压铸镁合金表面晶粒不均匀性的模拟研究

以AM50镁合金为研究对象,通过实验数据建立压铸件表面晶粒形核及生长模型,利用计算机模拟程序分析了不同压铸工艺对镁合金铸件表面不均匀性、中心压室预结晶含量及最终微观组织的影响。     

镁合金冷室压铸组织中压室预结晶组织的研究

镁合金压铸过程中压室预结晶对压铸件最终凝固组织及使用性能具有重要的影响。针对冷室压铸工艺,论述了压铸镁合金凝固组织中压室预结晶组织的研究所取得的成果,包括压室预结晶的组织特征,压室预结晶现象的试验及模拟验证,压铸件不同位置及工艺参数下压室预结晶的形态、分布和含量,以及压室预结晶组织对镁合金压铸件力学性能及断裂失效的影响机制。最后分析了现有冷室压铸镁合金压室预结晶研究所存在的不足并对未来的发展方向进行了展望。     

压铸镁合金AM60B的微观组织特征

镁合金压铸件中的压室预结晶组织及离异共晶组织对铸件最终使用性能有着重要的影响.针对冷室压铸工艺,采用AM60B镁合金研究不同低速速度及高速速度下压铸实验件的凝固组织特征.通过OM、EBSD、XRD、SEM及EDS等实验手段进行分析和定量统计.结果表明:在低速压射阶段,压室中熔体的过热度消失,达到初生相形核条件;在高速充填型腔过程中,压室预结晶组织向铸件中心偏聚,使得其心部含量高于表层;同时,压铸件靠近浇口端压室预结晶组织含量高于远离浇口端;低速速度越小,压铸件中压室预结晶组织含量越高;高速速度越小,压铸件中压室预结晶颗粒越粗大,枝晶形貌越完整.在压铸凝固后期,初生a-Mg晶界处出现离异共晶组织,共晶β-Mg17Al12相在压铸件中心及表层分别呈网状和粒状分布,且压铸件中心及缺陷带位置的共晶组织含量高于表层.     

镁合金压铸过程界面传热行为及凝固组织结构的表征与模拟研究

本文系统介绍了镁合金压铸界面换热行为以及凝固微观组织结构的实验表征及计算模拟方面的研究进展,包括:(1)一种基于换热系数的边界设定模型,由此发现了压铸界面换热系数可以分为初始升高、高值维持、快速下降及低值保持4个阶段;(2)压室预结晶流动分布预测模型,据此得到了压室预结晶组织的主要分布规律及其对镁合金铸件缺陷带形成的影响;(3)考虑压室预结晶组织的压铸镁合金形核模型及生长模型;(4)结合离异共晶形核及生长机制建立的镁合金压铸工艺条件下微观组织演变的数学模型;(5)镁合金枝晶组织的三维形貌和生长取向的研究,发现镁合金枝晶组织呈现十八个分支的形貌特征,分别沿着基面的112ˉ0方向和非基面的112ˉ3方向生长,由此建立了镁合金枝晶各向异性的生长模型,实现了镁合金枝晶组织的三维模拟研究。     

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。     

用Cellular Automaton方法模拟压铸镁合金AM50的微观组织

基于Cellular Automaton(CA)方法的基本原理,建立了压铸镁合金AM50形核和等轴晶粒生长的二维数学物理模型,能够对任意晶向的等轴晶晶粒生长进行模拟。模型耦合了宏观传热与微观组织模拟计算,镁合金AM50阶梯块压铸件的温度场通过热传导反算法计算求得。对阶梯块压铸件不同阶梯表面的微观组织进行了模拟,并与金相试验结果进行了对比,它们在晶粒尺寸上吻合较好。     

镁合金铸造成形过程宏观／微观模拟

通过研究镁合金压铸过程中界面热,采用热传导反算法确定压铸过程的界面换热系数,研究镁合金压铸过程中工艺参数及凝固过程对铸件界面换热系数的影响规律,建立镁合金压铸过程界面换热边界条件的处理模型,以实现镁合金压铸过程中凝固过程的准确预测。通过实验研究镁合金压铸过程中凝固组织,建立了镁合金压铸过程中形核模型。采用CA方法,建立了镁合金枝晶生长模型,以实现镁合金凝固组织的预测。采用相场方法研究了镁合金枝晶生长形貌。     

