超声冲击改善MB8镁合金焊接接头超高周疲劳性能机理探究

通过超声疲劳试验探究超声冲击对MB8镁合金焊接接头超高周疲劳性能的影响。同时，从应力集中、残余应力、晶粒细化等三个因素来探究超声冲击改善MB8镁合金焊接接头超高周疲劳性能的机理。结果表明：在1.0×108寿命下，焊态试样疲劳强度为31.62MPa，冲击态试样疲劳强度为39.81MPa，冲击态试样疲劳强度相较于焊态提高了26%。这说明超声冲击可以明显提高MB8镁合金焊接接头超高周疲劳性能。焊态试样焊趾处应力集中系数Kt1=1.95，冲击态试样焊趾处应力集中系数Kt2=1.67，应力集中系数降低了14.4%，所以超声冲击可以降低焊趾处的应力集中程度。超声冲击后试样焊趾处的应力由残余拉应力转变成残余压应力。超声冲击细化焊趾表面晶粒改善MB8镁合金焊接接头的超高周疲劳性能。

Study on Mechanism of ultrasonic impact treatment to improve very high cycle fatigue property of MB8 magnesium alloy welded joint

The effect of ultrasonic impact treatment (UIT) on very high cycle fatigue properties of MB8 magnesium alloy welded joint was investigated by ultrasonic fatigue test. At the same time, the mechanism of UIT to improve the very high cycle fatigue properties of MB8 magnesium alloy welded joints was investigated from three factors, such as stress concentration, residual stress and grain refinement. The results show that the S-N curve of the fatigue life, in 1× 108 cycles, the fatigue strength of as-welded specimen is 31.62MPa , the fatigue strength of treated specimen is 39.81MPa, and the fatigue strength of treated specimen is 26% higher than that of the as-welded. This shows that UIT can obviously improve the very high cycle fatigue properties of MB8 magnesium alloy welded joint. The weld toe stress concentration factor of as-welded specimen Kt1 is equal to 1.95, the weld toe stress concentration factor of treated specimen Kt2 is equal to 1.67, and stress concentration factor is reduced by 14.4%. The UIT can reduce the stress concentration of the weld toe. After UIT, the stress at the toe of the specimen changes from residual tensile stress to residual compressive stress. UIT refine surface grain of welded toe to improve very high cycle fatigue properties of MB8 magnesium alloy welded joint.