Semi-solid Metal Research Articles

Investigation of Semisolid Welding Characteristics of 2A14 Aluminum Alloy

Semi-solid processing of metals is becoming popular as a new potential manufacturing technology for parts and components in automobile, electronic and machine industries.Up to now semisolid welding technology have not been used in industrial applications. A new process of semisolid metals welding is proposed; the process shows excellent formability and joining ability of semisolid metals, and differs from the process of melt welding or mechanical joining with screws or nails. In the process, semisolid 2A14 aluminum alloy was joint together and was rapidly solidified under pressure. The heat affected zone of the joint was decreased and the grain size of the joint was become small. The application range of the process is wide, i.e., from the electronic industry to arts. The advantages of this new semisolid welding process are (1) ease of joining, (2) good joining microstructure, and (3) ease of carrying out mass production.

Microstructure Formation and Mechanical Properties in AZ31 Alloy Processed by Continuous Rheo-Extrusion

Based on continuous casting and extrusion (CAXTEX) process, a semisolid metal forming process, continuous rheo-extrusion of magnesium alloy, was proposed. Effect of casting temperature on semisolid region distribution, microstructure formation and stability of forming process, as well as microstructure and mechanical properties of the AZ31 alloy fabricated by the process were investigated. Microstructure evolution from dendrite to rosette or spherical grains was observed with the application of large shearing force provided by the roll. The results indicate that semisolid region in the roll-shoe gap moves downward gradually with the increase of casting temperature, and proper casting temperature range of 730~750°C is suggested. Under the suggested casting temperature, 10×15mm sectional bar of AZ31 alloy with smooth surface and homogeneous striped microstructure has been obtained. As the product was aged for 14h at 180°C after 16h solution at 415°C, the ultimate tensile strength and elongation could reach 305MPa and 11.5%, respectively.

A Constitutive Model for Thixotropic Plastic Forming of Composites

A new constitutive model for semi-solid composites forming was derived with analyzing constitutive relationships of semi-solid metal and composites forming. Parameters in the constitutive model were determined by the multiple nonlinear regression method. A constitutive relationship of semi-solid SiCp/AZ61 composites thixotropic plastic forming was proposed. The calculated results were good agreement with the experimental ones. The proposed constitutive model has the higher forecast precision and practical significance. The construction of constitutive model provides references for the thixotropic forming theory, simulation and technology.

Study on Filling Capacity of Semi-Solid Magnesium Alloy

The Semi-Solid Processing technology is an innovation manufacturing process. The filling capacity of the semi-solid metal has very important effects on the properties and qualities of the work pieces. Lots of heat factors have effects on the filling capacity of semi-solid metal. The filling capacity of semi-solid AZ91D magnesium alloy has been studied by means of fluidity test. The results indicate that the filling length increases with the increasing of the temperature and the filling capacity of semi-solid magnesium alloy with shearing is better than that of without shearing. The lower temperature, the more obvious the effect of shearing on the filling capacity is. However, if the temperature is very low, the flow of the slurry is poor and the effect of shearing on filling capacity decreases. The microstructures of semi-solid magnesium alloy after shearing is spherical and homogeneous.

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 RHEOMETAL™ 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.

Observation of interface of two kinds of bi-metal composite parts prepared by thixo-forging

Two kinds of bi-metal composite parts (Sn-15%Pb and Pb-22%Sn bi-metal system, and Al-7%Si and SiC p/6061 MMC bi-metal system) were prepared by the strain-induced melt activated thixo-forging. The interfaces of the bi-metal composites were observed by OM and SEM. The observations show that the semisolid metals keep independence during thixo-forging. The solid phases in the semisolid slurries maintain their original morphologies after thixo-forging. The liquid phases near the interface mix together and form a thin layer. The interfaces are bonded firmly with the metallurgical bonding. No oxide layers are found at the interfaces. Strengths of the interfaces were investigated by the micro-hardness test. The experimental results show that the composite interfaces have high strength. However, the agglomerated enhancing particles cause fine defect on the interface of the Al-7%Si and SiC p/6061 MMC bi-metal composite.

Development of aluminum rheo-extrusion process using semi-solid slurry at low solid fraction

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.

