A novel approach to melt purification of magnesium alloys

A novel low-cost method for melt purification of magnesium alloys,the melt self-purifying technology(MSPT),has been developed successfully based on a low temperature melt treatment(LTMT)without adding any fluxes.The iron solubility in the molten liquid of magnesium and its alloys,and the settlement velocity of iron particles were calculated.It is shown that the low temperature melt treatment is an effective method to decrease the impurity Fe content in magnesium and its alloys.Without any additions,the Fe content in the AZ31 alloy was reduced to 15 ppm from the initial 65 ppm,and the Fe content in the AZ61 melt was decreased to 20 ppm from the initial 150 ppm after the low temperature melt treatment.The results also showed that the Fe content in AM60 and AM50 dropped to 15 and 18 ppm,respectively,from the initial 150 ppm after the low temperature melt treatment.For ZK 60,the Fe content in the melt down to less than 5 ppm was achieved.After the low temperature melt treatment,the Si content in the above alloys was also decreased obviously.

Semi-continuous casting of magnesium alloy AZ91 using a filtered melt delivery system

A 3-D numerical simulation of an industrial-sized slab caster for magnesium alloy AZ91 has been carried out for the steady state operational phase of the caster.The simulated model consists of an open-top melt delivery system fitted with a porous filter near the hot-top.The melt flow through the porous filter was modeled on the basis of Brinkmann-Forchimier-Extended non-Darcy model for turbulent flow.An in-house 3-D CFD code was modified to account for the melt flow through the porous filter.Results are obtained for four casting speeds namely,40,60,80,and 100 mm/min.The metal-mold contact region as well as the convective heat transfer coefficient at the mold wall were also varied.In addition to the above,the Darcy number for the porous media was also changed.All parametric studies were performed for a fixed inlet melt superheat of 64℃.The results are presented pictorially in the form of temperature and velocity fields.The sump depth,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles are also presented and correlated with the casting speed through regression analysis.©2015 Production and hosting by Elsevier B.V.on behalf of Chongqing University.

As-cast structure and tensile properties of AZ80 magnesium alloy DC cast with low-voltage pulsed magnetic field

The effects of a low-voltage pulsed magnetic field on the solidified structure and mechanical properties of DC casting AZ80 magnesium alloy were investigated.The results showed that the solidified structure of the DC casting AZ80 magnesium alloy was refined obviously by the low-voltage pulsed magnetic field and significant grain refinement in the DC casting ingot of AZ80 magnesium alloy was achieved.Meanwhile,the morphology of the dentritic in the DC casting ingot was transformed from coarse dentritic to fine rosette with the application of low-voltage pulsed magnetic field.The ability of deformation of the ingot was enhanced and especially the plasticity of the ingot center after upsetting was improved greatly by more than 80%after deformation.

Numerical simulation of standing waves for ultrasonic purification of magnesium alloy melt

It was attempted to enhance and accelerate the separation of oxidation inclusions from magnesium alloy melt by virtue of ultrasonic agglomeration technology.In order to investigate the feasibility and effectiveness of standing waves for ultrasonic purification of magnesium alloy melt,numerical simulation and relevant experiment were carried out.The numerical simulation was broken into two main aspects.On one hand,the ultrasonic field propagations within the cells with various shapes were characterized by numerical solutions of the wave equation and with a careful choice of geometry a nearly idealized standing wave field was finally obtained.On the other hand,within such a standing wave field the agglomeration behavior of oxidation inclusions in magnesium alloy melt was analyzed and discussed.The agglomeration time and agglomeration position of oxidation inclusions were predicted with numerical simulation method.The results show that the oxidation inclusions whose apparent densities are close to the density of the melt can agglomerate at wave nodes in a short time which to a great extent enhances and accelerates the separation of oxidation inclusions from magnesium alloy melt.

