A multi-scale multi-physics modeling framework of laser powder bed fusion additive manufacturing process

A longstanding challenge is to optimize additive manufacturing (AM) process in order to reduce AM component failure due to excessive distortion and cracking. To address this challenge, a multi-scale physics-based modeling framework is presented to understand the interrelationship between AM processing parameters and resulting properties. In particular, a multi-scale approach, spanning from atomic, particle, to component levels, is employed. The simulations of sintered material show that sintered particles have lower mechanical strengths than the bulk metal because of their porous structures. Higher heating rate leads to a higher mechanical strength due to accelerated sintering rates. The average temperature in the powder bed increases with higher laser power. The predicted distortion due to residual stress in the AM fabricated component is in good agreement with experimental measurements. In summary, the model framework provides a design tool to optimize the metal powder based additive manufacturing process.     

Heat treatment of PM parts by Hot Isostatic Pressing

Hot Isostatic Pressing (HIP) is a technology that has been around for 60+ years. By using high temperature and high gas pressure, dry metal and ceramic powders can be consolidated and a volume decrease can be achieved. Later developments include rapid cooling and rapid quenching to enable higher productivity and high-pressure heat treatment. This paper shows the advantages of having HIPing and Heat Treatment combined for Powder metallurgy parts.     

Non-ammonia enrichment of rare earth by magnesium oxide from rare earth leaching liquor in magnesium salt system

In order to solve the problem of ammonia-nitrogen pollution in the enrichment process of the ionadsorption type rare earth ore,the technology of non-ammonia precipitation with magnesium oxide precipitant was carried out.It is determined that the rare earth precipitation efficiency is 99.6% and the purity of rare earth concentrates is only 85.89 wt%under the optimum precipitation conditions.And the contents of MgO,SO_3 and Al_2O_3 in the rare earth concentrates are 5.12 wt%,6.77 wt%and 1.78 wt%,respectively.Furthermore,the thermo-decomposition process of precipitates was investigated by TGDSC,XRD and FI-IR.The thermal decomposition process consists of two stages:the dehydration of rare earth hydroxide and alkaline rare earth sulfate within 900 ℃ and the thermal decomposition of RE_2O_2SO_4 at 900-1300 ℃.Therefore,a high-temperature calcinations method for removing SO_3 from precipitates is proposed.When the precipitates are calcined at 1300 ℃ for 2 h,the rare earth concentrates with a purity of 92.03 wt%can be acquired.Moreover,the content of SO_3 in the concentrate is only 0.46 wt%.In the MgO precipitation and high-temperature calcinations process,the raw material cost is low and the quality of rare earth concentrates is acceptable.It could have great significance for nonammonia enrichment of rare earth from the rare earth leaching liquor,and finally solve the problem of ammonia nitrogen in the extraction process of the ion-adsorption type rare earth ore within magnesium salt system.     

粉末冶金法制备AZ91镁合金的组织及热性能

采用粉末冶金方法制备了AZ91镁合金,研究了烧结温度对合金的致密度和热导率的影响规律,并对烧结样品的物相和显微组织进行了分析.研究发现,AZ91镁合金的最佳烧结温度为610℃,致密度可以达到97.4%,实验条件下所获得的最高热导率可达到63.1W·m^-1·K^-1.X射线衍射和扫描电子显微镜结果分析表明,烧结合金组织主要由α-Mg固溶体和β-Mg₁₇Al₁₂相两相组成,其中β-Mg₁₇Al₁₂相表现出离异共晶β相和非连续析出β相两种主要存在形态.     

剧烈塑性变形对块状镁合金微观组织和力学性能的影响

等通道转角挤压（Equal channel angular pressing, 简称ECAP）可以使镁合金产生较大的塑性变形.通过有限元方法模拟了等通道转角挤压工艺及其相关工艺参数,研究了工件的应变和载荷分布情况,并建立了累积变形结果、微观组织细化和力学性能的数学模型.通过分析得到了晶粒细化和力学性能的关系,对累积变形的特点分析,预测了晶粒细化后的尺寸和力学性能.      

