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.

Design and Verification of an Auxiliary System for High Vacuum Die Casting

Vacuum die casting is the optimal method to produce high quality aluminum alloy components.At present,there are still very few systematic studies on vacuum die casting theory and equipment design.On the basis of the existing theories of the vacuum die casting pumping and venting systems,a simplified model is established in this research.The model has an aggregate unit consisted of ＂vacuum pump ＋ buffer tank＂ and a cylindrical container（including the shot sleeve,cavity and exhaust channel）.The theoretical analysis is carried out between the cavity pressure and the pumping time under different volume models.An auxiliary system for high vacuum die casting is designed based on the above analysis.This system is composed of a vacuum control machine and a new vacuum stop valve.The machine has a human-computer control mode with ＂programmable logic controller（PLC） ＋ touch screen＂ and a real-time monitoring function of vacuum degree for buffer tank and die cavity.The vacuum stop valve with the ＂compressed gas ＋ piston rod ＋ labyrinth groove＂ structure can realize the function of whole-process vacuum venting.The new system shows great advantages on vacuuming the cavity with a much faster speed by making tests on an existing die casting mold and a 250 t die casting machine.A die cavity pressure less than 10 kPa can be reached within 0.8 s in the experiment and the porosity of castings can be greatly decreased.The systematic studies on vacuum die casting theory and equipment have a great guiding significance for high vacuum die casting,and can also be applied to other high vacuum forming in related theoretical and practical research.

A study of metal/die interfacial heat transfer behavior of vacuum die cast pure copper

High pressure die casting copper is used to produce rotors for induction motors to improve efficiency.Experiments were carried out for a special"step-shape"casting with different step thicknesses.Based on the measured temperature inside the die,the interfacial heat transfer coefficient(IHTC)at the metal/die interface during vacuum die casting was evaluated by solving the inverse problem.The IHTC peak value was 4.5×10^3-11×10^3 W·m^-2·K^-1 under the basic operation condition.The influences of casting pressure,fast shot speed,pouring temperature and initial die surface temperature on the IHTC peak values were investigated.Results show that a greater casting pressure and faster shot speed could only increase the IHTC peak values at the location close to the ingate.An increase of pouring temperature and/or initial die surface temperature significantly increases the IHTC peak values.

Synergistic effects of composition and heat treatment on microstructure and properties of vacuum die cast Al-Si-Mg-Mn alloys

The purpose of this study was to prepare high-quality Al-Si-Mg-Mn alloy with a good combination of strength and ductility employing the vacuum-assisted high-pressure die cast process. An orthogonal study of heat treatments was conducted to design an optimized T6 heat treatment process for both Al-10%Si-0.3%Mg-Mn and Al-11%Si-0.6%Mg-Mn alloys. The results demonstrate that no obvious blisters and warpage were observed in these two alloys with solid solution treatment. After the optimal T6 heat treatment of 530°C×3 h + 165°C×6 h, Al-11%Si-0.6%Mg-Mn alloy has better mechanical properties, of which tensile strength, yield strength and elongation reached 377.3 MPa, 307.8 MPa and 9%, respectively. The improvement of mechanical properties can be attributed to the high density of needle-like β″(Mg_5Si_6) precipitation after aging treatment and the fine and spherical eutectic Si particles uniformly distributed in the α-Al matrix.

Vacuum assisted high-pressure die casting of AZ91D magnesium alloy at different slow shot speeds

The effects of vacuum assistance on the microstructure and mechanical properties of high-pressure die cast AZ91 D alloy at different slow shot speeds were evaluated. Plate-shaped castings of AZ91 D alloy were carried out on a TOYO BD-350V5 cold chamber die casting machine incorporated with a self-improved TOYO vacuum system. It was found that the vacuum pressure in the die cavity at the beginning of mold filling increases with the increase of slow shot speed, following a cubic polynomial curve, resulting in a decline in the porosity-reduction ability of vacuum assistance with the increase of slow shot speed. The externally solidified crystal（ESC） contents in conventional and vacuum die castings behave similar against the slow shot speed. The tensile properties of vacuum die castings were strongly influenced by the ESC content at relative low slow shot speeds. With the increase of slow shot speed, the influence of the gas porosity level in vacuum die castings would get prominent.