AZ91D压铸镁合金的三维微观组织模拟

针对工程上应用广泛的AZ91D压铸镁合金,建立了其凝固过程中微观组织演化的数学物理模型．采用改进的三维微观元胞自动机（CA）模型,耦合三维溶质场计算,结合压铸镁合金进行了微观组织模拟．模拟结果再现了在多晶粒同时生长的情况下,初生晶间的溶质扩散、溶质富集直至共晶转变的全部演化过程．应用此模型模拟了实际AZ91D压铸件不同部位的微观组织,模拟结果与金相观察结果符合较好．     

基于角点生长算法的镁合金晶粒组织模拟

建立了一种新的模型来模拟镁合金凝固组织,该模型采用了概率性元胞法(正方形元胞、四邻胞配置).凝固过程中的形核由高斯分布形核模型控制;枝晶的晶向随机定义;枝晶尖端的生长速度由KGT模型计算后拟合成三次多项式;元胞的捕捉和状态转换规则由针对镁合金密排六方特性设计的角点生长算法控制.利用该模型计算了单个及多个晶粒的生长过程,并通过多个晶粒模拟结果与实验结果的对比验证了模型的准确性.     

薄壁镁合金手机后盖压铸工艺的数值模拟

运用专业铸造软件Pro CAST对镁合金AZ91D薄壁手机盖压铸件的充型和凝固过程进行数值模拟分析,以铸件凝固后存在于铸件中的缩孔缩松的总和为标准,研究浇注温度、压射速度和模具温度等工艺参数对压铸件质量的影响。获得较优的压铸工艺参数,为提高镁合金手机盖的压铸质量提供依据。模拟结果表明:手机盖压铸件最小缺陷的压铸工艺参数是:浇注温度650℃,模具温度220℃和压射速度2.5 m/s。依据优化后的参数进行压铸试验,压铸件质量良好。     

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

The structures in vacuum-assist high-pressure die casting(HPDC) AM60 B 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.     

Microstructure and mechanical properties of melt-conditioned high-pressure die-cast Mg-Al-Ca alloy

A new shape casting process, melt-conditioned high-pressure die-casting （MC-HPDC） was developed. In this process, liquid metal was conditioned under intensively forced convection provided by melt conditioning with advanced shear technology （MCAST） unit before being transferred to a conventional cold chamber high-pressure die-casting （HPDC） machine for shape casting. The effect of melt conditioning was investigated, which was carried out both above and below the liquidus of the alloy, on the microstructure and properties of a Mg-Al-Ca alloy （AZ91D＋2%Ca （mass fraction）, named as AZX912）. The results show that many coarse externally-solidified crystals （ESCs） can be observed in the centre of conventional HPDC samples, and hot tearing occurs at the inter-dendritic region because of the lack of feeding. With the melting conditioning, the MC-HPDC samples not only have considerably refined size of ESCs but also have significantly reduced cast defects, thus provide superior mechanical properties to conventional HPDC castings. The solidification behaviour of the alloy under different processing routes was also discussed.     

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.     

Characterization and Modeling Study on Interfacial Heat Transfer Behavior and Solidified Microstructure of Die Cast Magnesium AlloysEI北大核心CSCD

Magnesium alloys are widely used in various fields because of their outstanding properties. High-pressure die casting (HPDC) is one of the primary manufacturing methods of magnesium alloys. During the HPDC process, the solidification manner of casting is highly dependent on the heat transfer behavior at metal-die interface, which directly affects the solidified microstructure evolution, defect distribution and mechanical properties of the cast products. As common solidified microstructures of die cast magnesium alloys, the externally solidified crystals (ESCs), divorced eutectics and primary dendrites have important influences on the final performance of castings. Therefore, investigations on the interfacial heat transfer behavior and the solidified microstructures of magnesium alloys have considerable significance on the optimization of die-casting process and the prediction of casting quality. In this paper, recent research progress on theoretical simulation and experimental characterization of the heat transfer behaviors and the solidified microstructures of die cast magnesium alloys was systematically presented. The contents include: (1) A boundary-condition model developed based on the interfacial heat transfer coefficients (IHTCs), which could precisely simulate the boundary condition at the metal-die interface during solidification process. Accordingly, the IHTCs can be divided into four stages, namely the initial increasing stage, the high value maintaining stage, the fast decreasing stage and the low value maintaining stage. (2) A numerical model developed to simulate and predict the flow patterns of the externally solidified crystals (ESCs) in the shot sleeve during mold filling process, together with discussion on the influence of the ESCs distribution on the defect bands of die cast magnesium alloys. (3) Nucleation and growth models of the primary alpha-Mg phases developed by considering the ESCs in the shot sleeve. (4) Nucleation and growth models of the divorced eutectic phase, which can be used to simulate the microstructure evolution of die cast magnesium alloys. (5) The 3D morphology and orientation selection of magnesium alloy dendrite. It was found that magnesium alloy dendrite exhibits an eighteen-primary branch pattern in 3D, with six growing along < 11(2)over bar0 > in the basal plane and the other twelve along < 11(2)over bar3 > in non-basal planes. Accordingly, an anisotropy growth function was developed and coupled into the phase field model to achieve the 3D simulation of magnesium alloy dendrite.     

