【作者】 胡茂良；

【导师】 吉泽升；

【作者基本信息】 哈尔滨理工大学 ， 材料学， 2008， 博士

【摘要】 每合金是迄今在工程应用中最轻的金属工程结构材料,被誉为“21世纪最具发展前途的绿色金属材料”。镁合金作为结构材料应用受到限制的最主要因素是成本仍然很高,造成成本高的原因是镁合金废料回收率低,回收费用高,且利用传统再次熔炼的方法产生大量的烧损和废渣,重熔过程中须加覆盖剂和精炼剂。本文以固相再生的方法再生AZ91D镁合金屑和边角料,在固相再生过程中,镁屑或边角料经塑性变形直接成形,具体工艺为先冷压或热压,再热挤出成形,是一项新的再生镁合金技术。由于晶粒细化和氧化相的均匀分布,再生的镁合金具有较好的力学性能。研究了AZ91D镁合金屑固相再生的最佳工艺,分析了固相再生过程中AZ91D镁合金屑变形的基本特征。AZ91D镁合金屑在冷压成坯过程中,压力为300³10MPa,坯料密度可达到1.49g/cm³以上,形成少量新的结合面。采用屑粒尺寸（4⁶）mm×（3.5⁴.5）mm×（1.45¹.55）mm、挤压温度为400℃和挤压比25:1再生试样有较好的综合力学性能,与铸态热挤出试样的力学性能相当。再生试样随挤压比的增大,抗拉强度和延伸率同时增大,挤压比达到25:1以上时,延伸率又随挤压比的增大而减小。研究了再生试样中氧化相含量与AZ91D镁合金屑的表面积之间关系,指出了氧化相含量与屑的表面积成直线关系,适量的氧化相使再生试样有较高的抗拉强度和较好的延伸率,适量的氧化相在试样中可以作为一种强化相;过多的氧化相反过来影响镁合金的延伸率,过多的氧化相在拉伸过程中易产生微孔,降低了试样的延伸率。研究了AZ91D镁合金边角料固相再生的最佳工艺,给出了AZ91D镁合金边角料蚀洗工艺。采用间接挤压工艺时,挤压温度为450℃时,晶粒尺寸均匀,块与块之间结合较好,抗拉强度和延伸率分别达到350.24MPa和11.82%。挤压比达到25:1以上时,晶粒不仅变得细小,而且没有出现未打碎的结合面;随挤压比的增大,试样抗拉强度增大,挤压比40:1时,抗拉强度达到378.05MPa。采用直接热挤出研究表明,与相同条件下间接挤压相比,试样的抗拉强度和延伸率均降低。研究了AZ91D镁合金边角料固相再生试样中结合面与抗拉强度之间关系。当结合面呈连续分布成曲线时,结合面厚度0 < w≤7μm时,AZ91D镁合金抗拉强度的预测公式为σ_w = - 40 w/3 + 345 + 40/3;结合面厚度7μm < w <11μm时,AZ91D镁合金抗拉强度的预测公式为σ_w = -30 w+ 475。当结合面呈不连续分布成曲线时,曲线上打碎段的长度所占测量曲线长度的比例0.1≤l≤0.7时,AZ91D镁合金抗拉强度的预测公式为σ_l = 150l + 245;曲线上打碎段的长度所占测量曲线长度的比例0.7 < l< 0.9时,AZ91D镁合金抗拉强度的预测公式为σ_l = 75l + 297.5。研究了固溶时效处理对再生AZ91D镁合金组织和力学性能的影响。经T6处理后,抗拉强度和延伸率明显提高。随着时效时间的延长,析出的β-Mg₁₇Al₁₂相数量明显增多,更长的时效时间β-Mg₁₇Al₁₂相并没有明显增大。随着时效时间的延长,试样维氏硬度增加;用AZ91D镁合金边角料和屑固相再生试样的维氏硬度均高于铸态热挤出试样的维氏硬度。研究了工业化基础试验中固相再生AZ91D镁合金的组织及力学性能。热压温度400±10℃,保温300s,压力为400MPa;热挤温度450±10℃,保温1小时,挤压速度0.4mm/s。固相再生试样的抗拉强度和延伸率分别能达到346.2MPa和9.62%,挤出试样的厚度、挤压流线和氧化层的分布将严重影响试样的力学性能。更多还原

