Semi-solid Metal Research Articles

Preparation of Non-Dendritic Microstructure of AZ31 Alloy Using Isothermal Heat Treatment of the Refined Billet

Semisolid metal forming (SSF) has been regarded as a potential process to shape the wrough Mg alloy base on the thixotropy and low deformation resistance of non-dendritic semisolid slurry. Therefore, it is necessary to develop the slurry production method of wrought Mg alloy as most current researches are focused on the cast alloy. In this paper, the AZ31 billet was refined by the self-inoculation method, and the partialy remelt method was employed to produce the non-dendritic microstructure. Results indicate that both the morphology and size of the primary Mg phase in AZ31 were refined by the grain refining treatment. When the billet were reheated under 595°C and 610°C with different holding times, the morphology of primary Mg phase undergo an evolution pattern from rosette or tiny dendritic to spherical, and with the prolonged holding time, the size of primary Mg phase increased and the shape turn to irregular. Moreover, the result show that the high holding temperature shorten the evolution time from irregular to spherical and the deteriorative grains obtained earlier compared to the low holding temperature.

Microstructural Evolution of Semi-Solid Type 304 Stainless Steel Deformed Severely by ECAP

Thixoforming or semi-solid metal forming offers many advantages in comparison with casting and conventional forging. However, due to the high-melting temperature and related difficulties, there is relatively a few experimental data on the semi-solid processing of steels. Therefore, this study is subjected to study the microstructure evolution during partial remelting of 304 stainless steel which is priorely deformed in solid state by equal channel angular pressing (ECAP) at room temperature. ECAP is a promising technique to induce a great homogeneous strain in bulk metals known as a new method for strain induced melt activation (SIMA) process for preparing thixoforming ingot. Simultaneously, the effect of number of passes during ECAP was also discussed in terms of the microstructure change. The results showed that the microstructure was well refined with formation of fine twins and partial martensite along with many fine equiaxed grains which in turn lead to fine and spheroidal semi-solid slurry. It can be confirmed that ECAP followed by reheating was an effective method to produce semi-solid ingot for thixoforming.

Preparation of Globular Microstructure Aluminum Alloy Using Cooling Slope Casting as Feed Material for Equal Channel Pressing Process

Equal channel angular pressing (ECAP) could be used to achieve ultra fine grains in bulk aluminum alloy through severe plastic deformation. Typically a feed material of as-cast aluminum alloys is used with a typical hypoeutectic solidification structure, consisting of primary aluminum dendrites and interdendritic network of lamellar eutectic silicon. On the other hand, semi-solid metal casting provides non-dendritic and globular microstructure which is one of a considerable factor in obtaining homogenous microstructure after ECAP. This work is an attempt to produce aluminum alloy with globular microstructure using cooling slope semi-solid casting process which is believed suitable as a feedstock for ECAP. The aim of this work described in this paper was to understand of microstructural evolution of aluminum structure during cooling slope casting process. Two experiments were carried out. A sample was casted via a cooling slope into a vertical cold mild steel mould at pouring temperature of 640°C. Cooling slope length of 250 mm, slope temperature of room temperature and tilt angle of 60o was applied. Another sample was casted directly into a vertical cold mild steel mould at pouring temperature of 640°C. The primary α-Al phases in the sample that casted without cooling slope was mostly in dendritic throughout the section of sample whilst the primary α-Al phases transformed completely into non-dendritic in the sample that was casted via the cooling slope. Therefore, the transformation is believed resulted from the effect of cooling slope

Simulation studies on deformation behaviour of AA2017 alloy in semi-solid state using FEA

Thixo-forming is one of the near net shaped manufacturing processes in which the final product is made between the liquidus and solidus temperatures. Forming the product at this temperature range provides large benefits compared to conventional forming methods. In the present work, deformation mechanism of AA2017 alloy in the semi-solid state has been studied. As the mechanical behaviour and deformation mechanism of semi-solid metal is completely different from that of the solid state forming, it is necessary to investigate its nature at the semi solid range. To analyse the metal flow, the stress behaviour and the corresponding strain induced, extensive finite element analysis (FEA) based simulation studies have been performed. The commercial software, DEFORM 2D was used for the simulation. The flow behaviour was modelled by formulating an Arrhenius type constitutive relation.

