Microstructure control of AZ31 alloy by self-inoculation method for semisolid rheocasting

A novel rheocasting process, self-inoculation method （SIM）, was developed for the microstructure control of semisolid wrought Mg alloy. This process involves mixing between liquid alloy and particles of solid alloy （self-inoculants）, subsequently pouring the mixed melt into a special designed multi-stream fluid director. The primary phase with dendritic morphology in the conventionally cast AZ31 alloy has readily transformed into near spherical one in the slurry produced by SIM from melt treatment temperature between 690 ℃ and 710 ℃ and self-inoculants addition of 3%-7%. Achievement of the non-dendritic microstructure at the higher melt treatment temperature requires more self-inoculants addition or decreases in the slope angle of fluid director. Primary phase in the slurry thus produced has attained an ideally globular morphology after isothermal holding at 620 ℃ for 30 s. The increasing holding time leads to decrease of shape factor but the coarsening of particle size. The spheroidization and coarsening evolution process of solid particles during the isothermal holding were analyzed by Lifshitz-Slyozov-Wagner （LSW） theory.

Dry sliding wear behavior of rheocast hypereutectic Al-Si alloys with different Fe contents

The effect of iron content on wear behavior of hypereutectic Al?17Si?2Cu?1Ni alloy produced by rheocasting process was investigated. The dry sliding wear tests were carried out with a pin-on-disk wear tester. The results show that the wear rate of the rheocast alloy is lower than that of the alloy produced by conventional casting process under the same applied load. The fine particle-likeδ-Al4(Fe,Mn)Si2 and polygonalα-Al15(Fe,Mn)3Si2 phases help to improve the wear resistance of rheocast alloys. As the volume fraction of fine Fe-bearing compounds increases, the wear rate of the rheocast alloy decreases. Moreover, the wear rate of rheocast alloy increases with the increase of applied load from 50 to 200 N. For the rheocast alloy with 3% Fe, oxidation wear is the main mechanism at low applied load (50 N). At higher applied loads, a combination of delamination and oxidation wear is the dominant wear mechanism.

Microstructure and property of rheocasting AZ91 slurry produced via ultrasonic vibration process

The microstructure and mechanical properties of rheocasting AZ91 magnesium alloy were investigated. The semisolid slurry of this alloy was prepared by ultrasonic vibration （USV） process and then shaped by high pressure diecasting （HPDC）. The results show that fine and spherical a-Mg particles were obtained by USV at the nucleation stage, which was mainly attributed to the cavitation and acoustic streaming induced by the USV. Extending USV treatment time increased the solid volume fraction and average particle size, the shape factors were nearly the same, about 0.7. Excellent semisolid slurry of AZ91 magnesium alloy could be obtained within 6 rain by USV near its liquidus temperature. The rheo-HPDC samples treated by USV for 6 min had the maximum ultimate tensile strength and elongation, which were 248 MPa and 7.4%, respectively. It was also found that the ductile fracture mode prevailed in the rheocasting AZ91 magnesium alloy.

Research progress on microstructure evolution of semi-solid aluminum alloys in ultrasonic field and their rheocasting

The effects of ultrasonic vibration(UV)treatment on microstructure of semi-solid aluminum alloys and the application of UV in rheocasting process are reviewed.Good semi-solid slurry can be produced by high-intensity UV process for aluminum alloys.The microstructures of Al-Si,Al-Mg and Al-Cu alloys produced by rheocasting assisted with UV are compact and with fine grains.The mechanical properties of the UV treated alloys are increased by about 20%-30%.Grain refinement of the alloys is generally considered because of cavitation and acoustic streaming caused by UV.Apart from these mechanisms,a hypothesis of the fuse of dendrite root caused by capillary infiltration in the ultrasonic field,as well as a mechanism of crystallites falling off from the mould-wall and crystal multiplication by mechanical vibration effect in indirect ultrasonic vibration are proposed to explain the microstructure evolution of the alloys.

Microstructural characteristics and mechanical properties of Al-2024 alloy processed via a rheocasting route

This article reports the effects of stirring speed and T6 heat treatment on the microstructure and mechanical properties of Al-2024 alloy synthesized by a rheocasting process. There was a decrease in grain size ofα-Al particles corresponding to an increase in stirring speed. By increasing the stirring speed, however, the globularity of matrix particles first increased and then declined. It was also found that the hardness, compressive strength, and compressive strain increased with the increase of stirring speed. Microstructural studies revealed the presence of nonsoluble Al15（CuFeMn）3Si2 phase in the vicinity of CuAl2 in the rheocast samples. The required time for the solution treatment stage was also influenced by stirring speed;the solution treatment time decreased with the increase in stirring speed. Furthermore, the rheo-cast samples required a longer homogenization period compared to conventionally wrought alloys. Improvements in hardness and compres-sive properties were observed after T6 heat treatment.

