Process window for manufacturing of semi-solid aluminium alloys

Abstract

The demands of the automotive industry for lightweight technologies stimulate research on new materials and the development of corresponding innovative production methods. The thixocasting process is based on standard high pressure die casting with the exception that mould filling takes place in the semi-solid state. In comparison to other processes, thixocast parts generally have better mechanical properties. For an industrial application and for the safe handling of the partially solidified alloys in series manufacturing, an exact knowledge of the material behaviour is necessary. This paper attempts to give an overview of the relevant parameters influencing the successful processing of semisolid metals with thixotropic properties. With this aim, several parameters considered to be of major importance for the process were selected and analysed and correlated with the desired end properties. The influence of temperature to obtain an optimally heated semi-solid billet with thixotropic properties was the first parameter to be evaluated. Subsequently, the effect of wall thickness (25–0.5 mm) and metal velocity during mould filling on the mechanical properties, grain texture development and segregation phenomena was studied and analysed. As a result of the investigations, a preliminary process window for the successful processing of semi-solid aluminium alloys was defined.     

Microstructure and high temperature tensile properties of Al–Si–Cu–Mn–Fe alloys prepared by semi-solid thixoforming

The differences in the microstructure and elevated temperature tensile properties of gravity die cast, squeeze cast, and semi-solid thixoformed Al–Si–Cu–Mn–Fe alloys after thermal exposure at 300 °C were discussed. The results demonstrate that the elevated temperature tensile properties of semi-solid thixoformed alloys were significantly higher than those of gravity die cast and squeeze cast alloys, especially after thermal exposure for 100 h. The ultimate tensile strength (UTS) of semi-solid thixoformed alloys after thermal exposure at 300 °C for 0.5, 10 and 100 h were 181, 122 and 110 MPa, respectively. The UTS values of semi-solid thixoformed alloys were higher than those of heat resistant aluminum alloys used in commercial applications. The enhanced elevated temperature tensile properties of semi-solid thixoformed experimental alloys after thermal exposure can be attributed to the combined reinforcement of precipitation strengthening and grain boundary strengthening due to thermally stable intermetallic phases as well as suitable grain size.     

On the microstructure formation in chromium steels rapidly cooled from the semi-solid state

Semi-solid metal forming requires precise knowledge concerning the microstructural parameters in the mushy state. For many light metals, the liquid and solid fraction and the size, shape and contiguity of the solid phase can be evaluated easily in the “quenched from the freezing range” condition. For iron-based alloys, however, determining them is more difficult or even impossible because steels may undergo different phase-transformations during cooling. Due to the high processing temperatures, diffusion during quenching is also of more importance. Here we describe the phase formation during rapid cooling from the semi-solid state of two different steel grades, tool steel X210CrW12 and bearing steel 100Cr6. For both of these steels the microstructure in the as-quenched state does not directly reflect the condition in the semi-solid state, and thus no metallographic evaluation of the microstructural parameters is possible. It is also shown here that the microstructure of semi-solid processed steels is completely different from that of the conventionally treated species.     

The technologies of titanium powder metallurgy

Titanium alloys exhibit attractive mechanical properties but they are expensive. This paper reviews the current status of titanium powder metallurgy which offers near-net shape cost-effective approaches to the fabrication of components. For more information, contact F.H. Froes, University of Idaho, McClure Bldg., Room 437, Moscow, ID 83844-3026, e-mail imap@uidaho.edu.     

