The powder's thermal behavior on the surface of the melting pool during PTA powder surfacing

The thermal behavior of ceramic powders on the surface of a melting pool during the plasma transferred-arc powder surfacing are theoretically analyzed. The results reveal that for a ceramic powder particle, its experienced heat power from melting pool is much greater than that from arc space during the powder PTA surfacing. It means the heating by the melting pool has taken a much more important part in the particle's thermal behavior than the heating by plasma arc column for a ceramic particle. However, different from the heating by the plasma arc column, the particle's heated power on the 100 A melting pool surface is greater than that on the 200 A melting pool surface.     

Using contrast material techniques to determine metal flow in screw extrusion of aluminium

Screw extrusion is a new continuous solid state processing method for aluminium. In this process, small pieces of aluminium are continuously inserted into a container wherein a rotating screw pushes the material forward. The material is heated, consolidated and extruded through a die in front of the end of the screw in one single continuous process. Understanding the material flow and the pressure generating mechanism is vital for optimizing screw, container and die designs, thereby increasing capacity, material quality and process stability. Full scale experiments using a prototype extruder were chosen as the means of investigation. In this paper, it is reported on the development of a novel contrast material technique to visualize material flow in the screw channels and the extrusion chamber. Extrusion of Ø 10 mm aluminium profiles was performed using both single and double flight screws. The experimental technique provided a means for evaluating the frictional conditions between the aluminium and the surface of the extruder. Different feeding schemes were used to identify regions of high material flow rate together with dead metal zones in the screw channels and in the extrusion chamber. It is demonstrated how newly fed material interacts with and displaces material already present and how the screw pushes material into the extrusion chamber. The instantaneous extrusion velocity was measured and significant variations were detected for all feed rates. An explanatory model of the extrusion pressure generating mechanism in the extrusion chamber and the screw channel is presented linking the material flow and the consolidation process to the extrusion velocity variations.     

Numerical simulation of flow characteristics in new co-rotating triangle arrayed triple screw extruders

Triangle arrayed triple screw extruder (TATSE) is a new equipment of polymer processing, which is different from traditional single and twin-screw extruder in flow and mixing characteristics. The studies about TATSE are very limited and all have been in the conditions of qualitative analysis due to its complicated geometry. In order to understand the flow rule of polymer melt and optimise the design of TATSE, in this paper, 3D flow of polymer melt in TATSE was simulated by using finite element method (FEM). A special study of flow in particular central region of TATSE was carried out by the analysis of the velocity distribution, pressure distribution and partial residence time distribution (RTD). The extrusion characteristics of TATSE, namely the abilities of pumping capability, distributive mixing, dispersive mixing, power consumption and productivity ratio, were compared with those of twin-screw and four-screw extruder. The results showed that there was circumfluence in central region but no stagnation in it. Although the velocity and pressure gradients were small relatively, the ability of pressure generation for TATSE was not evidently lower. TATSE had better operative abilities and high productivity ratio than intermeshing twin-screw extruder in the capacities of pumping capability and mixing.     

Microstructure and properties of AlSi7Mg alloy fabricated by selective laser melting

The AlSi7Mg alloy was fabricated by selective laser melting (SLM), and its microstructure and properties at different building directions after heat treatment were analyzed. Results show that the microstructure of SLM AlSi7Mg samples containes three zones:fine grain zone, coarse grain zone, and heat affected zone. The fine-grain regions locate inside the molten pool, and the grains are equiaxed. The coarse-grain regions locate in the overlap of molten pools. After T6 treatment, the microstructure at the molten pool boundary is still the network eutectic Si, but the network structure becomes discrete, and is composed of intermittent, chain-like eutectic Si particles. The yield strength at three directions (xy, 45°, z direction) of the AlSi7Mg alloy samples fabricated by SLM is improved after T6 heat treatment. The fracture mechanism of the samples is a mixed ductile and brittle fracture before heat treatment and ductile fracture after heat treatment.     

Mathematical modeling of melting rate in twin-wire welding

The paper treats four mathematical models for calculation of melting rate in arc fusion welding with a wire in coil form. The mathematical models permit calculation of melting rate in direct current welding with single-wire and double-wire electrodes (both polarities).

For single-wire welding the models treated have been improved with regard to the ones published in the literature, for twin-wire welding, these are the first models for calculation of melting rate. The mathematical models have already been tested in practice and the results obtained show that they are very accurate, simple and applicable to practice.     

