两种典型金属零部件激光增材制造技术基础比较研究

发布时间：2018-09-18 09:36

【摘要】：激光增材制造高性能金属零部件技术具有成形结构复杂、成形精度高、成形性能优良等特点,是当前复杂精密金属零部件或大尺寸主承力金属构件一次性整体成形最具前景的应用技术之一。它不仅是铸造、锻造、焊接与机械加工等传统加工方法的有益补充,而且开启了一种全新的金属零件制造模式,具有重要的战略研究意义。目前,激光增材制造高性能金属零部件技术主要有两种典型方法,一种是基于同步送粉的激光熔覆沉积技术(Laser Cladding Deposition,简称LCD技术),另外一种是基于预置铺粉的激光选区熔化技术(Selective Laser Melting,简称SLM技术)。然而,上述两种技术因其成形方式与工艺参数的差别,导致二者在熔池形貌、冷却速率、凝固组织以及力学性能等若干材料成形基础方面存在较大差异,这为深入理解高性能金属零部件激光增材制造技术的成形原理及应用背景带来了一定的困难。为此,本文将对LCD与SLM技术在上述基础问题方面表现出来的差异性展开详细的对比研究,主要研究成果如下：采用大光斑、中光斑、小光斑LCD工艺以及微光斑SLM工艺成形316L不锈钢单道熔覆层试样,并由此将激光增材制造技术分为四大工艺类型展开研究。引入熔池的形状系数,特别是穿透系数,来对熔池的形貌变化进行分析。在LCD与SLM不同工艺条件下,熔池形貌与尺寸完全不同,导热模式与热影响效应也不同,进而最终影响到合金的组织与性能。尤其是在LCD与SLM工艺中,熔池形貌特征随归一化热焓的变化表现出相互独立的趋势,预示着LCD与SLM凝固过程的差异所在。采用LCD与SLM技术大、中、小、微四种不同光斑大小的工艺方案成形316L不锈钢样品,测量合金在不同工艺条件下的一次枝晶臂间距大小。通过一次枝晶臂间距与冷却速率之间的经验关系式,计算出熔池冷却速率的大小。结果显示,不同工艺条件下熔池的冷却速率相差1～4个数量级,其中,微光斑SLM工艺与大光斑LCD工艺熔池冷却速率的差异最高达到4个数量级。能量输入与熔池形貌的变化是导致冷却速率出现较大差异的主要原因。冷却速率大小决定晶粒尺寸大小。在LCD与SLM不同工艺方案成形316L不锈钢中,柱状晶宽度、长度及单个柱状晶的平均体积均随能量输入的增加而增加,且柱状晶的纵横比也逐渐增大。然而,柱状晶尺寸随冷却速率的变化却表现出不同的趋势。在LCD成形过程中,柱状晶尺寸与冷却速率满足经典的λ=a+b／(?)(λ代表晶粒尺寸大小,于为冷却速率,a,b为常数)线性关系,这与传统的凝固理论相似。而在SLM成形过程中,柱状晶尺寸与冷却速率平方根的倒数却满足三次函数关系,与传统凝固理论不符合。进一步分析表明,在LCD工艺中,由于冷却速率与过冷度较小,导致晶粒的形核率与生长速率比值相对较小,再加上熔池散热方向的单向性等原因,使得LCD工艺比SLM工艺更容易形成粗大的柱状晶组织。SLM与LCD成形过程中的多重热循环过程使合金组织结构变得更加复杂。在高功率SLM成形1Cr18Ni9Ti不锈钢中,随着熔池穿透系数的增加,熔覆层所经历的热循环次数越多,导致液相合金的冷却速率降低、过冷度减小,根据柱状晶的生长机制可知,这将导致柱状晶尺寸变粗大。同时,多重热循环过程还具有多次固溶处理的效果,这使得合金在胞壁区域的偏析现象减少,合金内元素成分更加均匀。同时,在小光斑LCD成形Inconel 718合金中,随着能量输入的增加,熔池中液相的温度梯度增加,成分过冷区减小,合金的组织形貌从树枝晶转变为树枝晶/胞晶共存,再转变为胞晶。而且,晶粒内亚晶界区域的拉夫斯相逐渐增多,基体内的二次沉淀相逐渐减少。组织结构的差异导致SLM成形构件的力学性能优于LCD成形构件的力学性能。在LCD与SLM成形316L不锈钢中,SLM成形样品的显微硬度比LCD成形样品高100 HV左右；SLM成形样品的最大抗拉强度比LCD成形样品高180 MPa; SLM成形样品的屈服强度是锻件标准的2.5倍,而LCD成形样品的屈服强度是锻件标准值的1.5倍。在高功率SLM成形1Cr18Ni9Ti不锈钢中,随着热循环次数的增多,由于晶粒尺寸与成分均匀性的双重变化,样品的拉伸性能变化差异较小；而在小光斑LCD成形Inconel 718合金中,随着柱状晶生长方向与样品沉积高度方向之间的夹角增大,样品的拉伸性能变差,且当试样沉积高度方向与拉伸方向垂直时的拉伸强度要比二者平行时的拉伸强度高约50 MPa。
[Abstract]:Laser augmentation is one of the most promising technologies for one-off integral forming of complex precision metal parts or large-sized main bearing metal components. It is not only a tradition of casting, forging, welding and machining. At present, there are two typical methods for manufacturing high-performance metal parts by laser augmentation. One is laser cladding Deposition based on synchronous powder feeding. The other is laser selective melting (SLM) technology based on pre-laid powder. However, because of the difference of forming methods and technological parameters, there are great differences between the two technologies in pool morphology, cooling rate, solidification structure and mechanical properties of some materials. It is difficult to understand the forming principle and application background of laser augmentation manufacturing technology for high performance metal parts. In this paper, the differences between LCD and SLM technology in the above basic problems are studied in detail. The main research results