5A06铝合金鞍型横梁等温挤压近净成形工艺

针对某5A06铝合金鞍型横梁,设计了等温挤压近净成形试验。通过Deform有限元分析研究了鞍型横梁等温挤压过程中金属流动规律和缺陷产生原因,并进行了毛坯尺寸和热挤压件图的优化。设计了鞍型横梁等温挤压近净成形模具,可实现在普通压力机上加热管加热方式升温、成形后多顶杆同步脱模功能。通过等温挤压近净成形方法制备了长为580 mm、最薄壁厚为4 mm、筋高为8 mm、筋宽为6 mm的薄壁复杂多筋鞍型横梁,该构件90%以上部位尺寸精度可达到±0. 3 mm,其余部位尺寸精度可达±0. 5 mm,抗拉强度达340 MPa,伸长率约为25%。对等温挤压件装配部位进行少量机加,可获得3种不同尺寸的鞍型横梁。     

阻燃镁合金材料的研究现状及发展趋势

综述阻燃镁合金材料的研究现状,分析合金元素Ca、Be、Ce、Bi的作用,对阻燃镁合金的一些新发展情况作了介绍.     

铝合金半固态压铸工艺参数及性能研究

应用具有不同厚度的简单板材模具来系统研究压铸工艺参数对A356铝合金半固态压铸件性能的影响,以优化半固态压铸工艺及其参数.试验结果表明,对A356铝合金半固态压铸,其性能随压射压力的增大而提高,当压射压力大于100 MPa,则性能基本上不再提高.压铸速度过小或过大会降低压铸件的性能.另外建压时间、坯料加热温度、模具预热温度以及脱模剂等参数对半固态压铸件性能也有明显的影响.     

21世纪最具发展前景的近净成形技术——半固态加工

就半固态加工技术的制坯机制、半固态充填流变学、高压下结晶凝固和力学成形理论进行了讨论,在此基础上, 就目前流变成形和触变成形的工艺过程、特点以及应用前景作了分析,指出由于半固态加工与其他成形工艺相比具有许多 优势,是一种潜在的、正逐步为世人认识的最具发展前景的近净成形技术之一。     

Mg-RE合金的阻燃能力研究

主要研究了不同稀土元素对工业纯镁阻燃能力的影响,结果表明:一定量的Ce、Y、Nd、Dy稀土元素可使工业纯镁的燃点值有不同程度的提高,其中Y和Dy对阻燃能力的提高作用更为明显;观察发现Mg-Ce和Mg-Nd合金的氧化膜存在裂纹,这是他们阻燃能力较小的原因。本文还对Mg-0.5Y-Ce和Mg-0.5Y-Dy合金的阻燃能力进行了研究,发现Mg-0.5Y-5Dy合金的氧化膜致密,韧性较好,阻燃能力最强。     

半固态成形轻合金的发展状况

半固态成形技术以其高效、节能、近净形生产以及成形件性能高等诸多优点，已成为21世纪最具发展前景的金属成形工艺之一，但是半固态成形轻合金应用品种的局限性，制约了半固态成形技术的发展。文中介绍了国内外半固态成形轻合金的研究、应用状况以及新型半固态轻合金的设计开发，并分析了半固态成形轻合金的发展前景。     

Mg-Y-Ce稀土阻燃镁合金的高温氧化行为研究

Y和Ce的同时加入使镁合金获得了优异的阻燃效果,实现了镁合金在大气条件下的无保护熔炼。AES和XRD表面分析结果表明,Mg-Y-Ce阻燃镁合金在高温下暴露于大气中时,合金表面会生成一层以Y2O3为主的氧化膜。干燥空气中,Mg-3Y-4.5Ce合金在773, 873和973 K下氧化时,其氧化动力学曲线遵循立方规律。基于高温氧化热力学,详细分析了Mg-Y-Ce合金在高温下的选择性氧化机制,并从电化学角度探讨了Y2O3膜的半导体特性及其对镁合金基体进一步氧化的阻碍机制     

Metastable structure of austenite base obtained by rapid solidification in a semi-solid state

