Effects of shot speed and biscuit thickness on externaly solidified crystals of high-pressure diet cast AM60B magnesium alloy

Standard mechanical test bars with a diameter of 6.4 mm and a gauge length of 50 mm were processed, and the microstructures of die cast AM60B alloy under different die casting process parameters were observed. The influences of the slow shot speed, the fast shot speed and the biscuit thickness on the externally solidified crystals （ESCs） were investigated. With the increase of the biscuit thickness, the number of the ESCs in the cast samples decreases. Under a low slow shot speed, larg ESCs are found in the cast structure and a high fast shot speed results in more spherical ESCs. The relationships between ESCs and process parameters were also discussed.

Effect of multi-step slow shot speed on microstructure of vacuum die cast AZ91D magnesium alloy

Two multi-step （two-step and three-step） slow shot speeds were used in the vacuum die casting process of AZ91D magnesium alloy. The vacuum pressure variation in the die cavity before mold filling was monitored by using a pressure sensor. The microstructures of the produced castings were analyzed with optical microscope and image analysis software. The experimental results demonstrate that, the vacuum pressure in the die cavity at the beginning of mold filling is significantly reduced by using three-step slow shot speed, resulting in a low gas porosity level in the produced castings. At an appropriate multi-step slow shot speed, the dwell time of the liquid metal in the shot sleeve before mold filling can be reduced and the flow of the liquid metal in the shot sleeve at the later stage of the slow shot process can be restrained, which cause a low externally solidified crystal content in the produced castings.

Characterization of the Grain Structures in Vacuum-Assist HighPressure Die Casting AM60B Alloy

The structures in vacuum-assist high-pressure die casting （HPDC） AM60B alloy were studied by using an optical microscope and a scanning electron microscope with an energy dispersive spectrometer. It was found that the HPDC under the vacuum could significantly change the morphology and distribution of the microstructure. For both conventional and vacuum-assist HPDC processes, the externally solidified crystals （ESCs） tended to aggregate in the center along the thickness direction of the castings. Besides, the aggregation was more pronounced, and the number of ESCs decreased, and the ESCs tended to become smaller and more globular, as the distance between the specimen location and runner increased. Compared with the conventional castings, the vacuum-assist HPDC can significantly reduce the size and amount of ESCs, and the ESCs tended to be more globular. For the distribution of ESCs along the thickness of the specimens, the aggregation tendency was more pronounced in vacuum-assist die castings than that in conventional castings. Besides, the distribution of ESCs at different locations was more converged in the vacuum-assist HPDC than that in the conventional HPDC.

高温高应变率下AM60B镁合金力学性能及失效行为

为了研究镁合金在高温、高应变速率下的变形行为及失效机制，采用分离式Hopkinson压杆在应变速率为1600—4500S^-1、温度为27—150℃范围内，对真空压铸AM60B镁合金进行了动态压缩实验，并采用金相显微镜和扫描电子显微镜对压缩后的组织进行了观察．结果表明：在所测试的应变范围内，随着应变率的提高，应力-应变曲线均呈现上升趋势，且最大应变也随之增加，表现出正应变率强化效应．在150℃时真空压铸AM60B镁合金变形能力最好；50℃时断裂强度最高．真空压铸AM60B镁合金在高温及高应变率下的断裂方式为以解理断裂为主并伴有韧性断裂的混合断裂方式．当变形温度低于150℃时，真空压铸AM60B镁合金在高应变率压缩下的变形机制主要是滑移．

Correlation between Porosity and Fracture Mechanism in High Pressure Die Casting of AM60B Alloy

X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiation, propagation and subsequent fracture of specimen. Results showed that four types of porosi- ties, including gas-shrinkage pore, gas pore, net-shrinkage and island-shrinkage, could be identified according to the formation mechanism and morphology characterization. During tensile deformation, it was shown that the gas-shrinkage pore and net-shrinkage, rather than gas pore or island-shrinkage, were the main sources for crack initiation. In addition, the crack propagated by interconnecting the po- rosities at the cross section with minimum efficient force bearing area. At these locations where externally solidified crystals （ESCs） were present, the crack would propagate along the ESC boundaries in an inter- granular mode, while at these locations without ESCs, the crack would propagate roughly along the direction perpendicular to the tensile stress in a combination of trans-granular and inter-granular modes.

Microstructure Characteristics of the Eutectics of Die Cast AM60B Magnesium Alloy

Under the cold-chamber high pressure die casting （HPDC） process, samples were produced with AM60B magnesium alloy to investigate the microstructure characteristics of the eutectics, especially focusing on the constitution, morphology and distribution of the eutectics over cross section of the castings. Attentions were also paid to study the effect of heat treatment on the eutectics in the die castings. Based on experimental analysis using optical microscopy （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy-dispersive spectroscopy （EDS）, it was determined that fully divorced eutectics consisting of c^-Mg and l%MglzA112 appeared at the grain boundary of the primary c^-Mg in the as-cast microstructure. Islands and networks of β-Mg17Al12 phase were observed in the central region of the castings, while the β-Mg17Al12 phase revealed a more dispersed and granular morphology on the surface layer. The two phases ratio β/α in the central region of the castings was approximately 10%, which was higher than that on the surface layer. Besides, the defect bands contained a higher percentage of the eutectics than the adjacent regions. After aging treatment （T6）, only α-Mg phase was detected by XRD in the AM60B magnesium alloy, though a small amount of precipitated β-MgITAI12 phase was observed at the grain boundary. In contrast to the microstructure of die cast AZ91D magnesium alloy under the same T0 heat treatment, no discontinuous precipitation of the β-MgITAI12 phase was observed in AMO0B magnesium alloy die castings.