Characterization of A390 aluminum alloy produced at different slow shot speeds using vacuum assisted high pressure die casting

The effects of vacuum assistance on the microstructure and mechanical properties of high pressure die cast A390alloy at different slow shot speeds were evaluated.Plate-shaped specimens of hypereutectic A390aluminum alloy were produced on a TOYO BD?350V5cold chamber die casting machine incorporated with a self-improved TOYO vacuum system.According to the results,the vacuum pressure inside the die cavity increased linearly with the increasing slow shot speed at the beginning of mold filling.Meanwhile,tensile properties of vacuum die castings were deteriorated by the porosity content.In addition,the average primary silicon size decreased from23to14μm when the slow shot speed increased from0.05to0.2m/s,which has a binary functional relationship with the slow shot speed.After heat treatment,microstructural morphologies revealed that needle-shaped and thin-flaked eutectic silicon particles became rounded while Al2Cu dissolved intoα(Al)matrix.Furthermore,the fractography revealed that the fracture mechanism has evolved from brittle transgranular fracture to a fracture mode with many dimples after heat treatment.

Effect of vacuum on porosity and mechanical properties of high-pressure die-cast pure copper

Pure copper tensile bars were produced by conventional die casting(HPDC) and vacuum-assist die casting(VADC) processes. Porosity and mechanical properties were investigated by using optical microscopy(OM), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), X-ray computed tomography(XCT) and tensile tester. Results show that porosities including gas porosity and shrinkage porosity could be observed in copper castings. Since the application of vacuum could reduce filling related gas entrapment and facilitate solidification due to the increased heat transfer between metal and die, both number and size of the entrapped gases, as well as shrinkage porosities were significantly reduced in vacuum-assist die castings of pure copper. The porosity fraction decreased from 2.243% to 0.875% compared with that of the conventional die casting. Besides, mechanical properties were improved significantly, i.e., by 15% for ultimate tensile strength and three times for elongation.

Effect of Injection Velocity on Structure Part Characteristic in A50 Die Casting Process with High Vacuum System

When diecasting large and thin Mg alloy parts,material defects occur,which include porosity,nonuniform mechanical properties,irregular surfaces,and incomplete filling.To resolve these problems,it is necessary to have uniform injection velocities and temperatures as well as control the melt.This study investigated the feasibility of producing large and thin components using a die caster by attaching a high vacuum system.In particular,the effects of injection velocity on surface quality and the mechanical properties of the products were investigated.Hence,an injection velocity scheme and a die structure capable of casting in a vacuum were proposed.As a result,it was found that the critical low injection velocity was 0.2 m/s to produce large thin Mg alloy structures having good mechanical properties.

Effect of different processing parameters on interfacial heat-transfer behavior in high-pressure die-casting process

Vacuum die casting can reduce the'air entrapment'phenomenon during casting process.Based on the temperature measurements at metal-die interface with different processing parameters,such as slow shot speed(VL),high shot speed(VH),pouring temperature(Tp)and initial die temperature(Tm),inverse method was developed to determine the interfacial heat transfer coefficient(IHTC).The results indicate that a closer contact between the casting and die could be achieved when the vacuum system is used.It is found that the vacuum could strongly increase the values of IHTC and decrease the grain size in castings.The IHTC could have a higher peak value with increasing the Tp from680to720℃or the VL from0.1to0.4m/s.In addition,the influence of the VH and Tm on IHTC could be negligible.

