【作者】 张金旺；

【导师】 许并社；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2009， 博士

【摘要】 镁合金作为21世纪最具生命力的新型环保结构材料,它的开发和应用对于实现人类可持续发展具有重要而深远的意义。但是,镁合金的合金系统还较少,现有镁合金的力学性能还远远不能满足人们的需求。本论文在现有镁合金系统的基础上,添加稀土元素,研究和开发了两种新型高强高韧镁合金材料,并对广泛应用的AZ31B镁合金的焊接冶金性能和焊接接头力学性能进行了深入研究。在AZ31镁合金系统的基础上,加入微量合金元素钇Y,研究了钇Y对AZ31合金的微观组织、室温力学性能和断裂机制的影响。AZ31镁合金中加入钇Y后,形成块状Al2Y化合物,该合金组织由α-Mg固溶体+Mg17Al12+Al2Y相组成。钇Y的加入使Mg-3Al-Zn合金系统韧性提高,铸造状态下的抗拉强度达到212 MPa,延伸率达到7%以上,拉伸断口出现撕裂棱和韧窝,使该合金系统的断裂机制由解理断裂转变为准解理断裂。分析了稀土元素镧La对AZ91镁合金的改性作用及其作用机理。稀土元素镧La的添加,可以有效地细化晶粒,使Mg17Al12相由原先连续、粗大的网状转变为细小的、致密的不连续状。当镧La的添加量在0.2%左右、精炼温度为993K的条件下,能得到理想的合金性能、最少的合金损耗。与国家标准AZ91镁合金材料比较,由于稀土元素镧La的添加,合金试样的硬度、冲击吸收功提高1倍以上,抗拉强度提高73%,延伸率提高了7倍。探究了AZ31镁合金材料TIG自熔焊、填丝TIG焊和MIG焊的焊接冶金性能。分析了三种焊接方法焊接裂纹产生原因及裂纹特征,AZ31B镁合金焊接热裂纹在接近焊缝中心处产生,裂纹向熔合区方向扩展,扩展速度很快并且过程中存在多次分叉裂纹,裂纹及其分叉都是沿晶扩展。研究了AZ31镁合金及其焊接接头的基本力学性能。试验所用轧制态AZ31镁合金板材静载抗拉强度为285.2 MPa,经过TIG焊接热循环的作用后,焊接接头的静载抗拉强度下降,比母材抗拉强度降低10%左右。横向非承载十字接头试件的抗拉强度为255.9 MPa,纵向非承载十字接头试件的抗拉强度为258.0 MPa。焊接接头显微硬度测试结果显示,焊缝区的显微硬度值较高,热影响区显微硬度最低。测试了AZ31镁合金板材在-80~340℃温度范围内的冲击韧性随温度变化的规律。结果表明:AZ31镁合金母材在-80~260℃温度范围内存在韧脆转变现象,以能量标准和延性标准测得的韧脆转变温度约为140℃。AZ31镁合金的冲击性能对缺口很敏感,试样从无缺口到有缺口,即使仅有很小的缺口,冲击韧性也突然下降,未开缺口的冲击试样的冲击韧性akv为50 J·cm-2以上,而缺口深度D = 0.1 mm的缺口试样,冲击韧性akv仅为15.95 J·cm-2。示波冲击试验结果显示AZ31镁合金母材和焊接接头的断裂过程极不相同。对AZ31镁合金板材及其TIG焊对接接头、纵向角接接头、非承载十字接头等三种接头的疲劳性能进行了测试。试验结果表明:在2×106的循环次数下,AZ31镁合金母材的疲劳强度为57.81 MPa,对接接头的疲劳强度为24.60MPa,纵向角接接头的疲劳强度为20.14 MPa,横向非承载十字接头的疲劳强度为17.25 MPa,镁合金焊接接头的疲劳性能远低于母材。扫描电镜观察疲劳断口发现,AZ31镁合金焊接接头断口中没有连续、清晰的疲劳辉纹,大部分区域都是由细小密集的解理台阶、扇形花样断口、河流状花样断口组成,在断口的很多地方都可以看到大小不一的二次裂纹。提出了采用超声冲击方法改善AZ31镁合金焊接接头的疲劳强度的措施。试验结果表明:焊接接头焊趾经过超声冲击后,纵向非承载十字接头的疲劳强度比焊态试样提高了54.5%,达到母材疲劳强度的84.2%;横向非承载十字接头的疲劳强度比焊态试样提高了43.6%,达到母材疲劳强度的68.6%。在一定应力范围的疲劳寿命延长到原来的3倍。研究了AZ31镁合金及其焊接接头在质量分数为3.5%的NaCl溶液中的腐蚀行为。电化学腐蚀结果显示,母材的自腐蚀电位Ecorr= -1.425 V,自腐蚀电流Icorr=1.093×10-2 A,年腐蚀率Ψ=139.8 mm/year,说明AZ31镁合金在Cl-溶液中极不耐蚀。焊接接头形成过程中存在的一些缺陷,加剧了AZ31镁合金焊接接头的腐蚀,焊缝金属的耐腐蚀性小于母材。采用化学镀的方法在镁合金表面获得金属涂镀层,采用XRD、SEM和TEM研究了镀层的成分和组织结构。镀层组织为非晶+少量微晶组织。耐腐蚀性能良好,原始镀层的最小腐蚀电流I corr为4.52μA/cm2,取对数约为0.66,此时的腐蚀电位Ecorr为-250mV。镀层耐腐蚀性能良好,连续盐雾八小时未出现腐蚀斑点。更多还原