【Abstract】 Magnesium alloy is the lightest metal structural material in the engineering application up to the present moment. It is known as the most developing recycling metal material in the 21st century. Magnesium alloy is limitedly used in metal structural material because of its high cost including of the lower rate of recovery of magnesium alloy scraps and the higher expense cost. Plenty of burnout and waste residue are produced using the common remelting method and covering flux and refining flux are added in the remelting process.In this paper, AZ91D magnesium alloy chips and scraps were prepared by solid state recycling. In the solid state recycling process, magnesium alloy chips and scraps were directly extruded into the products by plastic deformation. Cold-press or hot-press were carried out and then hot-extrusion was used. Solid recycling magnesium alloy presents high mechanical property due to grain refinement and homogeneous dispersion of oxide precipitates. Solid state recycling is a new efficient method for the recycling of magnesium alloy chips and scraps.Optimal parameters of AZ91D magnesium alloy chips prepared by solid state recycling were obtained and plastic characteristics in solid state recycling were analyzed. At first, AZ91D magnesium alloy chips were cold-pressed to form a compact billet with the pressure of 300³10MPa. The density of the billet was above 1.49g/cm³ and the surface of the chips exhibited some breaking and bonding. Recycled specimen prepared with chip dimensions of （4⁶）mm×（3.5⁴.5）mm×（1.45¹.55）mm, extrusion temperature of 400℃and extrusion ratio of 25:1 showed higher mechanical property. Its ultimate tensile strength and elongation to failure were almost the same as those of the as-cast and extruded specimen. Ultimate tensile strength and elongation to failure of recycled specimen increased with the extrusion ratio increasing. When the extrusion ratio is above 25:1, the elongation to failure of recycled specimen decreased with increasing the extrusion ratio.Relationship between oxygen concentration and total surface area of AZ91D magnesium alloy was studied for the recycled specimens. The accumulated oxygen concentration increased linearly with the total surface area. A small amount of oxide precipitation contributed to higher tensile strength and elongation to failure and resulted in dispersion strengthening. Excessive oxide may be prone to cause the early development of microvoids and contributed to lower elongation.Optimal parameters of AZ91D magnesium alloy scraps prepared by solid state recycling were obtained and corrosive washing technology of AZ91D magnesium alloy scraps was present. Adopting hot-press and hot-extrusion technology, the grain size was uniform and the bonding between scraps was better with the extruding temperature of 450℃. The ultimate tensile strength and elongation to failure can reach 350.24MPa and 11.82%, respectively. When the extrusion ratio exceeded 25:1, the grain became fine and the bonding layer didn’t appear. The ultimate tensile strength increased with the extrusion ratio increasing. When the extrusion ratio was 40:1, the ultimate tensile strength could reach 378.05MPa. Adopting hot-extrusion technology, comparing to hot-press and hot-extrusion technology, the ultimate tensile strength and elongation to failure decreased.Relationship between bonding layer and ultimate tensile strength was studied for the recycled specimens adopting AZ91D magnesium alloy scraps. When the bonding layer in the recycled specimens formed continuous curves, the ultimate tensile strength decreased with the thickness of the bonding layer increasing. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_w = -40 w/3 + 345 + 40/3 when the thixkness of the bonding layer is 0 < w≤7μm. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_w = -30 w+ 475 when the thixkness of the bonding layer is 7μm < w <11μm. When the bonding layer for the recycled specimens formed discontinuous curves, the ultimate tensile strength increased with the proportion of the length of the bonding layer accounting for the whole measured curve increasing. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_l = 150l + 245 when the thixkness of the bonding layer is 0.1≤l≤0.7. Forcastable equation of the ultimate tensile strength for the recycled specimens isσ_l = 75l + 297.5 when the thixkness of the bonding layer is 0.7 < l< 0.9.Effect of solution and artificial aging treatment on microstructure and mechanical property of recycled specimens was studied. The ultimate tensile strength and the elongation to failure obviously increased after T6. With the aging time prolonging, the amount ofβ-Mg₁₇Al₁₂ phase obviously increased andβ-Mg₁₇Al₁₂ phase didn’t become coarse. With the aging time prolonging, Vickness hardness of the recycled specimens increased. The Vickness hardnesses of recycled specimens adopting AZ91D magnesium alloy chips or scraps were higher than that adopting as-cast and hot-extrusion.Microstructure and mechanical property of recycled AZ91D magnesium alloy in the industrial experiment were studied. Ultimate tensile strength and elongation to failure prepared by solid state recycling were the highest and could reach 346.2MPa and 9.62%, respectively, with the optimal hot-press parameters of the temperature of 400±10℃, the holding time of 300s, the pressure of 400Mpa and the hot-extrusion parameters of the temperature of 450±10℃, the holding time of 300s, the extruding rate of 0.4mm/s. Mechanical property of the recycled specimens prepared by solid state recycling was related with the thickness of the recycled specimens, extrusion streamline and the distribution of the bonding layer.更多还原