Grain refinement offers several benefits in aluminum casting applications. Two methods are normally used to achieve a grain-refined microstructure: inoculation and dynamic nucleation. Inoculation is widely applied in industry, but is not an efficient process. Dynamic nucleation, achieved by application of localized forced convection with rapid cooling, is an alternative process. However, a deeper understanding of dynamic nucleation is required if this process is to be used commercially. This study aims to understand the grain refinement behavior of an aluminum alloy under the influences of inoculation and dynamic nucleation. A rapid quenching method was used to investigate the combined effects of inoculation and dynamic nucleation on the solid fraction, particle density and particle size of the secondary nuclei. In addition, the effects of the particle density of the secondary nuclei on the final cast microstructure were studied. The rapid quenching results show that dynamic nucleation by application of forced convection with localized cooling to the melt yields an increased solid fraction and particle density of secondary nuclei. The solid fraction and particle density are further increased by inoculation. This study also shows that increasing the convection level in an inoculated melt held at a temperature slightly above the liquidus temperature increases the effectiveness of dynamic nucleation, which consequently yields a finer microstructure of the final cast samples. The findings suggest that grain refinement can be effectively achieved by applying forced convection with localized cooling to create a low fraction solid of secondary nuclei in the melt prior to pouring and casting.
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Selected Journal Papers
Understanding the flow behavior of semi-solid slurries containing low solid fractions is key to the success in applying this process in the die casting industry. With these low initial solid fractions, the flow behavior of semi-solid slurries is quite complicated, making it difficult to model accurately. This present work developed and studied characterization methods for the flow behavior of semi-solid slurries at low solid fractions in high-pressure die casting. A new parameter, the ratio of gate speed to initial solid fraction (Vg/fs), was proposed to be correlated to the normalized flow interface length, blister area and tensile properties. Results from the flow pattern analysis suggest that the flow behavior can be controlled to achieve laminar flow by varying the initial solid fraction. Blister test results show the trend that slurry die casting conditions with high Vg/fs values exhibit high blister areas. Die casting conditions with excessively high gate speeds and insufficient solid fractions result in turbulent flow patterns and high levels of blister defect. The results of tensile test and fracture surface analysis are consistent with other analysis results. The samples formed by liquid die casting and slurry die casting with high Vg/fs values have gas porosity due to turbulent flow pattern during die filling. On the other hand, the samples formed by slurry die casting with too low Vg/fs values contain shrinkage porosity. This is because of insufficient time for shrinkage feeding due to a combination of a high solid fraction and a low gate speed. This study has demonstrated that die casting with slurries containing low initial solid fractions gives die casters another process parameter to adjust, which can help reduce and control the gas and shrinkage porosities.
Semi-solid slurry casting is a metal-forming process that involves transforming liquid metal into slurry having a low solid fraction and then forming the slurry into solid parts. To successfully apply this slurry-forming process, it is necessary to fully understand the flow behavior of semi-solid slurries. This present work applied the rapid quenching method and the modified gravity fluidity casting to investigate the flow behavior, which involves characterizations of the initial solid fraction, fluidity, and microstructure of semi-solid slurries. Three commercial aluminum alloys were used in this study: 383 (Al-Si11Cu), 356 (Al-Si7MgFe), and 7075 (Al-Zn6MgCu) alloys. The results show that the initial solid fractions can be controlled by varying the rheocasting time. The rapid quenching mold can be used to determine the initial solid fractions. In this method, it is important to apply the correcting procedure to account for growth during quenching and to include all the solid phases. Results from the fluidity study of semi-solid slurries show that the fluidity decreases as the initial solid fraction increases. The decrease is relatively rapid near the low end of the initial solid fraction curves, but is quite slow near the high end of the curves. All the three alloys follow this trend. The results also demonstrate that the slurries that contain high solid fractions of up to 30 pct can still flow well. The microstructure characterization results show that the solid particles in the slurries flow uniformly in the channel. A uniform and fine microstructure with limited phase segregation is observed in the slurry cast samples.
A rheocasting technique called the Gas Induced Semi-Solid (GISS) is being developed for commercial applications in Thailand. The process creates semi-solid metal slurries by applying the injection of fine inert gas bubbles through a graphite diffuser to induce localized convection and heat extraction. The slurries are then formed into parts using different casting processes such as die casting, squeeze casting, gravity casting, and semi-solid infiltration process. This paper reports some of the current applications of the GISS forming processes, including prosthetic adaptors, lapping plates, sacrificial anodes, and armor plates. Preparation of semi-solid slurries of the alloys used in these applications, which are aluminum 356, Sn-Sb, Al-Zn-In, and 7075 alloys, is also reported and discussed.