Effects of forming conditions on homogeneity of microstructure and mechanical properties of A6061 aluminum alloy manufactured by time-dependent rheoforging on a mechanical servo press

The different forming behaviors of the solid and liquid phases in semisolid metal slurry during deformation result in products with inhomogeneous quality. A mechanical servo press with the capability of multistage compression was employed to forge A6061 aluminum alloy in the semisolid state. A time-dependent rheoforging strategy including mechanical stirring, a fast first compression, short holding, and a secondary compression was designed to improve the homogeneity of rheoforged samples. The distributions of the microstructure and mechanical properties, such as the solid fraction, hardness, and deformation resistance, of samples manufactured under various experimental conditions were investigated. When the stirred semisolid A6061 slurry was forged in the temperature range from 625 to 628°C with a short holding time of 4s and the upper die preheated to 300°C, samples with a homogeneous microstructure and mechanical properties were manufactured. The homogeneity of rheoforged samples was attributed to the controllable free motion capability of the mechanical servo press and the adjustable fluidity and viscosity of the semisolid slurry.

Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography.

The behaviour of granular solid–liquid mixtures is key when deforming a wide range of materials from cornstarch slurries to soils, rock and magma flows. Here we demonstrate that treating semi-solid alloys as a granular fluid is critical to understanding flow behaviour and defect formation during casting. Using synchrotron X-ray tomography, we directly measure the discrete grain response during uniaxial compression. We show that the stress–strain response at 64–93% solid is due to the shear-induced dilation of discrete rearranging grains. This leads to the counter-intuitive result that, in unfed samples, compression can open internal pores and draw the free surface into the liquid, resulting in cracking. A soil mechanics approach shows that, irrespective of initial solid fraction, the solid packing density moves towards a constant value during deformation, consistent with the existence of a critical state in mushy alloys analogous to soils.

The Heat Treatment of Rheo-High Pressure Die Cast 6xxx Series Al-Mg-Si-(Cu) Alloys

Limited information is available regarding the heat treatment response of age-hardenable wrought Al-alloys produced by semi-solid metal forming. In this study, the effects of chemical composition, natural pre-ageing and artificial ageing on the T6 properties of rheo-high pressure die cast 6xxx series Al-Mg-Si-(Cu) alloys are investigated. A Cu-free alloy 6082 is compared with Cu-containing alloys 6013 and 6111. The effects of Mg:Si ratio of the alloys are also quantified by means of a comparison between alloys 6111 and 6013. Artificial ageing curves following either no or 5 days of natural pre-ageing are also presented.

SKH51의 반응고 상태에서의 가열 및 성형에 의한 부품 제조

The semi-solid metal forming process has been applied to realize a near-net shape fabrication of a high speed tool steel. A complicatedly shaped part out of SKH51 was successfully manufactured by introducing pertinent materials, tooling and processing conditions. A SKH51 billet with globular grains was heated at temperatures between 1300 and <TEX>$1350^{\circ}C$</TEX> using high frequency induction heater to get semi-solid microstructure before high rate injection of mushy metal into a die cavity for the forming process. It was necessary to control the preheating of dies between 300 and <TEX>$400^{\circ}C$</TEX> to maintain the homogeneous microstructure during the semi-solid metal forming process. Significant defects such as pores, high fraction of liquid fraction and segregation could be removed from the part by using air vents.