Application of shortened heat treatment cycles on A356 automotive brake calipers with respective globular and dendritic microstructures

Since the automotive industry has many possible applications for semi-solid metal (SSM)-high-pressure die casting (HPDC) parts, the newly developed heat treatment cycles, as well as the traditional heat treatment cycles, were applied to A356 brake calipers cast using a LK DCC630 HPDC machine. Vickers hardness measurements at a cross section of the brake calipers were performed, indicating that similar values can be obtained when using the significantly shorter heat treatment cycles. Finally, the typical tensile properties that can be obtained for SSM-HPDC A356 brake calipers are compared with those manufactured by gravity die casting. Results indicate that the differences in microstructures (globular or dendritic) do not have a noteworthy effect on the heat treatment response. This implies that the short heat treatment cycles originally developed for globular SSM-HPDC A356 castings can be successfully applied to dendritic liquid A356 castings too.

Analysis of semi-solid response under rapid compression tests using multi-scale modelling and experiments

Simulating semi-solid metal forming requires modelling semi-solid behaviour. However, such modelling is difficult because semi-solid behavior is thixotropic and depends on the liquid-solid spatial distribution within the material. In order to better understand and model relationships between microstructure and behavior, a model based on micromechanical approaches and homogenisation techniques is presented. This model is an extension of a previous model established in a pure viscoplastic framework to account for elasticity. Indeed, experimental load-displacement signals reveal the presence of an elastic-type response in the earlier stages of deformation when semi-solids are loaded under rapid compression. This elastic feature of the behaviour is attributed to the response of the porous solid skeleton saturated by incompressible liquid. A good quantitative agreement is found between the elastic-viscoplastic predicted response and the experimental data. More precisely, the strong initial rising part of the load-displacement curve, the peak load and the subsequent fall in load are well captured. The effect of solid fraction on mechanical response is in qualitative agreement with experiments.

Influence of surface liquid segregation on corrosion behavior of semi-solid metal high pressure die cast aluminium alloys

The corrosion behavior (in chloride medium) of the surface layer of SSM-HPDC plates of alloys 7075-T6 and 2024-T6 was compared with that of the wrought alloys 7075-T6 and 2024-T6. Potentiodynamic testing was performed in deaerated 3.5%NaCl solution. In separate tests, the open-circuit potential was monitored in aerated 3.5% NaCl for 30 min after immersion. The electrochemical tests show that there is no significant difference in the pitting potential of the wrought alloys and that of the Cu-enriched surfaces of the SSM-HPDC alloys.

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 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.

Semi-solid gravity sand casting using gas induced semi-solid process

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.

Influence of elevated Fe, Ni and Cr levels on tensile properties of SSM-HPDC Al-Si-Mg alloy F357

The microstructures and tensile properties of semi-solid metal high pressure die cast (SSM-HPDC) F357 alloys with low and high levels of Fe, Ni and Cr were compared in different temper conditions. ThermoCalc software was used to predict the different intermetallics that can be expected in the alloys, and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was used to investigate the actual intermetallics that formed. The influence of these intermetallics on tensile properties was quantified. The results show that lower strength is obtained in the alloy with high Fe, Ni and Cr levels. This is attributed mainly to the formation of more ?-Al 8FeMg 3Si 6 phase, which removes strengthening Mg atoms from solid solution. Also, the ductility of the high Fe, Ni and Cr levels alloy is decreased significantly due to microcracking of the higher volume fraction ?-Al 8FeMg 3Si 6 and Al 9FeNi phases. The combination of lower strength and ductility results in a decrease of the quality index of this alloy compared with the alloy with low levels of Fe, Ni and Cr.

Conception of tooling adapted to thixoforging of high solid fraction hot-crack-sensitive aluminium alloys

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.7< ϕ s<1), which involves the processing of alloys in the semi-solid state. Tooling has to be adapted to this particular process to benefit shear thinning and thixotropic behaviour of such semi-solid material. Tooling parameters, such as the forming speed and tool temperature, have to be accurately controlled because of their influence on thermal exchanges between material flow and tool. These thermal exchanges influence the high-cracking tendency and the rheology of the semi-solid material during forming, which affects parts properties and therefore their quality. Extrusion tests show how thermal exchanges influence quality of thixoforged parts made of 7075 aluminium alloys at high solid fraction by modifying process parameters like forming speed, tool temperature and tool thermal protector. Thus an optimum in terms of thermal exchanges has to be found between surface quality and mechanical properties of the part. A direct application is the evaluation of surface quality of thixoforged thin wall parts made of 7075 aluminium alloy.