Material processing in a semi-solid state with rapid solidification is an innovative technology, which enables us to produce complex-shaped semi products in one operation. Unconventional properties and microstructures can be obtained in this way. Material processing in a semi-solid state has been used for materials with lower melting temperatures, particularly for Al alloys.This paper concentrates on the development of new technologies for production of miniature thin-walled steel components with complicated shapes. Ledeburitic steel with 1.8% of carbon and 11% of chromium was chosen for this experimental study. This material was used to produce very small thin-walled semi products. From the initial structure consisting of primary and secondary carbides distributed in a ferrite matrix was obtained a microstructure with over 90% of metastable austenite after cooling from the semi-solid state. The main aim of this experimental program was to describe the effect of two different methods of heating to the semi-solid state. The first method used unique heating equipment, combining high frequency and resistance heating. The second method consisted of conventional heating in a furnace. The influence of the cooling rate on the development of the microstructure was investigated. If was found that both heating and cooling rates influence grain size and the size and the morphology of carbide network placed between the globular austenite grains.Structure analysis was performed with the help of light microscopy, laser scanning confocal microscopy and scanning electron microscopy. EDX analysis was applied to determine chemical profiles of phases and X-ray diffraction phase analysis was used to establish volume fraction of austenite in the final microstructure. Characterisation of individual structural components was further completed by hardness measurements.     

半固态A356合金的流变压铸充填性与组织分布

试验研究了压射比压和压射速度对半固态A356铝合金流变压铸充填性的影响。试验结果表明：压射比压和试片的壁厚对新型方法制备的半固态A356合金浆料的充填性影响较大，试片壁厚越大，压射比压越大，型腔越容易充满；为了保证充满型腔，对于10mm的试片，压射比压应≥15MPa，而对于5mm的试片，压射比压应≥20MPa。压射速度对半固态L356合金浆料充填性也具有较大的影响，较高的压射速度有利于半固态A356铝合金浆料的充填；为了保证充满型腔，对于10mm，的试片，压射速度应≥0．384m／s，而对于5mm的试片，压射速度应≥1．152m／s。流变压铸试片的组织分布很均匀，利于获得高质量的半固态A356合金压铸件.     

Microstructure evolution during semi-solid powder rolling and post-treatment of 7050 aluminum alloy strips

Semi-solid powder rolling (SSPR) combines semi-solid rolling with powder rolling to prepare high-performance metallic strips. Semi-solid powders were prepared under an inert atmosphere and subsequently rolled by a powder rolling machine. Conductive cooling between the pre-heated rollers and semi-solid powders results in a rapid solidification effect that is able to process alloys with a broad freezing range. The liquid in the semi-solid powders plays an important role in the microstructure evolution, which can improve the strength of strips. The 7050 aluminum alloy strips were obtained and used to evaluate the processing parameters and strip qualities for strips up to 100 mm wide and 1.5–2 mm thick. The process of semi-solid powder rolling was described and microstructure evolution during rolling and post-treatment was analyzed. The combination mechanism of semi-solid powders during rolling was also discussed. The results show that the best liquid fraction to prepare a strip ranges from 45 to 65%. Flowing and filling of liquid (>10%), densification by rolling and recrystallization (<10%) are the three combination mechanisms of the semi-solid powders during rolling. In addition, semi-solid powder rolled strips can be processed subsequently by hot rolling with the improved micro-hardness and relative density.     

Research on semi-solid thixoforming process of AZ91D magnesium alloy brackets for generators in JH70-type motorbikes

The investment on semi-solid die casting processes of AZ91D magnesium alloy brackets for generators in JH70-type motorbikes is introduced. The processes of low super-heat and cooling slope for the preparation of billets with non-dendritic microstructure, the remelting of billets for thixoforming and the parameters in the process of semi-sohd thixoforming have been researched. The results show that primary billets with non-dendritical structures can be prepared by forming great amount of nuclei in melt via the process of low super heat. By optimizing the remelting process through adjusting the current of the induced equipment, semi-solid billets with a structure of spherical grains were obtained from the primary billets with non-dendritical structure. The range of 580℃ to 583℃ is the proper remelting temperatures by which the billets have an expected thixotropy and can be transferred to a die-casting machine. The optimized parameters of semi-solid forming in a die-casting machine are as follows： the area of the ingate in the die is 383.5 mm^2, the speed of the pierce of the machine 5 m/s, the shot pressure of the pierce 75 MPa, and the maintenance pressure of the pierce 350 MPa. The castings of brackets for supporting generators in JH70 type motorbikes were formed by adopting the optimized processes and parameters mentioned above.     

