金属熔体电磁成形过程研究

以制备无污染的航空发动机叶片为背景 ,分析了金属熔体电磁成形定向凝固技术的原理 ,并以铝合金及 1Cr1 8Ni9Ti为研究材料 ,探讨了交流电磁场作用下金属熔体的感应加热熔化及约束成形过程 ,结果表明 :感应器结构决定其内部的磁场及电磁压力分布 ,感应器输入功率、熔体高度、上下液固界面位置、抽拉速度及冷却条件等参数综合影响金属加热熔化特性、熔体形状及其稳定性 ;通过控制合理的工艺参数 ,获得了截面为圆形及近似弯月面形、表面质量和内部定向组织良好的样件 .     

A new process for atomization of metal alloys

A laboratory apparatus was constructed to atomize metal alloys by forcing them through sintered ceramic disc filters. The process combines filtration of oxides and/or undesirable second phase particles with atomization. Sintered Al₂O₃ and SiO₂ disc filters with average pore sizes in the range of 36 to 150 m were used. The effectiveness of various filters in removing synthetic inclusions from Al-Ti-B melts was studied. 2 to 10 m size TiB₂ particles were successfully filtered out. Atomization experiments were done with pure aluminium and 7075 aluminium alloy. Spherical powders, 150 to 2000 m in size, of 7075 aluminium alloy with secondary dendrite arm spacings between 4 to 8 m, were cold compacted and extruded into a billet. Room temperature longitudinal properties of the billet in T6 condition were: YS 80×10³ psi, UTS 94.3×10³ psi, elongation 15% and R.A. 42%.     

棒材等离子喷涂制备非晶态合金雾化过程的数值模拟及工艺参数的制定

根据已有的火焰喷涂一维稳定流动学研究的成果,提出棒材等离子喷涂法制备非晶涂层的雾化机理并计算了棒材等离子喷涂法制备非晶时雾化粒子的中值粒径。同时改变棒材等离子喷涂中主气压力和辅气压力两个主要参数,优化了铁系非晶涂层制备工艺。结果表明,3mm Fe80P13C7及Fe72Cr8P13C7的晶体棒材通过大气等离子喷涂,在喷涂电压为55V,喷涂电流为600A,棒材与喷嘴的距离为4mm,送棒速率为50mm/min,喷涂距离为100mm,喷涂角度≈90°,主气压力为0.95MPa,辅气压力为0.35MPa喷涂可制备非晶涂层。     

镁合金冷金属过渡熔池动态行为数值模拟

为研究冷金属过渡焊(CMT)的周期性能量输入及焊丝抽送行为对镁合金熔池动态行为的影响,在FLUENT软件中建立了焊丝-熔滴-熔池多相流数值分析模型。提出一种在流体体积法(VOF)多相流计算域中划分熔滴与熔池区域并判断其接触状态的方法,结合动网格技术实现自动响应的焊丝抽送,在熔池区域加载间歇性的热源、电弧力,采用镁合金CMT堆焊实验所得参数进行数值模拟。结果表明,在CMT能量输入周期的短路阶段,由于焊丝的回抽,熔池被向上提拉并在焊丝端部形成了液桥,内部液态金属在马兰戈尼力的作用下,由边缘流向中间,由下方流向上方,焊丝持续回抽至脱离熔池后,熔池受到液桥断裂的反作用力,液态金属快速向后方流动使熔池形状发生改变,可知焊丝回抽与马兰戈尼力是熔池动态行为的主导作用力,此外,焊缝熔深的模拟结果为0.53 mm,与实际成形焊缝的熔深存在3.47 %的误差,验证了模型的有效性。     

Impact of process flow conditions on particle morphology in metal powder production via gas atomization

Additive manufacturing processes as for instance selective laser melting or electron beam melting are becoming more common and just turning into standard manufacturing processes for metal components. Nevertheless, these processes are still new compared to classic powder metallurgy manufacturing routes such as pressing and sintering. Hence not all necessary requirements for the powders in use are fully known yet. This makes an increase in control of the powder properties a crucial task to achieve. To reach this goal one must understand the different influences on the powder production process from the beginning of the whole production route. In this work, the influence of the spray chamber flow on the particle morphology is examined. The nozzle system used to produce the metal powders is a close-coupled atomization system with a convergent-divergent gas nozzle configuration. The particle morphology as well as the particle size distribution have been analyzed to examine the influence of the atomization gas flow compared to an additional use of a coaxial gas flow. To review the changes of the flow patterns, computational fluid dynamic simulations have been performed. The particle trajectories were calculated to assess the change in particle behavior as well. Atomization experiments have been conducted with an AISI 52100 (1.3505) steel in a small batch atomization plant to evaluate the influence of the change in flow on the particle size distribution and circularity. The experimental results show that a use of additional coaxial gas leads to an increase in particle circularity up to 10% for relevant particle sizes. An approach for the quantification of satellite occurrence is given by examination of the shift of the particle size distribution to smaller diameters.     