are as follows: LCD technology with large spot, medium spot and small spot is adopted. The shape coefficient of the molten pool, especially the penetration coefficient, is introduced to analyze the morphological changes of the molten pool. In particular, in LCD and SLM processes, the morphological characteristics of the molten pool show an independent trend with the change of normalized enthalpy, indicating the difference between the solidification process of LCD and SLM. The primary dendrite arm spacing of 316L stainless steel was measured under different process conditions. The cooling rate of molten pool was calculated by the empirical relationship between primary dendrite arm spacing and cooling rate. The difference of cooling rate between spot SLM and large spot LCD is up to four orders of magnitude. The variation of energy input and pool morphology is the main reason for the large difference of cooling rate. The average volume of columnar crystals and single columnar crystals increase with the increase of energy input, and the aspect ratio of columnar crystals increases gradually. However, the change of columnar crystal size with cooling rate shows a different trend. In the process of SLM forming, the reciprocal of columnar crystal size and the square root of cooling rate satisfies a cubic function, which is not consistent with the traditional solidification theory. Due to the relatively small ratio of nucleation rate to growth rate and the unidirectional heat dissipation direction of molten pool, it is easier for LCD process to form coarse columnar structure than SLM process. With the increase of penetration coefficient, the more thermal cycles the cladding layer undergoes, the lower the cooling rate and undercooling degree of the liquid alloy. According to the growth mechanism of columnar crystals, the columnar crystals become coarser and coarser. At the same time, with the increase of energy input, the temperature gradient of the liquid phase in the molten pool increases, and the supercooled zone decreases. The microstructure of the alloy changes from dendrite to dendrite/cell coexistence and then to cell. In 316L stainless steel formed by LCD and SLM, the microhardness of SLM is about 100 HV higher than that of LCD, and the maximum tensile strength of SLM is about 100 HV higher than that of LCD. The yield strength of SLM is 2.5 times higher than that of forging standard, while that of LCD is 1.5 times higher than that of forging standard. However, the tensile strength of Inconel 718 Alloy formed by LCD with small facula decreases with the increase of the angle between the growth direction of columnar crystal and the deposition height of the sample, and the tensile strength of the sample is about 50 MPa higher when the deposition height is perpendicular to the tensile direction.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TH16

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