【Abstract】 As a promising and environmentally-friendly structural material in 21st century, the development and application of magnesium alloy is of far reaching importance for human sustainable development. But the existing magnesium alloys are far from the demand due to the lesser alloy systems and lower mechanical properties. In this dissertation, two new styles magnesium alloy with high tensile and high tough were developed by adding rare-earth element into AZ31 alloy system. Moreover, the welding metallurgy performance of AZ31B alloy and mechanical properties of the welded joint were studied.The effect of Y addition on microstructure, room temperature mechanical property and fracture mechanism of AZ31-Y alloy were studied. Massive Al2Y was formed and the microstrure was composed ofα-Mg, Mg17Al12 and Al2Y. The toughness of AZ31-Y alloy was improved, and the tensile strength and elongation of as cast AZ31-Y was 212Mpa and 7%, respectively. On the other hand, the tearing ridge and dimple appearanced on the tensile fracture, which revealed that the fracture mechanism transformed from cleavage fracture to quasi-cleavage fracture.The modification effect and modification mechanism of La in AZ91 alloy was studied. With additon of La element, the grain was effectively refined, and the continous, massive Mg17Al12 of being reticular transformed to be uncontinous, fine and dense. At the refining temperature of 993K, the perfect properties and the least alloy waste were realized When La addtion was 0.2%. Comparing with standard AZ91, The rigidity and impact energy, the tensile strength and the elongation of AZ91-La alloys were improved 100%, 73% and 700% respectively.The welding metallurgy performance of TIG self fluxing welding, TIG filling wire welding and MIG welding of AZ31 alloy were investigated. And the formation and characteristic of cracks in the above three welding were analyzed. Welding hot cracks of AZ31 alloy formed near the centre of the welded seam, and then growed rapidly into fusion area, in the course which branch cracks happened. The cracks and branch cracks both growed along the grain boundaries.The mechanical properties of AZ31 magnesium alloy and its welded joint were studied. The tensile strength of AZ31 plates as rolling used in testing is 285.2 Mpa. After the TIG welding, the tensile strength of welded joint reduces about 10% Compared with base metal. The tensile strengths of transverse and longitudinal cruciform welding joints are 255.9 Mpa and 258.0 Mpa respectively. Moreover, the microhardness testing result of welded joint reveals that the microhardness in the weld joint zone is higher, while that in heat-affected zone is at the least.Effect of temperatures with a rang of -80 ~ 340℃on impact toughness evolving were studied. The result indicates that in AZ31 alloy tough-brittle transition phenomena happens at about 140℃which is measured with energy standard and ductility standard. The impact property of AZ31 alloy is very sensitive to indentation, and no matter how small the indentation is, the impact toughness of AZ31 alloy decreases markedly. For example, the impact toughness of the sample with a indentation of 0.1mm depth is 15.95 J/cm2, while the impact toughness of the sample without any indentations is higher than 50 J/cm2. the result of instrument impact test reveals that the fracture processes of AZ31 magnesium alloy parent metal and welded joint are quite different.The fatigue properties of base metal, TIG butt joint, transverse cross joint and longitudinal cross joint were tested. The results indicate that when the cycle index N is 2×106 times, the fatigue resistances of base metal, butt joint, transverse cross joint and longitudinal cross joint were 57.81 MPa, 24.60 MPa, 17.25 Mpa and 20.14 Mpa, respectively. It is obvous that the fatigue resistances of the welding joints is far lower than that of base metal. Moreover, SEM analysis of the fatigue fracture finds that there is no continuous and legible fatigue stripe on the fractures of the welding joints which is mainly composed of dense cleavage step, sector pattern fracture and river pattern fracture where there are secondary cracks with different sizes.The was put forward to improv fatigue strength of welding joint of AZ31 alloy. After treated by the ultrasonic peening method, Fatigue life is improved three times in certain stress extent. The fatigue resistance of longitudinal cross joint (2×106) is improved 54.5%, which is 84.2% of parent metal and that of transverse cross joint (2×106) is improved 43.6%, which is 68.6% of parent metal.The corrosion behaviors of AZ31 magnesium alloy and its welded joints in 3.5wt% NaCl solution were studied. The corrosion potential (Ecorr), corrosion current (Icorr), corrosion rate (Ψ) of AZ31 alloy were -1.425 V, 1.093×10-2 A and 139.8 mm/year respectively, which reveals that AZ31 magnesium alloy is very easily corrupted in cl- solution. On the other hand, the corrosion resistance of the welded joint is lower than that of AZ31 parent alloy because due to some defects in the welded joint.Metal coated layer was obtained on the surface of magnesium alloy by electroless plating method, and the chemical composition and structure of coated layer were studied using XRD, SEM and TEM. The coated layer is mainly composed of amorphous structures and a few microcrystalline structure. The corrosion current and corrosion potential of the original coated layer are 4.52μA/cm2 and -250 mV respectively. Moreover, there are no corrosion spots when the coated layer is exposedin in salt mist for 8h.更多还原