双螺杆制备半固态坯料装置的设计

双螺杆制备半固态坯料的思想来之于注塑行业挤出机的原理,与其不同之处在于半固态坯料制备中,更为注重的是其剪切和混合效果。本文基于以上思想,从双螺杆的设计起步,确定了双螺杆和与之对应的捏合块规格和参数,并对螺杆工作产生的轴向力进行了计算,选择了相应的止推轴承。此外,还估算了半固态坯料制备生产能力,解决了轴承径向尺寸和双螺杆中心距之间的矛盾。     

压铸镁合金缺陷带的组织特征及形成机理

研究了压铸镁合金缺陷带的组织形貌及分布特征,建立缺陷带与压室预结晶(ESCs)、气缩孔及压铸工艺参数之间的对应关系,在此基础上探讨了缺陷带的形成及演化机理。结果表明,压铸镁合金截面凝固组织以缺陷带为界可划分为3个有明显组织特征差异的区域。随着压铸低速速度提高,镁合金凝固组织中ESCs含量逐渐降低,而缺陷带组织愈加明显并向压铸件中心靠拢,缺陷带宽度减小,其内部孔洞更加聚集;高速速度越大,压铸镁合金凝固组织中ESCs越分散,缺陷带向压铸件中心靠拢,压铸在无高速速度情况下,镁合金凝固组织横截面出现双缺陷带现象。压铸充型过程中金属液流的形态决定了缺陷带的分布位置和发展趋势,在高速金属液的剧烈冲刷及增压压力的作用下,靠近金属液流外轮廓的晶粒发生破碎或转动,在晶粒间形成大于剩余金属液体积的间隙,随着凝固的进行,形成沿液流轮廓分布孔洞聚集的缺陷带组织。     

钢塑共挤模具过渡拖动段成型分析与模具结构优化设计

建立了钢塑共挤模具的三维模型,通过在Gambit中进行有限元前处理,导入Polyflow,对过渡拖动段成型的基本规律进行了数值模拟,获得了聚合物熔体在模具内的速度场、温度场分布,在特定流量下,分析了不同钢衬拖动速度对聚合物流动的影响规律,并在此基础上设计了钢塑共挤模具冷却系统。     

平原泵闸工程消力池CFD水力优化

在分析了表示泵站过水流道断面流速分布均匀性公式不足的基础上,结合河道过流特点,建立了断面有效过流面积率和过流均匀度两个表示过流断面水流流态的公式,并以此作为优化目标函数,利用三维稳态数值模拟方法对某单侧布置的泵闸合建工程消力池和导流翼墙进行了水力优化,经模型试验验证了数模结论的正确性.针对工程的具体情况,得出的结论是:过度增加消力池长度对消能效果产生负作用,可以采取消力池内设置辅助消能工,缩短消力池翼墙的措施改善消能效果;对河床采取抛石防护措施,解决海漫段末端最大水流速大于河床抗冲流速的问题.     

Derivation of current density distribution by arc pressure measurement in GTA welding

Abstract

Arc pressure is an important factor in understanding physical arc phenomena, especially its effects on the penetration, size and shape of GTA welding. The purpose of the present study is to derive the current density distribution on the anode base metal from the measured arc pressure distribution in GTA welding using the results from previous investigators. Using the measured arc pressure distribution on the anode base metal in GTA welding from the central part to the circumference and the equations of Lin et al. and Maecker, the current density distribution was derived. Applying the derived equation from the present work, the current density distribution was calculated from the low current region to the high current region by means of shield gas mixing ratio. It is compared and discussed with the practical welding current and the derived current by numerical integration of the current density distribution from the central part to the circumference region.     

The non-hydrostatic response of polymer melts as a pressure medium in sheet metal forming

The polymer injection forming process is a recent invention for producing plastic?Cmetal hybrids. It is a combination of injection molding and sheet metal hydroforming process in which polymer melt serves as a pressure medium. This paper presents the experimental investigations on the non-Newtonian nature of thermoplastic melt as pressure medium. The objective of this work is to identify the presence of non-hydrostatic pressure distribution within the cavity and its influence on the final shape of the formed sheet metal component. Experiments are conducted with center-gated injection mold under varying processing conditions. The development of localized cavity pressure during the process is recorded and evaluated against the final shape of formed sheet metal. It has been observed that higher injection rate, higher injection temperature, and higher melt flow index of the processed polymer is necessary for the uniform pressure distribution and subsequently uniform forming of the sheet metal.     