Evolution of microstructure in semi-solid slurries of rheocast aluminum alloy

Semi-solid metal processing is being developed in die casting applications to give several cost benefits. To efficiently apply this emerging technology, it is important to understand the evolution of microstructure in semi-solid slurries for the control of the theological behavior in semi-solid state. An experimental apparatus was developed which can capture the grain structure at different times at early stages to understand how the semi-solid structure evolves. In this technique, semi-solid slurry was produced by injecting fine gas bubbles into the melt through a graphite diffuser during solidification. Then, a copper quenching mold was used to draw some semi-solid slurry into a thin channel. The semi-solid slurry was then rapidly frozen in the channel giving the microstructure of the slurry at the desired time. Samples of semi-solid 356 aluminum alloy were taken at different gas injection times of 1, 5, 10, 15, 20, 30, 35, 40, and 45 s. Analysis of the microstructure suggests that the fragmentation by remelting mechanism should be responsible for the formation of globular structure in this rheocasting process.

Evolution of rheocast microstructure of AZ31 alloy in semisolid state

Semisolid rheoforming （SSR） is a promising technology for the production of Mg wrought alloy in foundry settings. In order to realize SSR, it is necessary to characterize the grain structure evolution during slurry preparation. In this paper, slurry of AZ31 alloy was produced by a novel rheocast process known as self-inoculation method （SIM）. Interrupted quenching technology was applied to investigate the primary a-Mg evolution during continuous cooling and isothermal holding. Results indicate that the initial microstructure of slurry produced by SIM is a mixture of irregular grains, which becomes ideally globular when the slurry slowly cools to 620 ~C and isothermally held for at least 30 s. The local solute diffusion leads to dendritic fragmentation and forms separated particles. During prolonged holding, the particle surface gradually becomes smooth because of protuberance melting and groove advancement. Coarsening of a-Mg grains in isothermal holding was analyzed using Lifshitz-Slyozov-Wagner theory. Results suggest that coalescence is most likely the dominant coarsening mechanism in the early stage while Ostwald ripening tends to be the principal one later. The EDS results indicate that a longer holding time leads to AI solute element segregation at the grain boundaries, but Zn distribution within liquid matrix has no obvious change.

Semi-solid A356 alloy slurry for rheocasting prepared by a new process

To obtain the semi-solid slurry with uniform and fine structure morphology that satisfies the requirement of rheocasting process, a new process for preparing semi-solid AI alloy slurry was developed, in which local chilling was combined with low superheat pouring and slight electromagnetic stirring （LSPSEMS）. The morphology and the size of primary a-AI in the A356 alloy slurry prepared with the new process, i.e., LSPSEMS with local chilling, were investigated using MIAPS image analyzing software, and the grain refinement mechanism was discussed. The results indicate that the semi-solid primary phase consists of particle-like or globular-like a-AI, and the morphology and grain size of primary a-AI in the slurry can be markedly improved by the new process. The fine primary a-AI distributes uniformly in the slurry, which satisfies the requirement of rheocasting. Compared with the alloy prepared by LSPSEMS, the average equal-area-circle grain diameter of primary a-AI in semi-solid A356 alloy ingot prepared by the new process is decreased from 85.6μm to 68.8 μm at the central area, 112.6 μm to 77.6 μm at the transition area and is 84.7 μm in the edge area, respectively. The corresponding shape factor of primary a-AI is increased from 0.78 to 0.83, 0.54 to 0.77 and 0.28 to 0.59, respectively. In addition, the pouring temperature could be suitably raised from 620-630℃ of the traditional process to 650℃ using this technique, which is convenient for practical operation. The mechanism of grain refinement, in the new process, is that the local chilling quickens up the temperature decrease in the center of the melt. The nuclei could not grow up in a short time so the finer grains are formed in the melt.

Shear band formation in shaped rheocast aluminium component at various plunger velocities

Significant progress has been made in recent years in understanding and modelling the rheology of semi-solid metals.These models show the effects of the microstructure in terms of size and morphology of globules on the material response.More recently it has been shown that semi-solid metals can behave as compacted granular materials such as sand.A particular signature of such deformation is that the deformation becomes concentrated into shear bands which are 10-20 grains wide.Such bands have also been observed in a range of cast products.Recently, it has been clearly shown that shear bands in high pressure die cast (HPDC) products are also the results of Reynolds dilatancy.Shear bands are also known to be a common feature in semi-solid metal products.The segregation banding in semi-solid metal (SSM) material and its dependence of plunger velocity were investigated.Shaped castings were made with the RHEOMETALTM process with a range of different plunger velocities.The microstructural characteristics were investigated, with a particular emphasis on shear bands.It is shown that ingate velocities influence the location and characteristics of the shear bands.