High-tenacity in-situ Ti/Zr-based bulk metallic glasses composites fabricated by industrial high-pressure die casting

The glass-forming ability and mechanical properties of metallic glasses and their composites are well known to be sensitive to the preparation conditions and are highly deteriorated by industrial preparing conditions such as low-purity raw materials and low vacuum.Here,we showed that a series of in-situ bulk metallic glass composites(BMGCs)which exhibit excellent ductility and segmental work hardening were successfully developed utilizing a high vacuum high-pressure die casting(HV-HPDC)technology along with industrial-grade raw materials.The tensile properties of these BMGCs are systematically investigated and correlated with the alloy microstructure.As compared with the copper mold suction casting method,the volume fraction difference of the dendrite phase for the BMGCs with the same composition is not significant when fabricated by the HV-HPDC,whereas the size of theβ-phase is generally larger.Insitu BMGCs with the composition of Ti_(48)Zr_(20)(V_(12/17)Cu_(5/17))19 Be 13 obtained by the HV-HPDC process show ductility up to 11.3%under tension at room temperature and exhibit a certain amount of work hardening.Two conditions need to be met to enable the BMGCs,which are prepared by vacuum die-casting to retain favorable ductility:(1)The volume fraction ofβphase stays below 62%±2%;(2)The equiaxed crystals with a more uniform size in the range of 5-10μm.Meanwhile,the results of the present study provided guidance for developing BMGCs with good ductile properties under industrial conditions.

基于真空压铸技术的机壳压铸模设计

通过对机壳压铸件的结构工艺分析,介绍了真空压铸技术下模具的浇注系统、成型零件、侧向分型与抽芯机构、推出机构、溢流与排气系统、温度控制系统等机构的设计要点,相比普通压铸的模具,能有效减低压铸原材料的损耗,提升压铸件质量,为类似模具的设计提供一定的参考借鉴。

Cu含量和热处理工艺对Al-Si-Mg-Mn-xCu铸造铝合金显微组织和力学性能的影响

采用三维X射线显微镜、光学显微镜、扫描电子显微镜、透射电子显微镜及硬度测试系统研究Cu含量及热处理工艺对真空压铸Al-Si-Mg-Mn-xCu合金显微组织和力学性能的影响。研究发现,虽然Cu含量增加会提高铸锭中气孔的密度和尺寸,但是Cu添加将促进凝固过程中含Cu初生相(Q-Al_(5)Cu_(2)Mg_(8)Si_(6)和θ-Al_(2)Cu)的形成,从而提高合金性能。合金中形成5种不同结构的初生相,包括共晶Si、α-Al(Fe,Mn)Si、β-Mg_(2)Si、Q-Al_(5)Cu_(2)Mg_(8)Si_(6)和θ-Al_(2)Cu相。随着Cu含量增加,θ相的面积分数迅速增加,α-Al(Fe,Mn)Si相面积分数首先降低,随后缓慢增加,而Q相的变化趋势与α-Al(Fe,Mn)Si相相反。这些初生相在热处理过程中会出现不同的演变规律。在随后的时效处理过程中,Q'和θ'相的协同析出能显著提高合金的时效硬化潜力。

热处理对高真空压铸AlSi10MnMg合金拉伸性能的影响

分别对高真空压铸AlSi10MnMg合金进行T5(180℃×2 h)、T6(515℃×1 h空冷+175℃×2 h)和T7(460℃×1 h水冷+175℃×2.5 h)热处理,研究了不同热处理条件下该合金的显微组织和拉伸性能。结果表明:经过T5热处理后的铸态试验合金铝基体相长大,圆整度较高,晶粒尺寸主要集中在20~80μm,硅相仍呈细长状,与铸态试验合金相比,试验合金的抗拉强度提高了8.9%,断后伸长率降低了9.1%;经过T6和T7热处理后,铝基体相形状不规则程度降低,晶粒尺寸分别为2~50,2~30μm,组织更均匀致密,硅相由铸态时的细长状演变为规则的圆形,试验合金的抗拉强度分别降低了26.0%和23.9%,断后伸长率分别提高了54.5%和72.7%;铸态和T5态试验合金的拉伸断裂方式均为韧脆混合断裂,T6和T7态试验合金的断裂方式则以韧性断裂为主,其中T7态试验合金拉伸断口中的韧窝细密且深,塑性更好。