Defect band formation in high pressure die casting AE44 magnesium alloy

The characteristics of defect bands in the microstructure of high pressure die casting (HPDC) AE44 magnesium alloy were investigated. Special attention was paid to the effects of process parameters during the HPDC process and casting structure on the distribution of defect bands. Results show that the defect bands are solute segregation bands with the enrichment of Al, Ce and La elements, which are basically in the form of Al₁₁RE₃ phase. There is no obvious aggregation of porosities in the defect bands. The width of the inner defect band is 4–8 times larger than that of the outer one. The variation trends of the distribution of the inner and outer defect bands are not consistent under different process parameters and at different locations of castings. This is due to the discrepancy between the formation mechanisms of double defect bands. The filling and solidification behavior of the melt near the chilling layer is very complicated, which finally leads to a fluctuation of the width and location of the outer defect band. By affecting the content and aggregation degree of externally solidified crystals (ESCs) in the cross section of die castings, the process parameters and casting structure have a great influence on the distribution of the inner defect band.

     

Determination of interfacial heat transfer coefficient and its application in high pressure die casting process

In this paper,the research progress of the interfacial heat transfer in high pressure die casting（HPDC）is reviewed.Results including determination of the interfacial heat transfer coefficient（IHTC）,influence of casting thickness,process parameters and casting alloys on the IHTC are summarized and discussed.A thermal boundary condition model was developed based on the two correlations：（a）IHTC and casting solid fraction and（b）IHTC peak value and initial die surface temperature.The boundary model was then applied during the determination of the temperature field in HPDC and excellent agreement was found.     

Effects of Gd Addition on the Microstructure and Tensile Properties of Mg-4Al-5RE Alloy Produced by Three Different Casting Methods

This work deals with the effect of 0.67 wt％ Gd addition on the microstructure and tensile properties of Mg-4Al-5RE (where RE represents LaCe mischmetal) alloy produced by sand casting (SC),permanent mold casting (PMC),and high-pressure die casting (HPDC).The results show that Gd addition could refine the grains,but its efficiency decreases by increasing the cooling rate due to the shifting from SC to PMC and finally to the HPDC method.Meanwhile,the acicular Al11RE3 phase is modified into the short-rod or granular-like shape under the three casting conditions.Such refined and modified micro-structures are due to the Al2(Gd,RE) phases,which act as the nucleation sites in both the α-Mg matrix and Al11RE3 phase.Also,the weakening grain refinement effect in the increased cooling rates can be attributed to the narrow constitutional undercooling zone.After Gd addition,the 0.2％ proof strength of the SC and PMC alloys increases by about 16.9％ and 12.7％,respectively,while in the HPDC alloy,it decreases by about 5.9％.The main factor in the strength increment of the SC and PMC alloys is the grain boundary strengthening due to grain refinement which is proved by modeling the related mechanisms,whereas weak secondary phases and grain boundary strengthening mechanisms in the HPDC alloy lead to strength reduction.After Gd addition,the elongation to failure of the SC,PMC,and HPDC alloys is significantly enhanced by about 34.8％,20.2％,and 12.3％,respectively,due to the crack resistance nature of the modified short-rod/granular Al11(RE,Gd)3 phase compared to the acicular one.     

模具充型过程中光滑粒子流体动力学模拟(英文)

讨论了基于光滑粒子流体动力学(SPH)的压铸充型模拟的实施过程。建立了一种区分流体粒子和入流粒子的入流边界条件。对人工黏度和移动最小二乘法在处理压力振荡中的作用进行了对比。对最终模型在模拟压铸二维与三维的充型过程进行了验证。将SPH和有限差分的模拟结果与实验结果进行了对比研究。结果显示SPH与实验更为吻合,表明了SPH在描述充型过程流态方面的有效性与精度。