Effect of electromagnetic parameters on melt flow and heat transfer of AZ80 Mg alloy during differential phase electromagnetic DC casting based on numerical simulation

Based on multi-physical field coupling numerical simulation method,magnetic field distribution,melt flow,and heat transfer behavior of aΦ300 mm AZ80 alloy billet during differential phase electromagnetic DC casting(DP-EMC)with different electromagnetic parameters were studied.The results demonstrate that the increase in current intensity only changes the magnitude but does not change the Lorentz force's distribution characteristics.The maximum value of the Lorentz force increases linearly followed by an increase in current intensity.As the frequency increases,the Lorentz force's r component remains constant,and the z component decreases slightly.The change in current intensity correlates with the melt oscillation and convection intensity positively,as well as the liquid sump temperature uniformity.It does not mean that the higher the electric current,the better the metallurgical quality of the billet.A lower frequency is beneficial to generate a more significant melt flow and velocity fluctuation,which is helpful to create a more uniform temperature field.Appropriate DP-EMC parameters for aΦ300 mm AZ80 Mg alloy are 10-20 Hz frequency and 80-100 A current intensity.

Magnesium alloys as extremely promising alternatives for temporary orthopedic implants-A review

Mg alloys are emerging as potential and very promising alternatives for replacing permanent metallic implant materials such as steels and titanium in applications where the implants need to be removed following healing through revision surgery.Use of Mg alloys for implant application is seen as a game changer and Mg alloys are almost perfect materials for the future in both engineering and biomedical applications.Present review therefore focuses on highlighting significance of Mg alloys in biomedical field and risks of using permanent metallic implants particularly when the implants are no longer required after the injury is healed.In this review,importance of orthopedic implants in present scenario,serious concern related to accidents that are causing permanent disabilities,demand in orthopedic implant market worldwide,potential applications of Mg based materials and their compatibility in biological environment is presented and discussed.In addition,degradation rate,major reactions associated with Mg based materials and effect of alloying elements on implant performance are also discussed based on in-vivo results.Recent advances in development of Mg alloys through various techniques and their performance in in-vitro conditions are also outlined.Possible ways to eliminate the limitations of Mg alloys include alloying,melt purification,surface alterations,surface modifications,chemical treatment,secondary processing etc.are discussed.Challenges and opportunities for Mg alloys to become ideal implant material is also addressed.

On numerical modeling of low-head direct chill ingot caster for magnesium alloy AZ31

A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach.Following the aluminum/magnesium DC casting industrial practices,the LH mold is taken as 30 mm with a hot top of 60 mm.The previously verified in-house code has been modified to model the present casting process.Important quantitative results are obtained for four casting speeds,for three inlet melt pouring temperatures(superheats)and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster.The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations.Specifically,the temperature and velocity fields,sump depth and sump profiles,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.

A Review on Casting Magnesium Alloys： Modification of Commercial Alloys and Development of New Alloys

The research and development status of casting magnesium alloys including the commercial casting alloys and the new types casting alloys are reviewed,with more attention to microstructure and mechanical properties of modified-AZ91,AM60 and WE43 alloys with various additions,and new types of low cost casting alloys and high strength casting alloys.The modification and/or refinement of Mg2 Si phase in Mg-Al-Si based casting alloys by various additions are discussed and new purifying technologies for casting magnesium alloys are introduced to improve the performance.The modified AZ81 alloy with reduced impurities is found to have the tensile strength of 280 ± 6 MPa and elongation of 16% ± 0.7%.The fatigue strength of AZ91 D alloy could be obviously improved by addition of Ce and Nd.The Mg-16Gd-2Ag-0.3Zr alloy exhibits very high tensile and yield strengths（UTS：423 MPa and YS：328 MPa）;however,its elongation still needs to be improved.

