Semi-solid Metal Research Articles

Study of the Microstructure Semi Solid Al-Si-Mg Casting

Semi-solid metal processing involves processing metallic alloys between the solidity and the liquidity. The microstructure must be non-dendritic and consist of spheroids of solid in a liquid matrix. In this work two potential routes are used: First, The superheated Al-Si-Mg alloy was continuously cooled down to various temperatures above its liquidity (620,630,640) o C . When the desired temperature of the melt has been reached, then mechanical stirring with rotational speed of (800)rpm for (30)sec. was started. The resultant slurry was bottom poured into a stainless steel mold placed below the crucible. The solidified samples were reheated to the semi-solid metal processing temperature, held there for (10) min, and rapidly quenched in into water tank to retain the semi solid microstructures. In the second set of experiments , the superheated melt was continuously cooled down to various temperature in semi solid range (610,600,595,590,580)o C, then mechanically stirred at rotating speed(800)rpm for (30)sec. the resultant slurry was bottom poured into a stainless steel mold placed below the crucible. The metallic mold is then rapidly quenched in water to room temperature. Metallographic examinations indicate that the mixing method leads to highly globular semi-solid slurries of fine particle size. The particle sizes of slurry quenched at various temperatures within the semi solid metal range are much smaller when compared to the reheated structures. The particle size gradually increases as the slurry temperature decrease.

Microstructural Evolution during DPRM Process of Semisolid Ledeburitic D2 Tool Steel

Semisolid metal processing is a relatively new technology that offers several advantages over liquid processing and solid processing because of the unique behaviour and characteristic microstructure of metals in this state. With the aim of finding a minimum process chain for the manufacture of high-quality production at minimal cost for forming, the microstructural evolution of the ledeburitic AISI D2 tool steel in the semisolid state was studied experimentally. The potential of the direct partial remelting (DPRM) process for the production of AISI D2 with a uniform globular microstructure was revealed. The liquid fraction was determined using differential scanning calorimetry. The microstructures of the samples were investigated using an optical microscope and a scanning electron microscope equipped with an energy dispersive spectroscopy analyser, while X-ray phase analysis was performed to identify the phase evolution and the type of carbides. Mechanical characterisation was completed by hardness measurements. The typical microstructure after DPRM consists of metastable austenite which was located particularly in the globular grains (average grain size about 50 μm), while the remaining interspaces were filled by precipitated eutectic carbides on the grain boundaries and lamellar network.

Efecto de las condiciones de enfriamiento sobre la fracción y morfología de la fase primaria épsilon en una aleación comercial ACuZinc5<sup>TM</sup>

The mechanical properties of ACuZinc5 TM alloys are influenced by the fraction solid of the primary epsilon (e) phase and their interrelations with the eta (η) phase. In this study the primary epsilon phase fraction results from cooling conditions similar to that applied in different casting processes have been measured. Additionally an analysis was made from the morphology’s changes and from the thermal analysis results to suggest the probably adequate moment to apply stirring in semisolid metal processing. From these results it is possible to design experimental thermal cycles to obtain an adequate distribution of e and η phases in the end castings microstructure.

Semi-solid Metal Flow Through Solid Network and Evaluation of Permeability for Aluminum and Zinc Alloys

Semi-solid Metal Flow Through Solid Network and Evaluation of Permeability for Aluminum and Zinc Alloys

Non-dendritic Structural Changes in Al–7Si Alloy Cast Through Rapid Slurry Formation (RSF) Process

Rapid slurry formation (RSF) is a semi-solid metal forming technique, which is based on a so-called solid enthalpy exchange material. The beauty of the process is that the slurry is controlled by relative added amounts of the alloy system used. It is not necessary to accurately control the heat flux to surrounding media such as, crucible or mould. In the present investigation 15 % solid fraction in Al–7Si alloy slurry has been produced by RSF method. The slurry so prepared was cast in gravity die cast mould with water cooling arrangement. Thus ingot of size 30 cm × 5 cm × 2.5 cm could be produced. The microstructure showed rosette and globular type α-Al morphology. Tensile properties evaluated were higher than as cast Al–7Si alloy. During slurry formation coarsening of particles was experimentally investigated and also theoretically described by Lifshitz-Slyozov-Wagner (LSW) theory and convective mass flow equations.