Injection Tests and Effect on Microstructure and Properties of Aluminium 7075 Direct Thermal Method Feedstock Billets

The success of semi-solid metal forming is dependent on a globular solid grain formation within a liquid phase. This paper presents experimental works concerning semi-solid metal processing of aluminium 7075 feedstock billets which were produced by direct thermal method. The flowability of feedstock billets was evaluated by an injection test processing unit. The feedstock billets were heated to a temperature of 620 °C by using a box furnace before transferred into a forming die. The formed feedstock billet was removed from the forming die after it was cooled to ambient temperature. Several analyses were conducted on the formed feedstock billets including dimensional measurement and microstructure analysis. The results show that the feedstock billets which contained highest amount of free secondary phase were most successfully formed. Microstructure analysis results also revealed the formation of more globular and larger α–Al solid grains in the same feedstock billets. In this experimental work, the feedstock billets with higher secondary (liquid) phase gives a significant effect to formability. It can be concluded that in order to get successful formability of direct thermal method feedstock billets, the billets need to have higher secondary phase content. Thus, the selection of proper feedstock billets is important to determine the success of the SSM processing.

Alloy Design for Semi Solid Metal Forming

Semi-solid metal forming is more than 40 years old but its full potential to near net shape form high strength aluminium alloys has been realised only to a limited degree. Alloys developed for traditional manufacturing processes were initially used but it became apparent that alloys specific to SSM forming needed to be developed. The main alloy development criteria revolved around SSM processing temperature, solid fraction (fs) versus temperature sensitivity and age hardening potential. This methodology while sound does not fully address the unique processing behaviour of SSM forming. By its very nature SSM requires the controlled solidification of a part of the melt before forming. From basic solidification fundamentals this results in the enrichment of the remaining liquid with alloying elements. During the forming process segregation of liquid phase essentially produces a component with very different compositions in the regions where the liquid solidifies last. From recent work completed on a wide range of standard alloy systems it has become apparent that this segregation effect has a significant impact on aging behaviour and strength. Low melting point structures formed in the these regions result in localised melting in the grain boundary region and along areas of gross liquid segregation during solution heat treatment, contributing to the poor mechanical properties. Although this behaviour can be addressed using modified heat treatment, this cannot be applied to all current alloy systems. Alloy design for SSM forming must take these phenomena into account in order to develop and or specify aluminium alloys with acceptable mechanical properties.

Direct Thermal Method of Aluminium 7075

The evolution of microstructure affect from different pouring temperatures and holding times using a direct thermal method is presented in this paper. The direct thermal method is one of the thermal techniques which are used to produce semi-solid metal feedstock. In this experimental work, aluminium 7075 alloy was used. The experiments were carried out by processing a sample with a 0.7 °C/s cooling rate to evaluate the formation of the microstructure. In direct thermal method experiment, a molten 7075 was poured into a cylindrical copper mould at different pouring temperatures of 680 °C and 660 °C meanwhile the holding time of 20 s, 40 s and 60 s before quenched into room temperature water. The sample processed by the cooling rate of 0.7 °C/s produced a large microstructure. The formation of a spheroidal microstructure was obtained with the combination of a suitable pouring temperature and holding time. The pouring temperature of 665 °C and the holding time of 60 s produced a finer and uniform microstructure that is suitable for semi-solid feedstock.

Investigation of Dissimilar Joint between 356 and 7075 of Semi-Solid (SSM) Aluminum Alloy by Friction Stir Welding

The objective of this research was to investigate the effect of parameters on the microstructure and mechanical properties of friction stir welded butt joints of dissimilar aluminum alloy sheets between Semi-Solid Metal (SSM) 356 and 7075. The base metal of SSM 356 and 7075 was located on the advancing side and the retreating side, respectively. The cylindrical pin was used as the welding tool geometry. Friction stir welded using different tool rotation speed (710, 1000 and 1400 rpm) and welding speed (80, 112 and 160 mm/min). The result indicated that the maximum average tensile strength of 205.13 MPa was achieved for the joint produced at tool rotation speed 1000 rpm, welding speed 160 mm/min. In addition, the metallurgy and welding structures of combined two types aluminium showed more definite than the base metal.