Isothermal coarsening of primary particles during rheocasting

The isothermal coarsening behavior of primary solid particles in A356 aluminum alloy semi-solid slurry produced by angular oscillation (AO) technique was investigated.The comparison between the calculation and experimental results shows good quantitative agreement with Lifshitz-Slyozov-Wagner theory.The results show that the variation in shape factor and solid fraction is not significant,the average particle size increases with increasing holding time at the expense of the particle density.Ostwald repining is most likely the predominant growth mechanism in the AO-treated semi-solid slurry during rheocasting.The differences of coarsening occurred in rheocasting and partial re-melting process were also discussed.

Mechanical properties and microstructural evolution of rheocast A356 semi-solid slurry prepared by annular electromagnetic stirring

Nowadays,having an effective technique in preparing semi-solid slurries for rheocasting process seems to be an essential requirement.In this study,semi-solid slurry of A356 aluminum alloy was prepared by three-phase annular electromagnetic stirring(A-EMS)technique under different conditions.The effects of stirring current,pouring temperature and stirring time on microstructural evolution,mean particle size,shape factor and solid fraction were investigated.The rheocasting process was carried out by using a drop weight setup and to inject the prepared semi-solid slurry in optimal conditions into the step-die cavity.The filling behavior and mechanical properties of parts were studied.Microstructural evolution showed that the best semi-solid slurry which had fine spherical particles with the average size of~27μm and a shape factor of~0.8 was achieved at the stirring current of 70 A,melt pouring temperature of 670℃,and stirring time of 30 s.Under these conditions,the step-die cavity was completely filled at die preheating temperature of 470℃.The hardness increases by decreasing step thickness as well as die preheating temperature.Moreover,the tensile properties are improved at lower die preheating temperatures.The fracture surface,which consists of a complex topography,indicates a typical ductile fracture.

Suitability of lost cores in rheocasting process

When treated in pressure die casting processes the semi-solid aluminum alloys demand resilient cores which can bear the stress occurring during filling and the final pressure phase.The design of permanent cores is highly restricted in order to maintain removability.Lost cores provide the possibility of complex,undercut geometrical shapes which is mandatory for a variety of casting components.However,eligible materials which show appropriate resilience and proper decomposition properties at the same time,and in the upper echelon of cost-effectiveness,are seldom known.With the semi-solid process suited between HPDC and GDC regarding the core stress,several common and economically efficient lost core systems could be used.A selection of potentially suitable materials was identified and tested.The range of testing comprises widespread sand core-systems(amin-pur-coldbox and CO2-water-glass) as well as materials less commonly used,namely salt,plastic and zinc.Different types of conditioning are applied to enhance the surface properties.The mechanical properties of the sand cores are enhanced by different heat treatment methods during curing.The cores are tested producing a research component cast on a HPDC machine with semi-solid A356 slurry fabricated in the cooling channel process.The cast component was analyzed regarding shape stability,core removability and surface as well as the structure quality.The results show the importance of the surface conditioning for the sand-cores while salt and zinc produce good parts comparable to the reference steel core quality.

Microstructure evolution behavior of AlSi9Cu3 alloy during rheocasting

The experimental and analytical approaches were taken to investigate the non-dendritic microstructure formation and evolution of AlSi9Cu3 alloy during rheocasting.The results show that the globular primary α(Al) particles free of entrapped eutectic form after rheocasting for 3 s,and could be morphologically stabilized during subsequent growth.The fine and globular particles underwent a coarsening process under quiescently continuous cooling in which the particle density decreases,the solid fraction increases,the average particle size increases with the increase of solidification time at a rate that closely followed the classical Ostwald ripening.

Effect of sodium modification on microstructure and mechanical properties of thick-walled AlSi6Cu2.5 rheocast component

The microstructure and tensile properties were investigated in a thick-walled section （approximately 45 mm×43 mm） of a rheocast component produced by the RheoMetalTM process. Due to the long solidification period of such components, it is expected that the A1-Si eutectic formed will be coarse. Therefore, sodium （Na） was used as a modifying agent to reduce the coarseness of the eutectic. Tensile test bars were machined from three different sets of materials： 1） non-modified melt, 2） modified melt cast directly after Na addition, and 3） modified melt cast 30 min after Na addition. The alloy used was a secondary AlSi6Cu2.5 alloy （STENAL Rheol）, specially developed for rheocasting. The material was studied in the as-cast condition as well as after a T6 heat treatment. The results show that the Al-Si eutectic is significantly refined by the Na addition, even after a fading time of 30 min. However, it is observed that the Na modification generally has a detrimental effect on the mechanical properties, despite the structure refinement. This is especially true in the T6 heat treated condition, where the yield strength is reduced by more than 30%. Some possible mechanisms for the degradation of mechanical performance are discussed.