铸造铝合金熔体处理技术发展现状

铸造铝合金具有轻质、高比强度、铸造性能好等一系列优点,是一种优异的轻量化材料,在汽车行业具有广阔的应用前景。合金熔体质量对铸件最终性能具有重要影响,铸造前对其进行净化、晶粒细化和变质处理是获得高性能铸件的必要前提条件。本文介绍了铸造铝合金熔体纯净化技术、晶粒细化技术以及共晶Si变质处理技术的发展现状,以期为先进铝合金熔体处理技术的开发与应用提供参考。

航空用高品质7050铝合金大规格扁锭熔铸技术研究

7050铝合金具有密度低、强度高、热加工性能优异等优点,在航空航天结构件上应用广泛。但在7050铝合金大规格铸锭半连续铸造过程中,存在热裂、成形困难、严重显微疏松和氧化夹杂等问题。本研究开发了大规格扁锭半连续铸造过程中合金熔体冶金质量控制技术,系统考察了精炼工艺、晶粒细化技术对大规格扁锭品质的影响规律,确定了大规格扁锭铸造工艺。结果表明,优化工艺制度能严控熔体内氢和夹杂物含量,采用Al-Ti-C晶粒细化工艺制度将晶粒度控制在1~1.5级,高质量的熔体质量缓解了合金凝固收缩应变,极高比例地解决了扁锭开裂问题。最终,形成了一套适用于大规格7050合金扁锭的熔铸工艺制度。

原铝液DC铸造2A12铝合金铸锭的高温均匀化研究

针对原铝液DC铸造制备的2A12铝合金铸锭,利用金相显微镜、扫描电镜及其附带的能谱分析仪、电导率测定、DSC测定分析及拉伸试验研究了其铸态显微组织,以及高温均匀化工艺对合金铸锭显微组织、轧材力学性能及电导率的影响。结果表明,原铝液DC铸造2A12铝合金铸锭的铸态组织具有明显的树枝晶状特征,其中α-Al构成树枝晶晶干,块状Al2Cu和AlFeMnCu合金相及具有明显共晶组织特征的层片状S相Al2CuMg均分布于晶干间;合金铸锭在490℃均匀化保温8 h之前,可溶合金相回溶入基体的速度很快,490℃均匀化时间超过12 h以后,可溶合金相回溶入基体的速度很慢,490℃均匀化处理保温时间延长至36 h,合金基体中仍残留大量低熔点合金相;497℃24h高温均匀化处理不能完全消除合金基体中低熔点产物,但经497℃12 h均匀化处理后合金基体中残留低熔点合金相量较经490℃36 h均匀化处理后合金基体中残留低熔点合金相量更少;497℃均匀化保温时间由12 h延长至24 h对其热轧板力学性能及电导率无明显提高作用。

电解原铝液DC铸造6×××系铝合金铸锭的枝晶组织

采用金相分析方法研究了电解原铝液直接合金化、DC铸造6063、6N01及6061铝合金Φ200 mm铸锭的显微组织尤其是枝晶分布规律。结果表明:3种合金铸态组织均呈明显的树枝晶状,存在严重的枝晶偏析,其枝晶直径及枝晶臂间距从铸锭心部向铸锭表面逐渐细化;3种合金铸锭的枝晶臂间距及枝晶直径随合金铸锭中合金元素含量增加呈现出逐渐粗化的趋势,即合金元素总质量分数最低为1.141%的6063铝合金铸锭枝晶直径及枝晶臂间距最细小,而合金元素总质量分数最高为2.05%的6061铝合金铸锭枝晶直径及枝晶臂间距最大。

镁合金熔体过滤净化工艺研究

采用在底流式镁合金熔炼炉中施加不锈钢丝网过滤的方法在半连续铸造工艺中实现镁合金熔体净化,以达到提高镁合金锭坯质量的目的。研究表明,该方法可以实现良好的净化效果。不仅过滤了绝大多数粗大夹杂物,而且对减少显微缩孔十分有效。采用低温铸造工艺,不仅减少了熔体氧化趋势,同时有利于增强过滤效果。采用此法过滤镁合金熔体在铸造开始和结束阶段效果较差。