A key success of semi-solid metal forming by the rheocasting route is having an efficient and effective technique to prepare semi-solid slurries. To achieve that it is necessary to be able to characterize the microstructure during the early stages at different solid fractions as the slurry is being cooled. This present work applied the rapid quenching method to study microstructure evolution during the early stages in a rheocasting process. Nine stages of microstructure evolution were characterized by applying rheocasting times of 5, 10, 12, 15, 20, 30, 35, 40 and 45 s during cooling. Analysis to evaluate the solid fraction, particle density, particle size, particle shape factor, and particle distribution was performed. The results show that the relationship between the solid fraction and the rheocasting time can be well fitted to a quadratic equation. For particle density, the relationship is better fitted to a linear equation. The change in particle size with time can be modeled using a ripening model with an upper bound value for the ripening coefficient. In addition, the particle distribution may be quantified by the dilation and counting technique to determine different levels of particle clustering. The results from this study suggest that this new characterization method can be used as a process and quality control tool in the rheocasting process. It can also be used to optimize the process and to study the mechanism of the rheocasting technique.
Several rheocasting processes are developed or applied worldwide in the metal forming industry. One of the new rheocasting processes is the gas induced semi-solid (GISS) process. The GISS process utilizes the principle of rapid heat extraction and vigorous local extraction using the injection of fine gas bubbles through a graphite diffuser. Several forming processes such as die casting, squeeze casting, gravity casting, and rheo-extrusion of the semi-solid slurries prepared by the GISS process have also been conducted. The GISS process is capable of processing various alloys including cast aluminum alloys, die casting aluminum alloys, wrought aluminum alloys, and zinc alloys. The GISS process is currently developed to be used commercially in the industry with the focus on forming semi-solid slurries containing low fractions solid (< 0.25) into parts. The research and development activities of the GISS process were discussed and the status of the industrial developments of this process was reported.
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 rheological 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.
The feasibility of semi-solid die casting of ADC12 aluminum alloy was studied. The effects of plunger speed, gate thickness, and solid fraction of the slurry on the defects were determined. The defects investigated are gas and shrinkage porosity. In the experiments, semi-solid slurry was prepared by the gas-induced semi-solid (GISS) technique. Then, the slurry was transferred to the shot sleeve and injected into the die. The die and shot sleeve temperatures were kept at 180 °C and 250 °C, respectively. The results show that the samples produced by the GISS die casting give little porosity, no blister and uniform microstructure. From all the results, it can be concluded that the GISS process is feasible to apply in the ADC12 aluminum die casting process. In addition, the GISS process can give improved properties such as decreased porosity and increased microstructure uniformity.
The semi-solid metal forming using high pressures has been applied for several years. In contrast, low pressure casting, such as gravity sand casting, has not been widely studied even though it may help reduce porosity defects and offer a better casting yield. A semi-solid gravity sand casting process using the Gas Induced Semi-Solid process was investigated. The results show that the process can produce complete parts with no observable defects. The ultimate tensile strength and elongation data of semi-solid cast samples are higher than those of the liquid cast samples. In addition, the semi-solid sand casting process gives a better casting yield. It can be concluded that the semi-solid sand casting of an aluminum alloy using the GISS process is a feasible process
The gas induced semi-solid (GISS) is a rheocasting process that produces semi-solid slurry by applying fine gas bubble injection through a graphite diffuser. The process is developed to be used in the die casting industry. To apply the GISS process with a die casting process, a GISS maker unit is designed and attached to a conventional die casting machine with little modifications. The commercial parts are developed and produced by the GISS die casting process. The GISS die casting shows the feasibility to produce industrial parts with aluminum 7075 and A356 with lower porosity than liquid die casting.
A simple and efficient rheocasting process that has recently been invented is being developed for aluminum die casting applications. The process called Gas Induced Semi-Solid (GISS) utilizes the combination of local rapid heat extraction and agitation achieved by the injection of fine gas bubbles through a graphite diffuser to create semi-solid slurry. In the GISS process, the die casting machine and the process cycle remain little changed from those of conventional die casting. The GISS unit creates a low solid fraction of semi-solid slurry in the ladle during the ladle transfer to the shot sleeve. The semi-solid slurry is then poured directly into the shot sleeve. This paper presents the detailed description of the process. The results of the semi-solid die casting experiments with ADC10 alloy using the GISS process are also reported and discussed.
A new approach to evaluate the solid fraction of semi-solid slurries is reported. The approach applies a thin-channel vacuum mold to rapidly quench semi-solid slurries at different rheocasting times. The slurry temperatures are analyzed from the cooling curves. The corresponding solid fraction data are obtained from standard quantitative metallography. A correction of the data is then conducted using the average growth layer of the solid particles. The analyzed solid fraction agrees well with the data from the Scheil model.