Microstructures and formation mechanism of hypoeutectic white cast iron by isothermal electromagnetic rheocast process

An investigation was made on the evolution of microstructures of hypoeutectic white cast iron slurry containing 2.5wt.%C and 1.8wt.%Si produced by rheocasting in which the solidifying alloy was vigorously agitated by electromagnetic stirrer during isothermal cooling processes.The results indicated that under the proper agitating temperatures and speeds applied,the dendrite structures in white cast iron slurry were gradually evolved into spherical structures during a certain agitating time.It also revealed that the bent dendrites were formed by either convection force or by the growth of the dendrites themselves in the bending direction;then,as they were in solidifying,they were gradually being alternated into separated particles and into more spherical structures at the end of the isothermal cooling process.Especially,the dendrites were granulated as the bending process proceeding,which suggested that they were caused by unwanted elements such as sulfur and phosphor usually contained in engineering cast iron.Convective-ow of the melt caused corrosion on the dendritic segments where they were weaker in strength and lower in melting temperature because of higher concentration of sulfur or phosphor.And the granulation process for such dendrites formed in the melt became possible under the condition.Certainly,dendrite fragments are another factors considerable to function for spherical particles formation.A new mechanism,regarding to the rheocast structure formation of white cast iron,was suggested based on the structural evolution observed in the study.

半固态金属成形应用的新进展与前景展望

论述了半固态金属坯料制备工艺、成形工艺、半固态金属成形件的性能和半固态金属成形在一些发达国家应用的最新进展，并展望了半固态金属成形在我国应用的前景及意义。

高能超声波制备铝合金半固态浆料技术的研究

研究了高能超声波制备半固态金属浆料的技术,包括制备工艺流程、非枝晶组织的生成机理,以及典型铝合金半固态浆料的制备工艺与组织。结果表明,采用超声波功率为1.2kW、频率为20kHz的钛合金振动头直接振动金属液的条件下,用0.5min左右的时间制备出铝合金半固态浆料。由于高能超声波在金属熔体中产生的声空化和声流的共同作用,制备出细小、圆整的非枝晶颗粒的半固态浆料。所制备的A390浆料在无P变质条件下初晶Si可细化到20μm左右;ZL101浆料的初晶α-Al非枝晶组织较圆整、细小,晶粒尺寸为80μm左右。

半固态合金流变铸造的研究进展

总结了 3 0余年来半固态合金流变铸造的研究和应用现状。介绍了传统机械搅拌式流变铸造工艺 ,由于其所制备的半固态合金浆料保存、输送麻烦 ,至今没有进入实际应用 ;评述了射室制备浆料式流变铸造工艺技术 ,其生产的压铸件经T6处理 ,力学性能与挤压铸件相当 ,但伸长率提高很多 ;介绍单、双螺旋流变射铸工艺 ,其特点是利用液态合金原料生产成本低 ,铸件气孔率低 ,但其所需设备和生产工艺还处在优化阶段 ;重点介绍了低过热度倾斜板浇注式流变铸造、低过热度浇注和弱机械搅拌式流变铸造的工艺特点和核心思想 ;

WSP流变铸造与球化处理对Al-18%Si-5%Fe合金组织和性能的影响

提出了波浪形倾斜板流变铸造(WSP)装置制备过共晶Al-Si-Fe合金的方法,对Al-18%Si-5%Fe合金流变铸造与球化处理进行研究。结果表明,WSP流变铸造可以明显改善合金中初晶硅、Al18Si10Fe5和Al8Si2Fe相的形貌。随着浇注温度降低,各相趋于细化和球化。WSP流变铸造过程中合金组织的形成源于3种机制:结晶雨带来的晶核增殖,斜板强冷却作用下晶体抑制生长和流动剪切作用下晶体破碎。WSP流变铸造与球化处理进一步改善Al-18%Si-5%Fe合金组织,合金维氏硬度达到87.5。在干摩擦条件下Al-18%Si-5%Fe合金的磨损率达到5.4 mg/h。

流变挤压铸造7075铝合金的组织及力学性能

以7075铝合金为例,对变形铝合金流变成型的组织和力学性能进行了研究,探讨了采用流变成型方法生产高性能变形铝合金的可行性。试验证明,通过斜坡冷却法直接获得半固态浆料,结合特殊的挤压铸造工艺,获得了组织致密,无微观气孔、微观缩孔的优良工件。通过3个成形比压水平(80MPa,87MPa,95MPa)和3种热处理制度下7075铝合金的组织性能对比,发现流变成形工件的性能远优于同等材质的金属型普通铸造工件,最高抗拉强度达到502MPa,接近于锻件水平,随着比压增大,工件的致密度随之提高,但强度提高不明显。不同热处理对于流变成形7075变形铝合金的抗拉强度影响不明显,但时效作用对于伸长率影响很大。