EFFECT OF POURING TEMPERATURE AND WATER COOLING ON THE MICROSTRUCTURE OF INGOTS PRODUCED BY COOLING SLOPE CASTING TECHNIQUE

ABSTRACTThe cooling slope (CS) casting is the simplest and cheapest technique for producing feedstock materials with non-dendritic microstructure required for semi-solid metal (SSM) processing methods such as thixoforming. In the present investigation, the effect of the pouring temperature and the water cooling on the thixotropic microstructure of commercial A319 Al-Si cast alloy was studied. The results showed that increasing the pouring temperature slightly reduces the bulk porosity of the CS ingots. The ingots poured with water-cooling exhibited slightly lower porosity content than those poured with without water-cooling. Generally, it has been found that the primary α-Al grains have generally higher shape factor near the edge of ingot than the middle and center. Ingots poured with water-cooling exhibited lower grain size and shape factor than those poured without water-cooling. Increasing the pouring temperature increases the size α-Al grains. Ingots poured with water-cooling exhibited higher hardness values than those poured without water-cooling.

The Effect of Direct Thermal Method, Temperature and Time on Microstructure of a Cast Aluminum Alloy

The direct thermal method is used for the creation of globular microstructures suitable for semi-solid metal forming. In this paper, both simulation and experimental results using direct thermal method are presented. ProCAST® software was used to estimate temperature distribution inside the aluminum billet. In validation work, molten aluminum A356 was poured into metallic copper tube molds and cooled down to the semi-solid temperature before being quenched in water at room temperature. The effect of pouring temperatures of 630°C, 650°C, 665°C, 680°C and holding times of 45s and 60s on the microstructure of aluminum A356 alloy were investigated. The simulation results showed that the average temperature rate within the copper mold, from initial pouring temperature to just before quenching, was approximately 1°C/s. Examination of the solidified microstructures showed that the microstructure was more spherical when lower pouring temperatures and holding periods were used. From the micrographs it was found that the most globular and smallest structures were achieved at processing parameters of 630°C and 45s.

Thin-Plate Forming by Thixo- and Rheoforging

Thin plates with a thickness of 1.2 mm are fabricated using two processes, thixoforging and rheoforging, which are semisolid forming techniques. The die design, formability, microstructure, and mechanical properties of the fabricated thin plates are analysed. A fan-shaped gate is designed by analysing the filling behaviour using semisolid material, and uniform filling behaviour of material is obtained by arranging nine overflows in product area. semisolid metal is prepared through a semisolid process in which reheating, a thixoprocess, and cooling with stirring, a rheoprocess, are applied. The semisolid material is injected into a forging die and is formed into thin plate at a punch speed of 300 mm/s and under a pressure of 100 MPa. Since semisolid material with a solid fraction below 45% has mainly small primary α-Al particles, the formability of the thin plate is improved. The formed thin plate also has good mechanical properties since the small and globular grains are evenly distributed. The thin plate formed from semisolid material with a solid fraction above 50% has poor mechanical properties owing to the large quantity of coarse primary α-Al particles. A rheoforged thin plate exhibits poorer mechanical properties than a thixoforged thin plate, but rheoforging produces a more precise thin plate.

Fabrication of Porous Aluminum Alloys with Aligned Unidirectional Pores by Dipping Pipes into Liquid and Semi-solid base Metals

The new fabrication method to fabricate porous aluminum alloys with aligned unidirectional pores was invented by dipping pipes into semi-solid slurry. Pure aluminum pipes were dipped into Al-4mass% Cu liquid and semi-solid slurry of base metal in an electric furnace. The base metal and dipped pipes were cooled and solidified together in the air. The pipes were bonded to the base metal and the original shape of the pipes was maintained by using the semi-solid slurry of the base metal. However, pipes are usually ruptured and buried in the liquid base metal during the process. On the other hand, it is possible to avoid the rupturing of pipes by using the semi-solid slurry because thermal damage of pipes can be reduced by an increase in solid fraction of the base metal. The analysis results of Electron Probe MicroAnalyser showed that Cu diffused into grain boundaries of pipes, which decreased the melting temperature of surface of pipes. As a result, the α phase melted partly and then the pipes were bonded with the base metal metallurgically. This dipping method can fabricate porous Al-Cu alloys with aligned unidirectional pores with porosity of 33% when nineteen pipes are used.