超音速气体雾化高硅铝合金粉末冷却速度计算

通过快速凝固技术制备合金材料,可以大幅度细化合金组织,其技术的冷却速度.本文通过对流换热原理对超音速气体雾化高硅铝合金粉末的冷却速度进行了理论计算,结果表明其冷却速度大约在104～107K/s之间,说明利用超音速气体雾化制备高硅铝合金粉末可以达到很高的冷却速度;另外,通过测定合金粉末内部凝固枝晶间距,并利用冷却速度和枝晶间距之间的经验关系,确定合金冷却速度,其结果与理论计算基本相符.     

Polymer-derived ceramic molten metal filters

Journal of Materials Science - This paper reports the synthesis and the performance of polymer-derived ceramic filters for molten metal filtration. Two different filter types were studied: foam...     

Effects of process parameters on surface morphology of metal powders produced by free fall gas atomization

There are a number of process parameters which affect the characteristics of metal powders produced by free fall gas atomization. In the following work effects of various process parameters like apex angle of atomizer, focal length of atomizer, number of nozzles, diameter of nozzles, diameter of liquid metal delivery tube, superheat of liquid metal and type of metal etc. were studied on their surface morphology. It was observed that shape of powder particles depends on apex angle, superheat of liquid metal, type of metal and particle size range within a powder collective. Other parameters like focal length of atomizer, number of nozzles, diameter of nozzles and diameter of liquid metal delivery tube were found to have no effect on the shape of powder particles. However, Surface porosity and solidification shrinkage were observed on almost all types of metal powders.     

熔盐电解钛精矿制备钛铁合金的脱氧历程

以攀枝花地区的钛精矿为原料通过CaCl2熔盐电解工艺直接制备FeTi合金。主要通过不同时间间断实验对电解过程进行跟踪,结果表明以钛精矿为原料电解制备FeTi合金的过程中首先得到金属铁和钙钛矿,然后是钙钛矿电解得到钛的低价氧化物,最后是钛的低价氧化物电解得到钛和形成FeTi合金,通过阴极失重实验验证了分段反应的结论。最后对钙钛矿的形成机理进行了实验研究,得到CaTiO3形成有两个途径：（1）熔盐参与反应,并且是主要成因;（2）熔盐CaCl2水解得到CaTiO3。     

Laser-induced breakdown spectroscopy of molten aluminum alloy

We have demonstrated that a fiber-optic laser-induced breakdown spectroscopy (LIBS) probe is suitable for measuring the concentration of minor constituents of a molten Al alloy in a laboratory furnace. For the first time to our knowledge we are able to record the LIBS spectra in several spectral regions of seven different molten Al alloy samples by inserting the LIBS probe inside the molten alloys, allowing us to obtain a ratio calibration curve for minor constituents (Cr, Mg, Zn, Cu, Si, etc.), using Fe as a reference element. A ratio calibration curve for Fe with a major element (Al) can also be obtained with which the concentration of Fe in the alloy can be determined. The effects of the surrounding atmosphere on the LIBS spectra of the molten alloy were investigated. Effects of focal length of the lens on the LIBS signals were also studied. LIBS spectra of a solid Al alloy recorded with the same LIBS probe were compared with the LIBS spectra of the molten alloy. Our results suggest that the LIBS probe is useful for monitoring the elemental composition of an Al melt in an industrial furnace at different depths and different positions inside the melt.     

金属粉末注射成形钛合金工艺研究

为了探究保障最终产品性能满足ASTM标准的粉末注射成形工艺,本文采用金属粉末注射成形方法,以德国Basf公司提供的喂料为原料,通过对注射参数优化获取1组最优注射工艺参数,用于后续脱脂及烧结工艺.利用拉伸、弯曲等力学实验,扫描电镜、金相光学显微镜等表征方法对材料的力学性能及微观结构进行表征,研究了注射、脱脂和烧结工艺参数...     

Visualization and modeling for water atomization of low melting point alloy

Liquid metal fragmentation by impinging fast water spray, so called water atomization, is widely used to produce metal powders efficiently. In the present paper, we conduct the high-speed visualization experiments and theoretical modeling for elucidating the mechanism of fragmentation and solidification processes, which are essentially important to control the metal powder characters. We successfully visualize the detailed sequential events from the water spray ejection, freely dropped molten metal of 42Sn-58Bi, followed by their collision, metal fragmentation in liquid phase, and solidification, leading to revealing the fragmentation processes as the impact of water spray and the vapor explosion. Quantified metal particle size convinces that the water atomization simultaneously proceeds fragmentation of metal in liquid phase with solidification. The experimental results of size distribution and mean diameter well validate the proposed physically-consistent theoretical modeling for the prediction of particle size.     