镁合金压铸真空抽气系统的设计与应用研究

设计了一种含有主、副排气通道的真空抽气系统,主排气通道较副排气通道横截面大,由真空阀控制其开闭,副排气通道采用横截面较小的齿状排气通道,在真空压铸过程中保持全程抽气。研究表明:采用主、副排气通道的真空抽气系统,能够获得外观完整、组织致密的镁合金真空压铸件,气孔率比齿状排气通道压铸件减少48%,T6热处理后气泡减少60%,铸件的抗拉强度、屈服强度和伸长率分别提高9.3%、10.2%和21.4%。

真空压铸工艺参数对AM50镁合金力学性能的影响规律

采用阶梯试验模具及AM50合金,进行了系统的真空压铸试验,实测了不同厚度的阶梯试样在不同工艺条件下的密度及力学性能,研究了高真空压铸工艺参数对AM50镁合金力学性能的影响规律。结果表明,随着型腔真空压力的降低,铸件密度、抗拉强度和伸长率均随之提高;铸造压力对力学性能的影响在真空压铸和常规压铸中遵循基本相同的规律,即增大铸造压力可以使铸件的致密程度、抗拉强度、屈服强度和伸长率得到提高;随着高速速度的增大,薄壁铸件的抗拉强度、屈服强度和伸长率均表现出明显的增加,这一点与常规压铸的规律相反。结合高真空和高速工艺,可以使薄壁铸件的抗拉强度和伸长率得到较为明显的提升。

基于数值模拟的镁合金真空压铸浇注系统设计与优化

设计出两种类型的浇口及浇注系统,运用有限元模拟软件对两种设计进行模拟,观察液态金属充型及凝固过程中流场和温度场的分布。根据凝固规律有效预测铸件中可能存在的缩孔及气孔缺陷的分布与尺寸,找出优化的浇注系统设计。结果表明:在浇注温度655℃、模具初始温度200℃、冲头压射速度3 m/s、真空度30 kPa情况下,具有阶梯分型面结构的浇注系统优于平直分型面结构;同时在优化设计基础上生产出具有致密微观结构的镁合金零件。

AM50镁合金真空压铸件孔洞分布与力学性能的关系

孔洞缺陷是压铸件力学性能预测中的重要因素。工艺条件在影响金属液充型凝固过程的同时,也对压铸件内部的孔洞尺寸、孔洞数量及分布情况产生影响,并导致力学性能变化。研究压铸件内部孔洞分布的特征及其对力学性能的影响规律,有助于理解压铸件组织和性能的内在联系,亦对研究真空压铸以及常规压铸工艺参数对压铸件力学性能的影响规律有所帮助。结合镁合金压铸件的缺陷带理论,应用超声扫描显微镜采集孔洞缺陷数据,对AM50镁合金真空压铸阶梯试样的孔洞分布进行统计分析,通过孔洞分布特征确定缺陷带的深度位置,研究缺陷带附近的孔隙率与压铸件力学性能的关系。

AZ91D镁合金真空压铸力学性能研究

对AZ91D镁合金的真空压铸工艺进行了实验研究,讨论了型腔真空压力和铸造压力等工艺参数对于铸件密度、抗拉强度、屈服强度和伸长率等力学性能指标的影响规律。研究结果表明:型腔真空压力和铸造压力是影响铸件密度和抗拉强度关键参数。铸造压力增加或真空压力降低时,铸件密度和抗拉强度增加。同时,研究了铸件在T6处理前后的力学性能,型腔真空压力为5kPa条件下的铸件在T6处理后未发生起泡现象,T6处理后试样的抗拉强度和屈服强度提高,伸长率降低。

镁合金减震塔的真空压铸研究

研究并对比了真空压铸生产的AZ91D和AM60合金减震塔铸件的微观组织、力学性能,考察了热处理工艺对其性能的影响。结果表明,AZ91D和AM60真空压铸件组织均由基体α-Mg和Mg17Al12相组成,固溶处理后Mg17Al12相消失,时效处理后其以非连续相形式析出。AZ91D和AM60铸件均能够热处理强化,但是AM60合金的强化效果弱于AZ91D合金。另外,真空压铸能有效消除铸件中的气孔,但无法完全消除缩松现象,不过仍可通过热处理提高真空压铸件的力学性能。

