基于材料性质转换的金属粉末直接选区激光烧结温度场数值模拟

基于MSC.Marc非线性有限元分析软件,建立金属粉末直接选区激光烧结过程的温度场有限元分析模型。模型中考虑了粉体—实体转化过程中热导率等物理性质随温度的非线性变化规律。对于表面对流和辐射散热条件,采用修正系数的方法,考虑激光冲击加热作用引起的极大温度梯度的影响。同时还采用在不同载荷步之间转换单元材料物理性质,以考虑激光烧结过程中的局部热源输入作用。对采用在不同载荷步之间转换单元材料物理性质的方法和不采用此方法进行模拟对比研究,结果表明：前者在烧结开始时在光斑中心附近具有极大的温度梯度,这与试验结果一致;前者热传播的主要范围集中在已烧结区域,而后者热传播的范围则是整个粉床;前者进入稳态烧结后光斑中心温度变化趋于平缓,而后者的光斑中心温度一直持续上升。这主要是由于已烧结部分的材料热导率比未烧结部分的粉末高约100倍造成的。     

Reducing ejection forces for parts moulded into direct metal laser sintered tools

With the development and refinement of Rapid Prototyping technologies, a gradual move into tooling applications has occurred. However, the design principles that are traditionally applied to mould tools are not always suitable for use with layer-manufactured tools. As these technologies develop, so must the ways in which they are used.Tools produced using Direct Metal Laser Sintering (DMLS) have previously been shown to be suitable for several applications, in particular for plastic injection moulding. However, there has been little work undertaken on the best procedures to follow when designing tools made using the DMLS process. This work has been performed to determine the effect of layer thickness, level of finishing and draft angle on the forces required to eject parts from DMLS injection mould tools.Results show that, although the choice of layer thickness and the level of tool finishing have a substantial effect on the ejection forces required, the level of draft included does not show an obvious trend towards increasing or decreasing the force.     

Sensitivity of direct metal laser sintering Maraging steel fatigue strength to build orientation and allowance for machining

This work deals with the effect of build orientation and of allowance for machining on DMLS‐produced Maraging Steel MS1. The experimental results, arranged by tools of Design of Experiment, have been statistically processed and compared. The outcomes were that, probably due to effect of the thermal treatment, machining, and material properties, the aforementioned factors do not have a significant impact on the fatigue response. This made it possible to work out a global curve that accounts for all the results, consisting in a high amount of data points. This can be regarded as one of the most complete and reliable fatigue models in the current literature. Fractographic and micrographic studies have been performed as well, to individuate the crack initiation points, usually located at subsurface porosities, and to investigate the location of internal inclusions and the actual martensitic microstructure along the stacking direction and on the build plane.     

Experimental investigation on machinability of DMLS Ti6Al4V under dry drilling process

Additive Manufacturing (AM) technology is being extensively utilized in many industries concerning its superior advantage of easily fabricating the 3D parts possessing relatively high geometrical complicacy. Nevertheless, the AM parts generally need conventional machining, such as drilling, to meet the practical requirements, indicating disparate machinability compared with those manufactured by conventional wrought processes. Hence, to investigate the effects of machining parameters on the drilling characteristics of AM parts is of great significance. This work experimentally studied the machinability of the additive manufactured Ti6Al4V by covering a variety of topics including the cutting forces, temperatures, machined surface roughness, tool wear modes and chip morphology with selected cutting speeds and feed rates under dry drilling process. The material utilized in experiments is the Ti6Al4V manufactured by Direct Metal Laser Sintering (DMLS) technology. The experimental results highlight the parametric influence on the machinability of DMLS Ti6Al4V and present the possibility to validly redesign the machining parameters accordingly.     

直接金属粉末激光成型异型水路在模具冷却系统中的应用

介绍了利用直接金属粉末激光烧结技术制作具有快速控温功能的异型水路的方法,该异型水路用在型腔模上不受传统加工工艺的限制,更能贴近成型制品轮廓,对制件的死角或不易散热区域,异型水路均能达到良好的散热效果。将异型水路设计应用于注射模冷却系统中,可使模具温度控制达到最佳的效果,塑件的质量也得以提高。     

Developments in investment casting process—A review

Investment casting has been used to manufacture weapons, jewellery and art castings during the ancient civilization. Today, its applications include jewellery/art castings, turbine blades and many more industrial/scientific components. The present paper reviews various investigations made by researchers in different stages of investment casting and highlights their importance. The paper initially highlights the investigations made on pattern wax properties, effects of blending, additives and fillers. Different ways through which pattern properties (like surface finish, dimensional accuracy, etc.) could be enhanced by properly controlling the injection processing parameters are thoroughly discussed. The paper also looks into the investigations made to enhance the strength, surface finish, etc. of ceramic shell for ferrous alloys/non-ferrous alloys as well as superalloys in investment casting. Investigations made on incorporation of nylon fibers and polymer additions confirm that a ceramic shell reinforced with nylon fibers attains additional permeability compared to the one with polymer additions.     

材料特性和工艺参数对DMLS制件质量的影响

分析了金属粉末直接激光烧结(DMLS)中三类粉末材料的粘结机制,讨论了材料特性和工艺参数对成型件质量的影响。建立了液相体积分数和颗粒尺寸与烧结件致密度之间的关系式,提出从材料特性和工艺参数方面改善烧结件质量的途径。最后讨论了金属粉末激光直接烧结成行进一步研究的三个主要方向。     

直接金属粉末激光烧结成形过程温度场模拟

在综合考虑热传导、热辐射及对流等热现象的基础上。初步建立了用于模拟激光烧结过程中传热行为的数学模型。根据Gusarov模型求得粉床的有效导热系数，表明导热系数与铺粉过程中粉末的堆垛方式以及烧结过程中的粘结率有关。在绝热边界条件和部分热物性参数恒定的合理简化下，利用解析法求解了热传导方程。利用FORTRAN语言编写了求解结果的运算程序，并对基模高斯激光近似作用下水雾化铁粉的烧结成形温度场进行了数值模拟，模拟结果较好地验证了实验结果。     

直接金属选区激光烧结热应力场有限元模拟

在考虑随温度变化而变化的热传导、比热容等热物性参数的作用下,建立了直接金属多道烧结的三维有限元分析模型。采用先进行温度场分析、再进行应力场分析的间接热力耦合策略。模拟结果显示,随着烧结过程的进行,由于已烧结部分的影响,最大热应力有减小的趋势;在扫描烧结道的前方有比后方更大的应力分布,这与实验结果相吻合。     

基于ABAQUS的金属粉末激光直接烧结温度场模拟

金属粉末激光直接烧结成形是激光快速制造技术重要的发展方向。为了掌握激光烧结中熔池的加热和冷却规律及各烧结道之间的相互影响,综合考虑热传导、热辐射和热对流及变热物性参数,基于ABAQUS平台建立了移动热源作用下的三维瞬态金属粉末激光末激光多道烧结温度场的有限元模型,利用用户子程序DFLUX实现热源移动,通过定义潜热来处理相变的影响,以水雾化铁粉作为烧结材料进行了模拟,摸拟结果与先前实验结果吻合较好。同时模拟结果表明此计算模型有助于估计实验激光功率密度,烧结密度及烧结深度;并为后续的残余应力的精确计算做好了准备。