Effects of rheoforming on microstructures and mechanical properties of 7075 wrought aluminum alloy

It is economically advantageous to cast wrought aluminum alloys directly into near-net-shape components.The objective of the present work is to take advantage of the rheoforming with 7075 alloy to improve the competitiveness of this emerging technology in the manufacture of wrought aluminum alloy.High quality semi-solid slurry was produced,in which primary α(Al) presents in diameter of 62 μm and shape factor of 0.78 and features no eutectics entrapped.Higher forming pressure results in small grain size,improved shape factor and higher density.Especially,rheoforming can effectively reduce the occurrence of hot tearing.The average yield strength and elongation of the rheoformed samples in the T6 condition are 483 MPa and 8%,respectively.     

铝合金半固态成形技术

介绍了铝合金半固态成形工艺；对半固态合金料的制备、成形方法及其优越性进行了讨论。简述铝合金半固态成形技术的研究和应用现状。     

节约型高强韧不锈钢及其薄带连铸制备技术

双辊薄带连铸技术自上世纪80年代至今已有近40年,但在常规不锈钢的工业化生产方面一直未取得成功。究其原因,主要归因于生产消耗、产品利润与产能之间的不匹配。近年来,国际国内开始将薄带连铸应用于制备节约型高强韧不锈钢,从而简化了热轧生产工艺,解决了因热塑性差而造成的钢板边裂和中裂等缺陷,大大提高了成材率。韩国浦项制铁采用薄带连铸技术生产出节约型双相不锈钢,耐腐蚀性能与SUS304奥氏体不锈钢相当,而抗拉强度可提高1倍,产品主要用于取代材料成本过高的奥氏体不锈钢。我国东北大学开发出高强韧节约型不锈钢的成分体系及其薄带连铸技术,制备出0.30~15 mm厚薄带,其强塑积高达62 000 MPa·%且其耐腐蚀性能较SUS304高出10%~15%,可以应用于核电、石化、铁路交通及汽车结构等领域。本文就薄带连铸高强韧不锈钢的现状、应用及未来发展趋势进行了综述和分析。     

半固态铸轧AZ91D镁合金板带的再加工组织性能

实验研究了半固态铸轧成形技术制备的AZ91D镁合金板带的再加工组织性能及特点。半固态铸轧实验是在自制的试验机上进行的,得到半固态铸轧AZ91D镁合金板带之后,分别进行冷轧、热轧和冲杯实验,加工后取样观察显微组织。实验结果表明,半固态铸轧的AZ91D镁合金板带材具有较好的加工性,最大冷变形可达到28%,热变形可达到47%。观察组织可见,横断面、纵断面固相颗粒在轧制作用下都有所变形,纵断面的变形更为严重,说明组织沿轧制方向塑性变形较大。     

添加0.5%富铈混合稀土AZ91D镁合金半固态组织的形成

半固态浆料的固相颗粒尺寸、形态和分布主要取决于熔化过程中液相的形成与演化过程。采用添加0.5%富铈混合稀土来改善AZ91D镁合金的铸态组织,研究在半固态等温热处理中的组织演变以及非枝晶组织制备与控制的机理。结果表明:稀土合金化处理可促进初生相在等温热处理过程中由枝晶向粒状晶的转变,可获得更加细小、均匀的球状固相颗粒,并且其粗化速度较慢。半固态等温热处理过程中,整个系统处于熔化和结晶的动态平衡,铸态组织中枝晶根部高溶质浓度区或系统的温度、浓度起伏是固相颗粒内液相形成的内在和外在条件。