Microstructural features associated with spray atomization and deposition of Al-Mn-Cr-Si alloy

An inert gas atomization process was employed in production of rapidly solidified powders as well as disc-shape preform by spray deposition of an Al₇₅Mn₁₀Cr₅Si₁₀ alloy. Microstructural features of atomized powders and spray deposited preforms were evaluated by scanning and transmission electron microscopy and X-ray diffractometry techniques. Solidification structure of powders revealed cellular and dendritic morphology, depending on their size. The interdendritic regions consisted of second phase particles. In contrast the spray formed alloy exhibited microstructural homogeneity with distribution of ultra-fine second phase particles of intermetallic compound. The structure of second phase intermetallics was identified as a cubic -Al(Mn,Cr)Si, in both the atomized powders and the spray-deposits. The formation of cubic phase is discussed as rational approximant structure of an icosahedral quasicrystal.     

Non-equilibrium solidification of undercooled droplets during atomization process

Thermal history of droplets associated with gas atomization of melt has been investigated. A mathematical model, based on classical theory of heterogeneous nucleation and volume separation of nucleants among droplets size distribution, is described to predict undercooling of droplets. Newtonian heat flow condition coupled with velocity dependent heat transfer coefficient is used to obtain cooling rate before and after nucleation of droplets. The results indicate that temperature profile of droplets in the spray during recalescence, segregated and eutectic solidification regimes is dependent on their size and related undercooling. The interface temperature during solidification of undercooled droplets rapidly approaches the liquidus temperature of the alloy with a subsequent decrease in solid-liquid interface velocity. A comparison in cooling rates of atomized powder particles estimated from secondary dendrite arm spacing measurements are observed to be closer to those predicted from the model during segregated solidification regime of large size droplets.     

Production of fine Nd-Fe-B alloy powder by pulsed plasma atomization

Extremely fine (<10 m) Nd-Fe-B powder was generated by pulsed plasma atomization processing. The number average was just over 3 m, as determined by computer-assisted scanning electron microscopy analysis of almost 3000 particles. Despite the fine particle size, alloys of stoichiometric Nd₂Fe₁₄B resulted in the properitectic formation of -Fe, deleteriously affecting the magnetic properties. Nevertheless, this work shows the possibility of generating extremely fine, isotropic powder for bonded and hybrid magnet applications.     

Microstructure and properties of Cu-Cr-Nb alloy powder prepared by argon gas atomization

The Cu-2Cr-1Nb (at%) alloy powder was prepared by close-coupled argon gas atomization. The morphology, microstructure and phase were characterized by SEM, TEM, HRTEM, SAED and XRD. The effects of heat treatment on the microstructure and microhardness of the powder were investigated. The alloy matrix had equiaxed crystal structure, and the second phase was Cr₂Nb intermetallic compound in sub-micron and nano-scale. Compared with large powder, the second phase of small powder is finer, and its distribution in the matrix is more uniform, the microhardness of the small powder is higher. After heat treatment at 773 K for 1 h, Cr and Nb precipitated and formed a large number of small precipitates, age hardening and peak aging phenomena occurred, and the peak hardness of the powder reached 130 HV. With the increase of heat treatment temperature, the second phase was coarsened. When the heat treatment temperature was 973 K, the short-time heat treatment hardening and long-time heat treatment softening were shown. However, after the heat treatment at 1223 K, only softening phenomenon was observed. This method can be used to prepare the excellent powder for Cu-Cr-Nb alloy, which can be used as a potential structural material for aerospace.     

Solidification microstructure of super-α2 alloy prepared by gas atomization

Ti–25Al–10Nb–3V–1Mo alloy has been prepared by gas atomization with atomizing pressure above 8 MPa. The cooling rate of 5 × 10³ ∼ 3 × 10⁵ K s^-1 was obtained according to secondary dendritic arm spacing, as the particle size is 30 ∼ 300 μm. It is known that in the big particles two dendrite morphologies can be found, cellular dendrite and well-developed dendrite, and in small particles there exists only well-developed dendrite. The surface of particles is characteristic of solidification that is identical to the observation on the cross-sectional views. The micrographs of cross-sectional views show that the big particles, whether of cellular dendrite or well-developed dendrite, solidify by multiple nucleation events while the small particles of well-developed dendrite solidify from a single nucleation event. The microstructure of the alloy consists of single β0 phase. This revised version was published online in November 2006 with corrections to the Cover Date.