Effect of Heat Treatment T6 on the Friction Stir Welded SSM 6061 Aluminum Alloys

The butt joints of semi solid metal 6061 were produced in as cast conditions by friction stir welding process (FSW). This experiment studied in pre/post heat treatment (T6) using the welding speed 160mm / min with tilt angle tool at 3 degree and straight cylindrical tool pin. The factors of welding were rotating speed rates at 710, 1000, 1400rpm and heat treatment conditions. They were divided into (1) As welded (AW) joints, (2) T6 Weld (TW) joints, (3) Weld T6 (WT) joints, (4) T6 Weld T6 (TWT) joints, (5) Solution treated Weld Artificially aged (SWA) joints and (6) Weld Artificially aged (WA) joints. Rotating speed and heat treatment (T6) condition were an important factor to micro, macro structure of metal and mechanical properties of the weld. Increasing rotating speed and different heat treatment condition impacted onto tensile strength due to the defects on joints. Therefore the optimum welding parameter on joint was a rotating speed 1400rpm, the welding speed 160mm/min, heat treatment condition of Solution treated Weld Artificially aged (SWA) which obtained the highest tensile strength 179.80MPa, as well as, the maximum average hardness of 92.7 HV at tool rotation speed 1400rpm, welding speed 160mm/min, heat treatment condition of Weld T6 (WT).

Investigation on Mechanical Properties and Microstructure of SSM 356-T6 Aluminium Alloy by Diffusion Bonding

SSM 356-T6 aluminium alloys generally present low weldability by fusion methods because of the sensitivity to weld solidification cracking, porosities, change microstructure in weld zone and other defects in the fusion zone. Diffusion bonding can be deployed successfully with aluminium alloys. This paper presents the technique to conserve the globular weld structure of SSM 356-T6 aluminium alloy. The effect of joining parameters on the microstructure and mechanical properties of diffusion bonding butt joints of semi-solid metal 356-T6 aluminium alloy were investigated by conditions as follows: contact pressure at 0.4, 0.9, 1.8, 2.4 and 2.7 MPa, for 3 hours holding time and temperature at 495°C under argon atmosphere at 4 liters per minute. The results showed that condition used contact pressure 2.4 MPa, with 3 hours holding time and temperature at 495°C, under argon atmosphere provided. The highest joint strength reaching to 182.2 MPa, which had joint efficiency of 61.34 percents compared with base material. In addition, microstructure in welded zone after welding is still in globular structure, but the grain size was increased when the higher bonding temperature was used. The results of this investigation have shown that an average hardness is around 121.2 HV.

Friction Stir Processing of SSM356 Aluminium Alloy

The aim of this experiment was to improve the mechanical properties of SSM 356 aluminum alloys by friction stir processing, a solid-state technique for microstructural modification using the heat from a friction and stirring. The parameters of friction stir processing for SSM 356 aluminum alloys were studied at three different travelling speeds: 80, 120 and 160mm/min under three different rotation speeds 1320, 1480 and 1750rpm. The hardness and tensile strength properties were increased by friction stir processing. The hardness of friction stir processing was 64.55 HV which was higher than the base metal (40.58 HV). The tensile strengths of friction stir processing were increased about 11.8% compared to the base metal. The optimal processing parameter was rotation speed at 1750rpm with the travelling speed at 160mm/min. Consequently, the application of the friction stir processing is a very effective method for the mechanical improvement of semi-solid metal aluminum alloys.

Modelling and Simulation of Die Casting Process for A356 Semi-solid Alloy

To describe the flow behaviors of semi-solid state alloy A356, two non-Newtonian constitutive equations are modeled in this paper. All the parameters in the above equations are obtained by an isothermal shear experiment. The CFD software PROCAST is used to simulate the process of die filling. The filling temperature is 585 oC (solid fraction is 0.4). A Newtonian type fluid which stands for the traditional casting is also performed. Based on the results of the simulation, it is found that the material in semi-solid state exhibits an apparently higher viscosity and thus, flows much more smoothly than the Newtonian fluid. In addition, the semi-solid metal alloy has a special die filling behavior compared with liquid filling which is of great importance in improving the quality of the final components.