超声冲击对镁合金焊接接头表面塑性变形及疲劳失效的影响机理研究

Magnesiun alloy is sensitive to cyclic loading and the fatigue properties of welded joint of magnesiun alloy worsen dramatically because of stress concentration, residual stress and coarse-grain in heat affexted zone. The fatigue strength of welded joint is much lower than that of the matrix of magnesiun alloy. The research target in this object is welded join of MB8 magnesiun alloy. The aim is to increase the fatigue properties of welded joint of magnesiun alloy. The welded joint of magnesiun alloy is treated by using ultrasonic impact method. The nano-microstructure in the surface and fine microstructure in the subsurface is obtained in the vicinity of weld seam, including the heat affected zone. The changing of microstructure, the forming process of nano-grain will be studied and the nano-grain refining mechanism in the surface of welded joint of magnesiun alloy will be revealed by using SEM and TEM during the process of plastic deformation. The changing of the three factors including surface microstructure in the vicinity of weld seam, stress concentration caused by undesirable weld seam geometry, and residual stress will be researched before and after ultrasonic impact treatment. The effects of ultrasonic impact treating on the fatigue properties of welded joint of magnesiun alloy and fatigue failure mechanism will be deeply researched. The effect of grain refinement, the changing of residual stress and stress concentration caused by autrasonic impact treament on the welded joint of magnesiun alloy will be determined. The inherent law of the effects of ultrasonic impact treating on the fatigue properties of welded joint of magnesiun alloy will be revealed. The theory evidences will be provided for the application of ultrasonic impact treatment in improving the the fatigue properties of welded structure of magnesiun alloy.

镁合金对循环载荷敏感，其焊接接头引起的应力集中、残余应力以及热影响区组织粗大，都会导致疲劳性能恶化，使焊接接头的疲劳强度远低于母材。本研究以MB8镁合金焊接接头为研究对象，以提高焊接接头疲劳性能为目标，采用超声冲击方法对焊接接头进行超声冲击处理，使焊缝附近（含热影响区）表面组织纳米化，近表层组织细化，借助于SEM和TEM等手段研究焊接接头表面在塑性变形过程中的微观组织演化及纳米晶粒形成过程，揭示镁合金焊接接头表面纳米化机理；通过超声冲击前后焊缝附近表面微观组织、焊缝形状引起的应力集中与残余应力三个因素的变化，就超声冲击对镁合金焊接接头疲劳性能与疲劳失效机理的影响展开研究，确定镁合金焊接接头超声冲击表面纳米化引起的晶粒细化以及残余应力与应力集中变化对接头疲劳性能的影响程度，揭示超声冲击对镁合金焊接接头疲劳性能影响的内在规律。为超声冲击应用于镁合金焊接结构疲劳性能改善提供理论依据。

镁合金构件的的疲劳性能较差，承载能力低。如何提高镁合金焊接接头的疲劳性能成为工程上重要课题。本项目以超声冲击表面纳米化为手段，以揭示冲击表面纳米化机理及对接头疲劳性能影响规律、失效机理为目的，在以下几方面进行重点研究：.采用有限单元法，对镁合金接头的应力集中系数进行计算，得出了接头应力集中系数与接头几何参数之间的关联性及接头具有最小应力集中系数时的最佳焊缝形状；采用SEM和TEM研究了超声冲击纳米化接头焊趾表面的微观组织和纳米化机理；对MB8镁合金接头的疲劳性能进行了研究，在循环周次为2×106时，焊态对接接头的条件疲劳极限值为38.34MPa，十字接头的条件疲劳极限值为32.07MPa；冲击态对接接头的条件疲劳极限为52.75MPa，十字接头的条件疲劳极限值为41.88MPa。无论是焊态还是冲击态，对接接头的条件疲劳极限值都明显要高于十字接头，说明接头形式对疲劳极限有较大影响。相比焊态试样，对接接头的条件疲劳极限提高了37.58%；十字接头的条件疲劳极限提高了30.59%。经过冲击处理的对接和十字接头，其疲劳寿命分别比原始焊态接头提高58-65倍和6-17倍。对接接头经循环超声冲击后的疲劳极限比一次冲击的疲劳极限提高约3%左右。接头残余应力由拉变为压，表面组织细化到纳米晶粒，镁合金的晶粒细化过程主要依靠位错运动、缠结，进而形成位错墙，冲击的一部分动能转变热能，使表层温度升高，为晶粒的动态回复再结晶创造了条件，在动态再结晶作用下，亚晶逐步演变成晶界清晰，随机取向的纳米晶。研究成果对提高镁合金焊接接头的疲劳性能起到了关键性作用，为扩大镁合金焊接结构的应用提供了理论和技术支持。.经4年研究，圆满完成申报书中的研究内容。发表“Fatigue Properties of Welded Butt Joint and Base Metal of MB8 Magnesium Alloy”、“Experimental research on wear resistance and microhardness of MB8 wrought magnesium alloy after ultrasonic impact treatment”等学术论文24篇（SCI3篇、EI6篇、ISTP1篇）。申请发明专利3项，授权实用新型专利1项，培养研究生5名，青年教师2名，目前正撰写专著一部