃for 16hr, the discontinuousβ-Mg₁₇Al₁₂ is precipitated on the grain boundary of the AM60 magnesium alloy without modification. However, there are less amount ofβ-Mg₁₇Al₁₂ precipitation phase on the grain boundary of AM60 magnesium alloy with modification. It suggests that Nd and C₂Cl₆ modification agent can delay aging process of the alloy. After heat treatment, the mechanical properties of AM60 magnesium alloy increase in comparison with as-cast condition, especially for tensile strength.With increasing cooling rate, the primaryα-Mg dendrite become smaller, secondary dendrite arm spacing decreases and the amount ofβ-phase increases. However, there is not obvious dispersion. Under the same modification condition, the cooling rate can improve the modification effect. With increasing cooling rate,α-Mg dendrite become much finer, the amount ofβ-phase decreases slightly and more dispersed distribution appears.