电冶碳化钨钢结合金在螺纹钢丝轧辊上的应用

介绍了用废弃的粉末冶金碳化钨基钢结合金作原料,采用电冶熔铸工艺制备出新型的电冶碳化钨基钢结合金,并将其用在冷拔轧螺纹钢丝轧辊上。结果表明,电冶钢结合金组织致密、强度高、韧性好,尤其是具有高的抗磨损性能,用其所制的钢丝轧辊成本低、使用寿命高。     

A simple model of throughput and pressure development for single screw

To be able to predict the throughput of a single-screw extruder or the metering time of an injection moulding machine for a given screw geometry, set of processing conditions and polymeric material is important both for practical and designing purposes. The model is based on viewing the entire screw simply as a pump, conveying a solid and a molten fraction. The evolution of the solid fraction is the essence of the plastication process, but under a particular hypothesis on solid bed acceleration, its influence on the throughput is nil. This allows getting a good estimate on the throughput and pressure development along the screw. Calculations are compared to a large set of experiments available from the literature. Consistent agreement with these published results is obtained, both for throughput and pressure along the screw. The effect of the plasticating process on the throughput is non-existent if the plastication length is short, and more visible if the plastication length takes a good part of the screw length (for instance, at higher screw rotation frequency). This diminishes the throughput value and widens the pressure peak. The model also shows that the screw geometry is the most important parameter, followed by polymer rheology and processing conditions. Melting properties and length seem to intervene to a lesser extent. Finally, the model is used for screw design, highlighting the influence of the compression zone on throughput.     

采用双联熔炼技术的灰铸铁熔炼材料的选用

总结了双联熔炼工艺中灰铸铁熔炼材料的要求和特点,并针对使用过程中容易出现的问题进行了分析,同时分析了熔炼材料对铸件性能可能造成的影响,为合理选用熔炼材料提供了依据。     

螺杆泵在液压机过滤冷却加热系统中的应用

大型液压机液压系统中的液压油必须保证一定的清洁度和温度,螺杆泵作为液压机过滤冷却加热系统的动力源,在整个系统中起着非常重要的作用.介绍了75 MN铝挤压机过滤冷却加热系统的组成及其工作原理,通过转速、结构形式和流量的要求确定了螺杆泵的型号,最后对螺杆泵在使用时的注意事项进行了总结.实践结果证明,螺杆泵的设计选型合适,该...     

Forming of aluminium alloy at temperatures just below melting point

In order to improve the mechanical properties of products, a forming process of a solid material at a temperature just below the melting point is proposed. The material is deformed at the semi-solid temperature due to the heat generation caused by plastic deformation. The tensile strength, elongation, hardness and toughness of the aluminium alloy (Al–7%Si–0.3%Mg) billet extruded at temperatures between 500 and 550 °C are compared with those of the billet extruded in the hot forming (450 °C) and the semi-solid (560 °C) temperatures. The billet temperature during forming is evaluated by the finite element simulation. The tensile strength and hardness of the billet extruded at 550 °C just below the solidus temperature are higher than those for a billet at 450 °C, and they are almost the same as those for a billet deformed at 560 °C in the semi-solid region. The elongation and toughness of the extruded billet at 550 °C are lower than those for a billet at 450 °C. The forming load at 550 °C is almost half of that at 450 °C. Cracking on the surface of the extruded billet occurs at a high punch speed. The calculated temperature when the solid billet is extruded in the semi-solid state agrees well with the experimental one at which the tensile and hardness are improved.     

Experimental and theoretical investigation on microwave melting of metals

Experiments were conducted for microwave heating and melting of lead, tin, aluminium and copper with the aid of susceptors and the detailed results were presented for various microwave power levels and sample loading. Aluminium and copper samples were heated in presence of inert gas to minimize oxidation. Compared to conventional melting, microwave melting was twice as fast and more energy efficient. Lumped parameter model of the heating process showed that the conversion of microwave to thermal energy was enhanced at higher temperatures, justifying this a favourable process for metal melting applications.     

发射药螺杆挤压机叶片强度分析及结构参数优化

对于发射药挤出成型用的螺杆挤压机的双锥度螺杆,在采用某种工艺的时候,药料的相互剪切和挤压作用大大提高。利用ANSYS有限元分析软件,通过模拟药料在挤压过程中的流动和应力分布情况以及流固耦合,对现有的螺杆进行强度分析,校核其是否适应新的工艺。以降低流道内最大压力差为优化目标,螺杆强度为约束条件,通过改变螺杆的结构参数,提高螺杆挤压机的挤出效率。结果表明：楔形角对流道最大压力差影响显著大于螺棱宽度;通过正交试验得到最佳参数组合为楔形角均为24°,螺棱宽度为1.5 mm,此挤出流道内压力差达到最小。     

Local melting in silicon driven by retrograde solubility

High point-defect enthalpy of formation leads to retrograde solubility for a number of solutes in silicon, especially d-shell transition metals. We present direct experimental evidence for a thermodynamic pathway leading to local retrograde melting in silicon driven by the retrograde solubility of low-concentration metallic solutes at temperatures above the bounding liquid-silicide–silicon invariant reaction. We experimentally demonstrate this local melting pathway using in situ synchrotron-based X-ray microprobe measurements of silicon supersaturated with nickel, wherein solute precipitation into liquid droplets is observed. No significant energy barrier is observed for the nucleation of the liquid droplets at the surface, suggesting that in the Ni–Si system, retrograde melting will occur upon supersaturation if favorable heterogeneous nucleation sites are available.     