Simulation of Cooling of Liquid Metal in an Inclined Slope to Predict the Condition for Semi Solid Forming and Its Experimental Validation

In semi solid processing, the preparation of slurry with globular microstructure is very important. A recent technique named the inclined slope casting, to produce the semi-solid metal slurry, has been evolved. The microstructure of casting depends on several process parameters like initial temperature of liquid metal, length of the slope, cooling rate and the rate of flow of liquid metal. In the present work numerical modelling of cooling of liquid was carried out. Modelling was done for different mass influx velocities, temperature of pouring and different inclinations of the slope set-up. In this work, a CFD model based on finite volume method for simulating flow along the inclined slope was carried out. Heat transfer, fluid flow and solidification were studied by varying the process parameters like slope angle and inlet pouring rate. The numerical results were verified with experiments for same conditions.

Fusion Welding of Rheocast Semi-Solid Metal (SSM) Processed Aluminium Alloy 7017

Near-net shape casting of wrought aluminium alloys has proven to be difficult due to a tendency towards hot tearing during cooling. Rheocasting, or semi-solid metal (SSM) processing followed by high pressure die casting (HPDC), has recently been shown to be an effective alternative to conventional die casting, yielding near-net shape wrought aluminium alloy castings with less risk of hot tearing. This casting process involves pouring the liquid metal into a processing cup, which is then transferred into a coil for induction stirring and simultaneous forced air cooling. When the metal reaches the semi-solid casting temperature, the resultant slurry is transferred to a high pressure die casting machine and cast to near-net shape. This modifies the as-cast microstructure, yielding a more globular primary phase and results in mechanical properties in the -T6 condition closely approaching those of wrought material in the same condition. Little information is currently available on the response of SSM-HPDC material to welding. This project investigated the influence of autogenous laser and gas tungsten arc welding on the microstructure and mechanical properties of aluminium 7017 after rheocasting. It is possible to successfully weld this material without solidification or liquation cracking. The effect of welding on the rheocast microstructure in the heat-affected zone and weld metal was shown, and the hardness and tensile properties of the resulting joints in the as-welded condition were tested and related to the microstructures achieved.

Property and Quality Optimization of Laser Welded Rheo-Cast F357 Aluminum Alloy

F357, a hypo-eutectic Al-7%Si-0.6%Mg alloy without beryllium, was processed with CSIR-Rheo technology to the Semi-Solid Metal (SSM) state and cast in plates by means of High Pressure Die Casting. The castings were either left in the as-cast (F) condition or subjected to T4 or T6 heat treatments prior to autogenous Nd:YAG laser welding. A welding operating window was established and within this window, the weld porosity and undercut were minimised. Butt welds complying with ISO 13919-2: 2001 could be produced by means of the optimum parameters of 3.8 kW laser power and a welding speed of 4 m/min with a twin spot laser beam configuration. The mechanical properties of age-hardenable Al-Si-Mg alloys are dependent on the rate at which the alloy is cooled after the solution heat treatment. The low heat input provided by the laser welding process resulted in high enough cooling rates to ensure that both the fusion zone and HAZ were in the T4 condition after welding. Tensile properties equivalent to the parent metal in T6 condition were obtained after subjecting welded T4 plates to conventional artificial ageing treatment.

Flow Behaviour of M2 Steel Alloy during Semi Solid Isothermal Rapid Compression

The key to all semisolid processes is spherical and non-dendritic particles suspended in a liquid matrix. This special kind of microstructure causes the semisolid material to behave like a non-Newtonian fluid dependent on time and shear rate. Semi solid metal processing gives less shrinkage and porosity, non-macro segregation, lower flow stress, good formability and increased die life compared with conventional production methods. Therefore, semi solid processing introduces a good option for massive forming of high temperature alloys. Here, by partial remelting of M2 tool steel alloy under rapid compression test, deformation mechanism of steel alloy and its correlation to rheological properties were investigated. Flow stress for M2 tool steel alloy corresponding to solid fraction above 55% derived at constant strain rate and holding time. In order to investigate the parameter involved in this process, in the second stage of experiments the ram speed and holding time are variable. The analysis of the derived curves shows that the semisolid slurries exhibit a pseudo plastic behavior dependent on stain rate and temperature. The classical power law model used to describe the rheological and thixotropic dependence.

R-HPDC in South Africa

The history of semi-solid metal forming and in particular rheo-high pressure die casting at the Council for Scientific and Industrial Research in South Africa is discussed. Processing flexibility is demonstrated on the Al-Si-Mg, Al-Mg-Si, Al-Cu-Mg and Al-Zn-Mg-Cu casting and wrought alloy systems as well as on high purity aluminium, unmodified Al-Si binary eutectic, metal matrix composites and magnesium alloys. Material properties are highlighted.