镁合金散热器真空压铸的抽真空技术研究

介绍了一种真空压铸抽真空系统及控制方法。该系统采用了大尺寸半过程排气通道与齿状全过程排气通道相结合的真空排气结构,采用快速响应的电磁阀控制半过程排气通道的关断,通过调节PLC可视化程序控制半过程排气的时间。试验结果表明:采用该抽真空系统可获得充型完整、外观良好的镁合金散热器,充型能力较普通压铸提高10%以上,气孔率减少70%,T6处理后起泡减少80%。铸件抗拉强度、屈服强度和伸长率分别提高17.8%、17.8%和37.9%。

基于数值模拟的镁合金真空压铸真空度对气孔率的影响

采用有限元模拟软件ProCAST对镁合金AZ91D汽车壳体零件进行不同真空度下的真空压铸数值模拟,讨论在其它工艺参数一定的条件下,不同真空度对铸件存在的气孔体积分数的影响。结果表明,在压射速度为2.4 m/s、浇注温度为655℃、模具初始温度为200℃条件下,型腔中气体压强值为30kPa时铸件中存在的气孔率最小。

轧制对真空压铸AM50薄板组织与性能的影响

为研究真空压铸态AM50镁合金薄板在不同轧制变形量下组织和力学性能的变化,利用小型轧机对该合金进行多道次轧制,并进行了微观组织观察和室温拉伸性能测试.结果表明:该镁合金在常温下可进行多道次的轧制,但每道次之间要进行310℃×10 min的退火处理,总变形量可达到45%;随总变形量的增加,轧制流线逐渐形成,晶粒变形成长条状,平均尺寸逐渐变小;变形量为40%时,沿轧制方向晶粒大约为2～4μm,轧制延展方向晶粒大约为5～10μm;随变形量的增大,材料的强度和塑性均在增加,而在总变形量达到40%时,材料的强度和塑性达到极值,屈服强度为282 MPa,抗拉强度为329 MPa,伸长率为8.8%.

T6处理对真空压铸AM50镁合金腐蚀行为的影响

为了研究不同时效工艺对真空压铸AM50镁合金耐蚀性的影响,通过静态失重实验和电化学实验研究了T6处理后AM50镁合金在Na Cl溶液中的腐蚀行为.结果表明,合金经过170和230℃时效处理后腐蚀速率的变化情况基本类似,均为先降低后升高,最后达到一个稳定值.但经过230℃时效处理后合金的耐蚀性远差于经过170℃时效处理后的情况,且达到腐蚀速率最低点所需时间更长.不同时效处理后的合金具有相似的电化学腐蚀行为,腐蚀电位无明显变化规律,而腐蚀电流密度与腐蚀速率的变化趋势近乎一致.腐蚀速率随腐蚀时间的延长而下降,并最终保持一个较为稳定的数值.此外,合金腐蚀速率的理论值和实测值相吻合.

镁合金AZ91D真空压铸的组织与性能

对镁合金AZ91D真空压铸下的组织与性能进行了研究。结果表明，真空压铸可以明显改善AZ91D合金组织中的孔洞分布，。提高铸件的密度。对铸件进行热处理（T4）后，铸件表面气泡相对于普通压铸而言有明显改善，可以热处理。热处理后（T4和T6），由于AZ91D具有固溶强化和时效强化效果，合金的性能相对于压铸态有大幅度的提高。

压铸工艺对汽车用镁合金组织与性能影响

采用不同浇注温度和型腔真空压力进行了AM50Ce汽车镁合金的真空压铸试验,并进行了显微组织和力学性能的测试与分析。结果表明:在650～730℃浇注温度和1～30 kPa型腔真空压力,随浇注温度升高,型腔真空压力减小合金的强度先提高后下降。与650℃浇注相比,型腔真空压力8 KPa690℃浇注时合金抗拉强度增大89MPa(从174到263MPa),平均晶粒尺寸减小16μm(从22到6μm);690℃浇注时,与30 kPa型腔真空压力相比,采用8kPa型腔真空压力时合金抗拉强度增大71MPa(从192到263 MPa),平均晶粒尺寸减小14μm(从20到6μm)。合金的浇注温度和型腔真空压力分别优选为690℃、8 kPa。

热处理工艺对镁合金真空压铸件的组织及性能的影响

研究了热处理工艺对真空压铸件的组织及性能的影响，结果证明，真空压铸镁合金可以通过热处理工艺来提高其力学性能，经固溶处理与时效处理后，铸件可以获得更好的综合力学性能。

镁合金摩托车轮毂压铸技术的开发与研究

运用抽真空和局部补压技术对镁合金摩托车后轮毂(简称镁轮毂)进行压铸。模具采用辋缘切向进浇,中心集渣设计,有效解决了轮毂夹杂与热节缩孔(松)。抽真空系统的设计与应用避免了镁合金出现氧化、卷气问题,使模具型腔真空度可达2.9kPa。局部补压设计解决了轮芯厚大部位凝固时镁液补缩不足产生的缩孔问题。同时对镁轮毂制程中易出现的冷隔、缩孔、开裂、夹渣、变形等缺陷进行了分析,提出了改善方案。实践证明,真空辅助高压铸造加局部补压能获得外轮廓清晰、尺寸合格、符合摩托车轮毂安全性能要求的产品。