旋流数对铝熔炼炉熔炼过程的影响

针对现有的圆形铝熔炼炉,结合铝熔炼炉熔炼过程的特点,在铝熔炼炉热平衡测试的基础上,建立了合理的铝熔炼炉数学模型,并运用计算流体力学软件FLUENT实现燃烧空间和熔池的耦合物理场的数值模拟。同时分析了不同旋流数对铝熔炼炉熔炼过程的影响,依据提出的优化准则,确实旋流数大于0.6时能够获得最佳的熔炼性能。     

氩弧焊修复激光成形TC11钛合金组织及高周疲劳性能

采用氩弧焊对沉积态激光成形TC11钛合金进行修复并进行双重退火热处理.观察分析了修复试样热处理前后各区域的组织,测试了高周疲劳极限并进行分析.结果表明,修复区宏观组织呈柱状晶形貌,热影响区和基材区呈柱状晶和等轴晶交替排列形貌.热处理前修复区和热影响区为超细针状α相分布在β相基体中,基材区为较细的网篮组织;热处理后各区域的微观组织均为板条状初生α相+β相转变组织,修复区初生α相长度显著大于其它区域.修复件高周疲劳极限下降约7.1%.断口分析表明:修复件疲劳源均为修复区气孔,气孔缺陷是疲劳性能下降的主要原因;疲劳源区呈α相解理、β相撕裂形貌.     

Characterization of an electrical melting point using a focusing transducer

The thickness resonances of an elastic plate normally insonified by an ultrasonic plane wave are well known. From these resonance frequencies, the plate thickness can be easily deduced. In this paper, we use a focusing ultrasonic transducer with a large bandpass to characterize an electrical melting point between two steel plane shells. This solder type is abundantly used in the manufacturing of cars. The solder is placed in the focal spot of the beam. The transducer can be parallely moved to the surface of the sheet steel. A number of time signals are recorded along a diametrical line of the melting point with a small step. For each time signal, two spectra are computed: the magnitude spectrum also called backscattered spectrum and the phase spectrum. The results are processed with graphic software. The different images allow the evaluation of the quality of an electrical melting point.     

蓄热式铝熔炼炉熔炼过程的数值模拟

为了更好地研究和优化铝熔炼炉的性能,针对现有的蓄热式圆形铝熔炼炉,在建立合理的铝熔炼炉基本模型的基础上,通过耦合用户自定义熔化模型和氧化烧损模型,运用计算流体力学软件FLUENT实现燃烧空间和熔池的耦合物理场的数值模拟。着重研究不同固液区和不同孔隙率对铝及铝合金熔炼过程的影响。结果表明,该模型较好地反映铝熔炼炉的熔炼现象,可运用该模型进行铝熔炼炉熔炼过程工艺参数的优化研究。同时获得了固液区和孔隙率对熔炼参数影响规律:铝液温度在固液区上升缓慢,而离开固液相线时,铝液温度上升速度加快,炉膛温度和氧化层质量随着熔炼时间分别呈周期性增加和呈抛物线增加;随着氧化层厚度的增加,铝液温度随着孔隙率的增加而增加变得缓慢。     

烧嘴对蓄热式铝熔炼炉熔炼过程的影晌

结合蓄热式铝熔炼炉熔炼过程的特点,运用FLUENTUDF和FLUENTScheme混合编程,耦合用户自定义熔化模型和燃烧器换向及燃烧量变化模型,实现了蓄热式铝熔炼炉熔炼过程的数值模拟。依据优化原则,获得了熔炼时间随影响因子的变化规律：熔炼时间随着旋流数、燃烧器倾角、空气预热温度或天然气流量的增加而缩短;熔炼时间随着燃烧器间水平夹角或空燃比的延长,先减小而后增加：熔炼时间随着燃烧器高度的增加而延长。     

Study of the operational parameters of plasma transferred arc melting and modelling of the process

Abstract

Plasma transferred arc (PTA) melting/alloying technique can be successfully used in various surface modification applications and has the potential for industrial use, replacing other more expensive techniques such as laser or electron beam. Such applications require tight control of the process parameters. There is a restricted field of PTA parameter values, where PTA melting can be successfully performed with smooth and unoxidised surfaces. The heat source efficiency of PTA treatment is strongly affected by welding current, arc length and travel speed of the arc and it takes values between 0·40 and 0·55, while melting efficiency takes values between 0·05 and 0·25, affected by the same parameters as arc efficiency. The microstructure and hardness of the melting zone of a PTA pass is associated with heat input and arc length.     

激光选区熔化致密度预测模型建立与工艺验证

致密度是评价增材制造成型件性能的重要参数之一,而高致密度成型件对应的工艺参数需要大量实验研究获得。针对该问题,本文建立了基于工艺参数的熔池致密度预测模型,能够有效地预测熔合不良引起的孔隙,进而为激光选区熔化工艺参数的选择和优化提供更多的参考。模型采用有限元分析和数值计算软件获得熔池尺寸以及模拟多层多道次的熔池拼接形貌,并预测工艺参数对应的致密度。同时在熔池预测模型中引入了波动系数、偏转角度以及层间扫描转角,以此兼顾熔池尺寸波动、熔池倾斜以及层间扫描转角对熔池模拟结果的影响。最终通过HR-2不锈钢选区激光熔化实验对预测模型的可行性进行验证。结果表明,模型预测结果与成型件熔池实验结果吻合较好,致密度预测结果与实测偏差在2%以内。