车身真空高压压铸技术研究

铝合金真空高压压铸技术在汽车行业逐步得到应用,研究分析了真空高压压铸材料、工艺、产品及现阶段重点问题。概括了热处理和免热处理材料的物理特点、化学成分及机械性能;介绍了真空高压压铸工艺流程和产线、工艺设计、工艺参数、成形性能;分析了产品性能、厚度、尺寸、连接、维修、模具寿命现阶段的主要问题;总结了真空高压压铸技术的优势并展望其发展前景。

乘用车车身大型压铸件设计与研究

为实现乘用车车身大型压铸件设计,从压铸工艺种类和特点出发,分析了高真空压铸在乘用车车身大型压铸件中的应用,并针对关键接头、重要梁系、主要加强件、减振器塔座、A柱模块、后地板总成、前舱总成等常见压铸件应用部位,从壁厚、加强筋和顶针布局方面分析了压铸件的设计原则和要求。

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%。

高真空压铸AlSi10MgMn合金淬火敏感性

通过分级淬火法测定了高真空压铸AlSi10MgMn合金的时间-温度-转化率(TTT)和时间-温度-性能(TTP)曲线,利用淬火因子分析法(QFA)预测了合金的力学性能,验证了该方法的准确性;采用透射电子显微镜(TEM)研究了等温淬火和时效过程中合金微观组织的演变规律。结果表明:合金TTT和TTP曲线的鼻尖温度约为370℃,淬火敏感区间为295~445℃;根据淬火因子分析法,合金的临界冷却速率为7.7℃/s,但在冷却速率较小时,该方法误差较大;合金在等温过程中产生了棒状β-Mg_(2)Si相,且随着等温时间的延长,β-Mg_(2)Si尺寸变大且数量增多,导致时效后得到的纳米相β?相减少,无析出带(PFZ)范围增大。

一体化压铸工艺在车身减震塔上的开发与应用

介绍了通过轻量化结构设计方式,将钢板冲焊结构减震塔设计成一个压铸件,重量由11.24 kg降到5.06 kg,实现减重55%。并采用高真空压铸工艺完成样件试制,材质选用AlSi10MgMn,尺寸范围540 mm×510 mm×300 mm,平均壁厚为3.5 mm,最小壁厚达2.5 mm,最大壁厚达5 mm。通过采用高真空压铸工艺、T7热处理等工艺,实现产品性能要求,本体取样检测力学性能满足屈服强度R_(p0.2)≥120 MPa,抗拉强度R_(m)≥180 MPa,伸长率A_(50 mm)≥10%。

某汽车铝合金减震塔真空压铸工艺设计与优化

针对某汽车铝合金减震塔铸件的结构特征,设计了真空压铸浇注系统,并数值模拟了其真空压铸过程。研究了浇注温度、压射速度、模具温度和真空度对铸件质量的影响规律,设计正交实验得到最优的工艺参数(浇注温度720℃,压射速度2.5 m/s,模具温度230℃,真空度25 kPa)。针对铸件壁厚较大区域存在的缺陷,设计了补缩冒口,有效减少铸件的缩松缩孔缺陷,获得了品质良好的铸件。

A356铝合金耐张线夹真空压铸工艺及应用

为提高输变电线路金具的极端气候条件下的服役性能,在不改变现有架空输电线路耐张金具产品结构的条件下,合理选择高强韧铝合金材料以及研究压铸工艺的优化。首先,提出采用材料的强塑积来评价输变电金具抗冲击和长期失效的材料稳定性,以确定材料选择;其次,通过有限元模拟铸造成形过程,优化模具的进料口和溢出口的位置,避免成形过程中的气体裹挟和输送缺陷,以提高产品的材料强韧性和零件的抗疲劳失效特性;然后,在现有的压铸机上增加真空设备及真空阀辅助系统和模具结构,实现真空压铸工艺;最后,优选采用水基氮化硼环保脱模剂,解决A356材料的脱模困难,提高模具润滑性和表面质量。用新材料和新工艺生产的耐张线夹真空压铸成形零件的抗拉强度不小于250 MPa,延伸率不小于7%,为提高现有的架空输电线路金具产品的运行可靠性奠定了基础。