振动场作用下聚合物熔体在L型挤出口模内的流动分析

利用ANSYS软件对受口模入口处周期性振动的压力驱动的非牛顿幂律聚合物熔体在“L”形异型材挤出口模内的流动过程进行了三维等温数值模拟,以曲线图、等值线图和等值面图的形式展示了不同条件下的反映流动全貌的速度场、压力场和应力场。结果表明:由于振动场的引入,熔体速度、压力、应力均产生振动,其频率和入口压力的振动频率相同,但振幅从口模入口至出口逐渐衰减,直至消失;压力从口模入口至出口近似线性递减,但在入口附近有一个急剧下降而后又短暂上升的过程,这是因振动的引入而产生的特殊现象。     

圆形微通道结构多相流流动形态试验及数值模拟研究北大核心

利用高速摄像观测系统对不同挡板结构的圆形微通道结构进行全流场流动显示实验。通过对重力影响、气液流量比、流动方向等变量的控制进行多相流流动形态分析;在此基础上进行数值模拟研究,得到气液多相流流动特性沿流道的压力、速度场等流动特性。结果表明:微通道复杂结构会显著影响气液多相流的流动形态,复杂结构微通道加剧了流体剪切流动特性,大大促进了气液两相流混合效果。     

Semisolid microstructure evolution of AZ91D alloy under high shear rate

Experimental work was undertaken to investigate the effect of rotation speed on the solidification microstructure of AZ91D alloy using a twin-screw slurry maker.Then numerical simulation was used to investigate the effect of rotation speed on the flow filed and temperature field of the melt.It was found that increasing the rotation speed could give rise to substantial grain refinement,which can be attributed to the increase of effective nucleation rate caused by the extremely uniform temperature and dispersing any potential clusters of nucleation agents in the bulk liquid under dispersive mixing.     

Fracture types and thickness distribution in superplastic sheets formed with plastic injection molding

This paper investigates the fracture types and thickness ratio distribution in superplastic Zn/Al sheets formed during a hybrid process combining superplastic sheet forming with plastic injection molding. Three types of sheet fractures, edge cracking, central cracking and combined cracking, are observed. The success of forming superplastic sheets with rib features using injection molding with various parameters, including melt temperature, injection pressure and mold temperature, are investigated. They are presented in the form of a molding area with areas of various fracture types. Central cracks occur when superplastic sheets are formed with injection molding with higher melt temperature, while edge cracks occur with higher injection pressure. When the melt flow is parallel to the sheet rolling direction, the areas of edge crack are enlarged. The sheet thickness ratio distribution is obtained for various injection parameters and rib depths. Observation of the sheet thickness distribution for variation parameters, the tendency of the sheet fracture type can be generalized.     

基于投影法求解三维不可压缩粘性方程的新算法

提出一种新的基于投影法的隐式有限差分算法,用于计算速度场和压力。这种方法的特点是将计算区域根据雷诺数分成几个区域;对于远离壁面的区域,热金属流按牛顿流计算;对于贴近壁面的区域,热金属流按非牛顿流计算。通过非参数统计和实验的方法证明新算法的正确性。数值模拟结果表明,新算法的计算速度要快于基于SOLA-VOF法的显式有限差分方法。     

Effects of harmonic electric field frequency on electromagnetic and flow fields of Ti-Al melt in a rectangular mold based on numerical simulation

A model was established based on Maxwell's equations and Navier-Stokes' equations to numerically simulate the electromagnetic field and flow field in a rectangular mold with sectional aspect ratio of 5:1.The FEM (Finite Element Method) and APDL (ANSYS Parametric Design Language) were employed for the model to execute the modeling,meshing,load applying and solving.The Ti-Al alloy melt was selected to illustrate and validate the effects of the harmonic field frequency on the distribution of the physical fields in the mold.The simulated results demonstrate that with an increasing frequency the electric current forms an ellipsoid cavity where it becomes much weaker,and that the melt flows more intensely with low frequency (less than 5 kHz) than with high frequency (more than 5 kHz).The melt is pinched from the central part in the mold to bipolar parts in which it forms two vortexes in each side.The maximum value of fluid velocity exists near the bipolar zone.     

The Effect of PA6 Content on the Mechanical and Tribological Properties of PA6 Reinforced PTFE Composites

Polytetrafluoroethylene (PTFE)/PA6 blends of different compositions, where PTFE acts as the polymer matrix and PA6 as the dispersed phase, were produced in a corotating twin-screw extruder. The effects of PA6 content on mechanical and tribological properties of the composites were investigated. The worn surface morphologies of neat PTFE and its composites were examined by scanning electron microscopy (SEM) and the wear mechanisms were discussed. The presence of PA6 particles dispersed in the PTFE continuous phase exhibited superior tribological characteristics to unfilled PTFE. The optimum value of wear reduction was obtained when the content of PA6 is 30 vol.%.     

Thermal decomposition kinetics of in situ reinforcing composite based on polypropylene and liquid crystalline polymer

A thermotropic liquid crystalline polymer (TLCP), a copolyester with a 80/20 molar ratio of p-hydroxy benzoic acid (HBA) and poly(ethylene terephthalate (PET), known as Rodrun LC5000, was melt blended with polypropylene (PP) using a twin-screw extruder. The thermal degradation of the blends containing various TLCP contents was investigated using thermogravimetry (TG). In nitrogen and in air, the TG profiles of PP revealed a single weight-loss step, whereas TLCP showed two and three major weight-loss steps in nitrogen and in air, respectively. In air, more complex TG curves were observed for the blends with TLCP loading. Simultaneous DSC thermograms revealed that the degradation processes for all neat polymers and the blends were endothermic in nitrogen and exothermic in air. The kinetic parameters (E_a, ln A, and n) calculated using isoconversional method indicated that the thermal stability of PP was significantly improved by in situ reinforcing with TLCP. Addition of SEBS-g-MA compatibilizer slightly enhanced the thermal resistance of blend.     