超声场作用下镁合金熔体净化工艺

采用XRD、SEM和图像分析软件对AZ80镁合金中夹杂的种类和形态进行了研究,并采用施加超声场来实现AZ80镁合金熔体的净化,以达到提高镁合金熔体质量的目的。研究表明,AZ80合金中夹杂主要为MgO,夹杂主要以颗粒状、团絮状、长条带状和块状形式存在。含有夹杂的AZ80合金熔体经超声处理可实现良好的净化效果,净化程度与超声功率和熔体温度有关。AZ80合金熔体在650℃经80W超声功率处理60s可以获得理想的净化效果。超声功率过大或超声处理时间过长均不利于合金熔体净化。

高性能铸造稀土镁合金的发展

从合金体系和纯净化工艺两个方面对高性能铸造稀土镁合金进行了综述。按时间顺序,对Mg-Nd、Mg-Y和Mg-Gd三个系列合金的研究现状进行论述,重点针对Mg-Gd系合金,着重介绍了其发展历史、力学性能、强化机制。通过纯净化前后的对比可以看出,纯净化可以有效地降低合金中的缺陷,提高合金的力学性能。最后分析讨论了高性能铸造稀土镁合金研究中存在的问题以及今后发展的主要方向。

AZ91镁合金旋转喷吹除气理论分析及实验研究

通过分析旋转喷吹除气过程中氢在惰性气泡表面和金属熔体/空气界面的质量传输,建立适用于镁合金旋转喷吹除气的理论模型。通过计算,分析工艺参数对除气效率的影响。结果表明:随着旋转喷吹转速由230r/min增大到330r/min时,熔体中气泡总表面积由1.49×10-2m2增大到3.44×10-2m2,这增大了熔体中氢向气泡的单位时间扩散量,从而使除气效率随着转速的增加而增大。利用减压凝固方法对旋转喷吹除气的效果进行实验验证,实验结果与理论模型计算结果吻合较好。该模型可以用来指导镁合金熔体除气工艺的优化。

镁合金铸造技术进展

综述了近年来国外在镁合金熔炼气体保护方法。

半连续铸造AZ70镁合金的纯净化及均质化

试验优化了传统的半连铸生产工艺,并研究了合金锭的均质化处理工艺。结果表明,工艺优化后,主要元素及杂质Fe的含量在达到设计要求的同时,不带入新的杂质,净化了镁合金基体。确定了最佳的均质化工艺参数为400～430℃保温10～15h。AZ70合金铸锭经410℃/12h均质化处理后,整个横截面的组织及力学性能均匀,且具有较高的塑性,其伸长率为8.16%。

压铸AZ91D镁合金激光重熔区氢气孔的形成机制

为研究压铸镁合金熔化焊接气孔的形成机理,对厚度为6mm的AZ91D压铸镁合金和厚度为2.2mm的AZ71热挤压镁合金进行CO2激光局部重熔。采用扫描电子显微镜(SEM)观察气孔形貌,利用粒径分析软件Nano measure 1.2测量气孔的尺寸。结果表明:压铸镁合金重熔区气孔问题突出,少数粗大的宏观气孔形状不规则,内壁粗糙,具有明显的金属冲刷痕迹,均来源于母材预存微观气缩孔;多数微观气孔内壁光滑、呈倒喇叭形,属于氢致气孔。分析了氢致气孔的形成机制,建立了氢气孔形成过程的模型,并同实验照片进行了比较,发现所建立的模型能够很好地用来解释氢致气孔的形成过程。

超轻Mg-Li合金熔铸工艺与轧制温度的研究

采用电阻炉熔炼锂盐覆盖氩气保护的方法成功地熔炼和铸造了锂质量分数分别为４．１６ ％ ，８．２ ％ ，１０．８ 的超轻镁－ 锂合金铸锭，其组织分别为α，α＋ β和β固溶体。铸锭经均匀化后在２００ ℃，３００ ℃，４００ ℃轧制，研究了轧制温度，含锂量与轧制裂边的关系。实验发现α相合金在２００ ℃发生板材横裂碎化，３００ ～４００ ℃之间轧制裂边减轻，在４００ ℃最轻。β相合金在各温度下十分容易轧制。α＋ β相合金塑性较好，裂边介于α相和β相合金之间，在２００ ～３００ ℃塑性最好。