Numerical Simulation on Semi-Solid Metal Slurry Preparation by A-EMS

A computational model coupling electromagnetic stirring with a macroscopic heat and fluid flow in semisolid A357 alloy slurry preparation by A-EMS was developed. Effects of electromagnetic field on flow field and temperature field were presented successfully. It is shown that more intensive magnetic field, velocity field and thereby temperature field were uniformly distributed in the stirred melt even at commercial frequency, and thus more uniformly fine microstructures were obtained in comparison with normal EMS.

Benefits of thixoforging hot-crack-sensitive aluminium alloys at high solid fraction

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.7<fs<1), which involves the processing of alloys in the semi-solid state. The use of semi-solid material with globular microstructure helps to decrease forming load. But the introduction of a liquid phase in the material is a source of defects that limit mechanical properties of thixoforged parts. One of these defects is the hot cracking for some hot crack-sensitive wrought aluminium alloys. Hot cracking occurs because of strain and deformation during last step of solidification. This paper shows how thixoforging process should help to lower hot cracking tendency without modification of the alloy composition by decreasing strain and deformation due to shrinkage. Different experimental tests by extrusion of 7075 aluminium alloys are used.

Comparison of heat treatment response of semisolid metal processed alloys A356 and F357

The heat treatment response of semisolid metal high pressure die cast Al–7Si–Mg alloys A356 and F357 was studied and compared. It was found that the heat treatment behaviour of alloy F357 is influenced markedly by the stability of the Mg containing π phase. This phase, which dissolves in alloy A356 during solution treatment, persists in F357 and decreases the amount of magnesium in solid solution. This is the likely origin of the decrease in the aging response of the F357 alloy. The tensile properties (yield strength and ultimate tensile strength) of the alloys correlate much better with the Mg concentration in solid solution than with the bulk Mg content of the alloys. The recently developed shortened T6 heat treatment cycles for rheocast A356 were tested on alloy F357. The optimum artificial aging treatment was determined to be 180°C for 4 h, regardless of the prior natural aging period.

Influence of temper condition on microstructure and mechanical properties of semisolid metal processed Al–Si–Mg alloy A356

The microstructures and mechanical properties of strontium modified semisolid metal high pressure die cast A356 alloy are presented. The alloy A356-F (as cast) has a globular primary grain structure containing a fine eutectic. Solution treatment results in spheroidisation of the eutectic silicon particles under the T4 and T6 temper conditions. The A356-T5 maintains the fibrous silicon morphology after artificial aging. A356-T4 has better ductility and impact strength than A356-T5 due to its spheroidised silicon morphology. The impact properties of semisolid metal high pressure die cast A356 are controlled mainly by the silicon morphology and alloy strength (hardness), whereas tensile strength is determined by the degree of solid solution coupled with precipitate formation during aging.

Research on fractal characteristics of primary phase morphology in semi-solid A356 alloy

Semi-solid slurry of A356 alloy was prepared by low superheat pouring and slightly electromagnetic stirring, and the fractal characteristics of morphology in semi-solid primary phase was researched. The fractal dimensions of primary phase morphology in the semi-solid A356 alloy were calculated by the program written to calculate the fractal dimensions of box-counting in the image of solid phase morphology in semi-solid metal slurry. The results indicated that the morphology of primary phase in semi-solid A356 prepared by low superheat pouring and slightly electromagnetic stirring is characterized by fractal dimension, and the primary phase morphology obtained by the different processing parameters has different fractal dimensions. The morphology of primary phase at the different position of ingot has different fractal dimensions, which reflected the effect of solidified conditions at different positions in the same ingot on the morphology of semi-solid primary phase. Solidification of semi-solid alloy is a course of change in fractal dimension.

Solid freeform fabrication of aluminum alloy components: Numerical simulations

A heat transfer model has been developed to simulate the evolution of temperature during semi-solid metal (SSM) solid freeform fabrication (SFF) process. The governing equations were solved numerically using finite difference method. Deposition of simple cubic components of A356 (Al-7%Si-0.45%Mg) aluminum alloy have been simulated. The effect of process parameters such as deposition pattern, deposition velocity, slurry cross-section and slurry temperature on the transient temperature distribution in these components have been studied. Out of the three deposition patterns studied, i.e. rotating, alternating and unidirectional, the alternating pattern enabled depositing larger interface-free components. Increasing the slurry flow rate resulted in the deposition of larger interface-free components.