超声及电磁场对铝合金熔体影响的数值模拟及试验研究

针对铝合金熔体的超声及电磁搅拌处理过程,创建了超声场、电磁场、流场和温度场的数学模型,并采用COMSOL Multiphysics软件求解模型,得到了超声场、电磁场和超声/电磁复合场对铝合金熔体内流场和温度场的分布规律,并通过试验对模拟结果进行了验证。结果表明,增加超声功率使得声压幅值和流场的流动速率均随之增加,同时会使熔体的温度梯度逐渐减小;施加电磁场后产生的电磁搅拌改变了熔体的流动方向且使熔体的流动速率显著增大,这对熔体的温度分布产生了较大影响;在超声和电磁复合场下,电磁场比超声场对熔体流场和温度场的影响更大,且占主导地位。数值模拟结果和试验结果符合较好。     

Effects of process parameters on morphology and distribution of externally solidified crystals in microstructure of magnesium alloy die castings

During the cold-chamber high pressure die casting(HPDC) process, samples were produced to investigate the microstructure characteristics of AM60B magnesium alloy. Special attention was paid to the effects of process parameters on the morphology and distribution of externally solidified crystals(ESCs) in the microstructure of magnesium alloy die castings, such as slow shot phase plunger velocity, delay time of pouring and fast shot phase plunger velocity. On the basis of metallographic observation and quantitative statistics, it is concluded that a lower slow shot phase plunger velocity and a longer delay time of pouring both lead to an increment of the size and percentage of the ESCs, due to the fact that a longer holding time of the melt in the shot sleeve will cause a more severe loss of the superheat. The impingement of the melt flow on the ESCs is more intensive with a higher fast shot phase plunger velocity, in such case the ESCs reveal a more granular and roundish morphology and are dispersed throughout the cross section of the castings. Based on analysis of the filling and solidification processes of the melt during the HPDC process, reasonable explanations were proposed in terms of the nucleation, growth, remelting and fragmentation of the ESCs to interpret the effects of process parameters on the morphology and distribution of the ESCs in the microstructure of magnesium alloy die castings.     

混凝土搅拌运输车搅拌罐内部流场的CFD分析

搅拌叶片是搅拌设备的主要部件,叶片周围的流动是相当复杂的三维、非定常、不可压缩粘性流动,搅拌罐内流体的流动状态直接影响搅拌叶片的性能.为了进一步获取搅拌罐内部三维流场的其它参数,为搅拌叶片的设计提供理论依据,以有限元法为依据,对搅拌罐内部流场的速度、压力进行CFD分析,初步得出了搅拌罐内部流场主要特征和分布规律.     

气辅共注射成型充填过程数值模拟中的移动界面追踪技术

气辅共注射成型工艺的充填过程中存在多个移动界面。为能对充填过程进行有效的模拟,针对凝固层界面、表层熔体前沿、内层熔体前沿和气体穿透前沿推进的特点,采用不同的方法实现其追踪。模拟结果与实验结果比较表明,该方法简单有效,能够较准确地预测内层熔体前沿的位置,而要更准确地预测气体穿透前沿的位置,还要进一步考虑注气延迟时间和保压冷却阶段气体在熔体中的“二次穿透”。     

A357铝合金半固态流变压铸数值模拟

利用商业模拟软件Flow-3D对A357铝合金半固态流变压铸过程进行了研究。对不同压射压力和不同压射速度下的半固态流变充型流动状态及气孔分布进行了模拟,并对其凝固过程进行了模拟,最后通过试验对其进行验证。模拟及试验结果显示,随着压射压力与压射速度的增大,半固态熔体流动状态从层流向紊流过渡;同时压射压力和压射速度的增大对气孔分布也有一定的影响。模拟结果与试验结果基本保持一致,表明利用Flow-3D对压铸工艺进行参数优化是可行的,实现了压铸件性能提高的目的。     

双吸平衡式双螺杆压缩机流致噪声分析

双吸平衡式双螺杆压缩机采用对称结构,能够大幅降低机械噪声,从而流致噪声成为主要的噪声。为了研究双吸平衡式双螺杆压缩机内部流体噪声的具体声压级分布,提出利用CFX计算得到双吸平衡式螺杆压缩机的瞬态流场,再将结果进行离散傅里叶变换,运用Actran计算流致噪声。分析计算结果发现:压缩机的噪声频率声压级与压缩机的排气频率及其倍频相关;压力脉动变化率越大声压级越大;转子区域的声压级大于排气区域,排气区域大于进气区域;不同于其他压缩机,双螺杆压缩机的高频声压级仍然较大。