A new technique to achieve grain refinement using gas bubbles to agitate a molten metal during solidification is reported. The effects of the processing conditions on the degree and homogeneity of the grain refinement are discussed. Results show that a fully grain-refined structure can be obtained using suitable processing conditions
Various processing methods exist for applying agitation to a molten metal during solidification to obtain metal slurries suitable for semi-solid metal processing. . In this paper, a new technique to achieve semi-solid metal structure using agitation during solidification is reported. The technique applies a new medium and means to efficiently create semi-solid metal structures. The results of a systematic study showing the feasibility and the necessary conditions to achieve the structure are discussed.
Aluminum-copper alloys offer both high strength and excellent ductility suitable for a number of automotive applications to reduce vehicle weight; however, the alloys are difficult to cast because of their tendency for hot tearing. In this work, semi-solid gravity casting of an aluminum-copper alloy, B206, was conducted in constrained rod casting molds to study the feasibility of using the process to reduce or eliminate hot tearing. To demonstrate the feasibility of gravity casting of the metal slurries, a fluidity test was also conducted. Results show that the hot tearing susceptibility of the aluminum-copper B206 alloy cast in semi-solid state is lower than those cast in liquid state with high superheat temperatures. The grain size of the semi-solid cast Al-Cu samples appears to be finer than those cast in liquid state with high superheat temperatures. In addition, the metal slurries had sufficient fluidity to fill the molds even with low gravity pressures. The results suggest that semi-solid gravity casting is a feasible process to help reduce hot tearing.
Other Journal Papers
Aged hardening of semisolid cast A356 Al alloy produced by gas induced semi-solid (GISS) process was studied. It was found that maximum hardness and tensile strength could be achieved from specimens aged at 165 °C for 18 h of which the average maximum hardness, the average ultimate tensile strength and the average percent elongation were 96.4 HRE, 312 MPa and 7.6%, respectively. The higher aging temperature of 195 °C for 3 h led to a slightly lower average tensile strength of 305 MPa together with a higher average elongation of 9.8%. The strain hardening exponent of specimens aged at both sets of conditions was lower than that of the as-cast specimen as well as the as-cast specimen aged at 225 °C for 15 min. The mechanical properties of the alloys in this study were comparable to those of typical thixoformed products. β″ phase was mainly responsible to the strengthening of the peak aged alloy. Elongated precipitates were formed in the specimen after prolonged aging at 195 °C for 16 h. The activation energy for the precipitation hardening process of the alloy derived in this research was 128,717 J/mol.
The creep rupture behavior of semi-solid cast 7075-T6 Al alloy produced by the Gas Induced Semi-Solid (GISS) process was investigated and compared to that of commercial 7075-T651 Al alloy. The semi-solid cast 7075-T6 Al alloy displayed lower minimum creep rate and longer creep rupture time than the commercial 7075-T651 Al alloy. On the basis of their stress exponent, n, values of 6.3, dislocation creep was seemingly the predominant mechanism controlling the creep deformation of both alloys. The creep rupture time of the semi-solid cast 7075-T6 Al alloy was distinctly longer than that of the commercial 7075-T651 Al alloy at stress regimes of 120–140 MPa. This difference was attributed to the lower precipitate coarsening and higher precipitate density in the semi-solid cast alloy. Creep cavities predominately controlled the creep rupture of the semi-solid cast 7075-T6 Al alloy despite the appearance of precipitate coarsening. The commercial 7075-T651 Al alloy creep rupture behavior was controlled by the combination of rapid precipitate coarsening and creep cavities. However, de-cohesion between insoluble particles and the matrix is evidently accelerated with increasing stress to 180 MPa, leading to cavity propagation and resulting in the convergence of creep rupture time in the semi-solid cast 7075-T6 Al alloy to that of the commercial 7075-T651 Al alloy.
Effects of solution heat treatment and age hardening on the microstructures and mechanical properties of rheocasting 7075 Al alloy produced by a novel technique, Gas Induced Semi-Solid (GISS) technique, were studied. This work reveals that the optimum solution heat treatment condition for the non-dendritic structured 7075 aluminium alloy was 450 °C for 4 h. Age hardening was performed at temperatures of 120 °C, 145 °C, 165 °C, and 185 °C under various time durations. The peak aging condition was the artificial aging at 120 °C for 72 h, at which a highest tensile strength of 486 MPa with 2% elongation was recorded. This higher strength was caused by higher number density and finer precipitate size of η/ phase than other aging temperatures. The main hardening phase was identified to be the η/ phase while early nucleation of η phase in the higher aging temperature specimens resulted in lower strengths of the alloy. The activation energy for the precipitate hardening process of the alloy derived in this research was 95,827 J/mol.
For many metal alloys, the process of metal cutting is accompanied by extensive plastic deformation and fracture. To study this process, quick stop sectional samples of hypoeutectic Al-Cu alloy chip formation, either as conventionally cast alloy or as "semi solid metal" are used. The type of chip formation is classified according to crack formation mechanism and propagation. During cutting, in all specimens used, quasi-continuous chips with built-up edge (BUE) are obtained. The formation of BUE is undesirable since it is a highly deformed body with a semi stable top which periodically breaks away giving rise to poor workpiece surface quality.