First-principles study of structure,mechanical and optical properties of La-and Sc-doped Y_2O_3

The electronic,mechanical and optical properties of La-and Sc-doped Y_2O_3 were investigated using firstprinciples calculations.Two doping sites of Sc and La in Y_2O_3 were modeled.The calculated values of the energy of formation show that the most energetically favorable site for a La atom in Y_2O_3 is a d-site Y atom,while for Sc a b-site Y atom is the more stable position.The calculated band gap shows a slight decrease with increasing La or Sc concentration.The calculated results for the mechanical and optical properties of Y_(2-x)R_xO_3(R = Sc or La,0x ≤ 0.1875)show that La-or Sc-doped Y_2O_3 would have enhanced strength,and thus an ability of resisting external shocks,and increased hardness and mechanical toughness.These improved mechanical properties are achieved without sacrificing the optical properties of the doped compounds.So the doping of La or Sc in Y_2O_3 is permissible in the preparation of Y_2O_3 transparent ceramics,of course,doping of La or Sc will benefit the sintering of transparent ceramics.     

宣钢单喷镁铁水脱硫技术应用与实践

介绍了宣钢炼钢厂采用单喷颗粒镁铁水脱硫工艺,改善了转炉冶炼原料铁水条件,同时对镁在铁水中的脱硫机理作了简要的叙述.该工艺在生产实践中不断优化,使铁水经过预处理后终点硫均达到0.010%(质量分数)以下,最低终点硫含量达到0.003%,满足了炼钢铁水要求.     

Effect of mechanical activation on the characteristics of ruthenium and aluminum powder mixtures

The effect of treatment in a high-energy mill-attritor on the structure of RuAl-based alloy powder mixtures and the exothermic effects in them is studied. The mechanical activation (MA) of aluminum and ruthenium powder mixtures is found to mill the conglomerates of hard disperse (0.5–2 μm) ruthenium particles in the initial mixtures and to produce composite granules. These granules consist of hard disperse ruthenium particles connected by plastic fcc aluminum particles. The structure of these granules differs from that of the layered granules that form during the MA of powder mixtures of two plastic fcc metals (nickel, aluminum). The cold working of the hard ruthenium particles, which have the hcp lattice and are deformed via twinning, occurs due to a decrease in the coherent domain size (to 120–80 nm) rather than to an increase in the dislocation density (as in the case of the MA of Ni-Al powders). Every granule contains all alloy (composite) components, including disperse or nanosize oxide particles, bound to the components that form an intermetallic matrix during reaction sintering. In granules of both types, MA increases the contact area between both metals entering into the reaction of RuAl (NiAl) formation and sharply decreases the diffusion path length of Al in Ru (Ni). This results in a decrease in the temperature of the onset of reaction alloy formation, which begins now in a solid phase, and in a decrease in the exothermic effect of the monoaluminide formation with the participation of a liquid phase (Al). MA for 15–16 h of powder mixtures provides a microuniform distribution of base and alloying elements and phases in the deformable alloys with an intermetallic matrix that are produced by reaction sintering.     

Microstructure and mechanical properties of A356 alloy with yttrium addition processed by hot extrusion

The effects of the rare earth element yttrium(Y) and hot extrusion on the microstructure and mechanical properties of A356 alloy were investigated by mechanical properties testing and microstructure observation. The results indicate that the addition of Y improves the microstructure of the as-cast alloy. The distribution of primary α-Al is uniform and orderly. The long needle-like eutectic Si phases and β-Fe phases turn to strips and short rods. When the content of Y increases to 0.2 wt%, the mean diameter of aAl(40.3 μm) and the aspect ratio of the eutectic Si phase(2.3) reach the minimum values, which are68.9% and 86.1% lower, respectively, than that of the alloy without Y addition. Under extrusion stress, the shape of the eutectic Si phase is changed from long rod-like to near grain-like after solution treatment.The size of the eutectic Si phase is significantly reduced. The needle-like β-Fe phases are squeezed and broken. The mechanical properties of the as-extruded alloy are significantly improved compared to the as-cast alloy. When the rare earth content is 0.2 wt%, the ultimate tensile strength, hardness and elongation of the alloy reach the maximum values, which are 328.2 MPa, 110.4 HV and 21.3%, respectively, and increase by 42.01%, 37.71% and 481.91%, respectively, in comparison to the as-cast alloy without Y addition.