Effect of the two-step solution heat treatment on the microstructure of semisolid cast 7075 aluminium alloy has been studied. The microstructure of the as-cast specimens mainly consisted of matrix-α (Al) and grain boundary (GB)-eutectic phase (α-Al + Mg(Zn,Cu,Al)2). After solution treating, coarse black particles were found to form in the single-step solution treated specimens at the condition of 450 °C for 8 h and 480 °C for 1 h, respectively. Two-step solution heat treatment resulted in the reduction of coarse black particle formation while maintaining the same amount of eutectic MgZn2 phase dissolution as the high temperature single-step solution treatment. Therefore, the two-step solution heat treatment enables alloying elements dissolved into the matrix without overheating and hence decreases coarse black particles. The optimum two-step solution heat treatment condition derived from this study was 400 °C for 8 h + 450 °C for 4 h.
The aim of this study is to determine the appropriate solution treatment temperature and time of semi solid 2024 Al alloy. Solution heat treatment at 450°C and 480 °C for various times, from 4 hours to 16 hrs, were applied followed by artificial aging at 220 °C for 1 hr. Microstructure of the semi solid cast 2024 aluminum alloy mainly showed globular grain structure which consisted of matrix-α (Al) and grain boundary (GB) - eutectic phases (α+Al2CuMg/Al2Cu). Eutectic GB phases was found to completely dissolved after solution heat treatment at 480°C for 14 hrs while sample solution treated at 450°C for the same time showed the existence of remaining GB phases. Prolonging heat treatment after 14 hrs at both temperatures resulted in the formation of coarse black particles at the grain boundaries which were identified as Mg2Si phases. Therefore the suitable solution treatment of the alloy in this study was at 480°C for 14 hrs.
Influence of temperature and time of solution heat treatment on the microstructures of rheo-casting 7075 aluminium alloy produced by a novel technique, the Gas Induced Semi Solid (GISS) technique, had been investigated in this study. The microstructure of the as-cast specimens mainly consisted of matrix-α (Al) and grain boundary (GB)-eutectic phase (α-Al + Mg(Zn,Cu,Al)2). After solution heat treatment at 480 °C for 1 h, MgZn2 phase at the grain boundary was observed to have dissolved and coarse black particles of Mg2Si were observed to form in the matrix. In comparison, when solutionizing temperature of 450 °C was applied, it took 4 h of solution treatment time in order to dissolve the same portion of GB phase and MgZn2 phase, and coarse black particles of Mg2Si were found to form in the 8 h solution treated sample.
A new approach to evaluate fluidity of semi-solid rheo-slurries was developed. The equipment was designed in order to reduce pouring error by using bottom tapping and heated tapping ladle. Commercial AC4C aluminum alloy slurries were tested in spiral sand mold by gravity casting. The slurries were prepared by introducing fine gas bubbles into molten metal above the liquidus temperature at different rheocasting times. Average fluidity and microstructures of cast spirals were reported. Results show that the spiral microstructure is non-dendritic and the fluidity of AC4C alloy decreases with increasing solid fraction. In conclusion, the spiral casting method using bottom tapping can be used to evaluate the fluidity of rheocast slurries in gravity sand casting.
An aluminum extrusion process is mainly used to fabricate long tubes, beams and rods for various applications. However, this process has a high production cost due to the need for investment of high-pressure machinery. The objective of this work is to develop a new semi-solid extrusion process using semi-solid slurry at low solid fractions. A laboratory extrusion system was used to fabricate aluminum rods with the diameter of 12 mm. The semi-solid metal process used in this study was the gas induced semi-solid (GISS) technique. To study the feasibility of the GISS extrusion process, the effects of extrusion parameters such as plunger speed and solid fraction on the extrudability, microstructure, and mechanical properties of extruded samples were investigated. The results show that the plunger speed and solid fraction of the semi-solid metal need to be carefully controlled to produce complete extruded parts.
The gas induced semi-solid (GISS) process was developed to create semi-solid slurry with fine and uniform globular structure. The combination of local rapid heat extraction and vigorous agitation by the injection of fine inert gas bubbles through a graphite diffuser in molten metal held at a temperature above its liquidus temperature changes the morphology of primary a(Al) from coarse dendritic to rosette-like and finally to fine globular. The GISS process produced semi-solid slurry at low solid fractions and then formed the slurry by a squeeze casting process to produce casting parts. The effects of primary phase morphology on the mechanical properties of Al-Si-Mg-Fe alloy were investigated. The results show that the ultimate tensile strength and elongation are affected by the shape factor and particle size of the primary a(Al).