A Novel Process in Semi-Solid Metal Casting

In this research a new process for semi-solid casting of ductile iron based on the high nucleation rate combined with locally mechanical stirring is presented. In this process at first fully liquid ductile iron was poured on the peripheral surface of a wheel rotating against pouring direction. At this stage, the solid crystals nucleated at the chilling surface were pushed to the melt by a heat resistance steel cutter and finally the semi-solid slurry was generated. Reheating treatment was done on the samples to achieve more efficiency of semi-solid casting process. The effects of the travelling distance of solid particles during casting, the reheating time and temperature were examined. The results showed that the process effectively changes the dendrite structure to globular one.

Applications of Semi-Solid Slurry Casting Using the Gas Induced Semi-Solid Technique

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.

Industrial Applications of Rheoforming Technology in China

Rheological forming, a semi-solid metal forming process, is one of the manufacturing technologies for near net shape forming. The technology has attracted global academic research interests in recent years. This paper presents the current status of industrial applications of the semi-solid rheological forming technology in the China mainland. A variety of semi-solid slurry preparation techniques have been adopted including electromagnetic stirring and low superheat pouring. Dedicated semi-solid rheological forming equipment developed by the local manufacturers have been highlighted. This paper also makes an attempt to review the crucial factors for successful industrial application of the semi-solid metal forming process.

Machinability Study of Stir Cast Hypoeutectic Aluminum-Silicon Alloys During Turning

The influence of Be and Cd (iron correctors) on mechanical properties and machining behavior of hypoeutectic Al-Si alloy (Al-7Si-0.5Mg-1.2Fe) processed by conventional and semi-solid metal (SSM) processing (stir casting) techniques is investigated. The alloys under investigation were prepared by controlling melt in an induction melting furnace. The stirring of SSM was carried out at a constant stirring speed of 400 rpm under constant cooling conditions from liquidus temperature. The turning operations were carried out under dry conditions on a CNC turning center using coated-carbide insert by varying cutting speed, feed rate, depth of cut, and approaching angle. An orthogonal array, the signal-to-noise ratio, and analysis of variance were employed to study the machining performance characteristics. The results indicate that Be/Cd modification of the alloy and selected cutting parameters significantly affect the machining characteristics. The feed rate, cutting speed, and Cd as an iron corrector have more effect on the machining behavior of the alloys under study.

Preparation of semi-solid A356 Al-alloy slurry by introducing grain process

A preparation technology of semi-solid metal slurry introducing grain process (IGP) was developed. The effects of processing parameters on the microstructures of the semi-solid A356 Al-alloy slurries were investigated, and the formation mechanism and morphology controlling of the spherical primary α(Al) grains were discussed. The results show that when the preparing slurry is 4 kg, IG size is 10 mm, dosage is 3.5% and dumping temperature (DT) is 611–617 °C, the mean diameter of the primary α(Al) grains in the semi-solid slurries can reach 40–75 μm and the shape factor can reach 0.82–0.89. When the IG size is 10 mm, DT is 613 °C and dosage is 2%–4%, the mean diameter can reach 45–82 μm and the shape factors can reach 0.78–0.88. With decreasing DT or increasing dosage properly, the primary α(Al) grain morphology is better. When QR=QA and Rh=Rc, as long as the DT is suitable, excellent semi-solid slurry can be produced. As a result of the IG melting, a large amount of dendritic fragments can become the direct source of the primary α(Al) grains. Meanwhile, many undercooled areas are formed, where abundant primary α(Al) grains are multiplied by heterogeneous nucleation.

Characterization of the microstructure evolution of a semi-solid metal slurry during the early stages

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 45s 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.

Thixotropic Behavior of Semisolid AlSi6Mg2 Alloy under Cyclic Shear Deformation

In the present work, the rheological model (CF model) developed by Chen and Fan[1] for semisolid metal slurries (SSMS) is applied to investigate the thixotropy of the AlSi6Mg2 alloy under cyclic shear deformation. The present investigation indicates that the semisolid AlSi6Mg2 alloy has the thixotropy by the technique of hysteresis loop. Specifically, the area of the hysteresis loop increases with decreasing the up-time, the initial shear rate and increasing resting time, solid volume fraction and maximum shear rate, respectively. These results agree qualitatively with the experimental data. Furthermore, the origin of the hysteresis loop is attributed to the difference between the agglomeration rate and the deagglomeration rate.

Composition segregation in semi-solid metal cast AA7075 aluminium alloy

The composition distribution and microstructural evolution associated with the characteristic globular grain structure in semi-solid metal cast AA7075 aluminium alloy has been investigated. The primary globular grains possess uniform composition over nearly 90% of their diameter with fluctuations only occurring in the near surface of the individual grains. The inter-globular regions are solute rich and contain non-equilibrium eutectic. The segregation pattern in semi-solid cast structures occurs over a much greater length-scale than the dendritic pattern prevalent in conventional castings and it is proposed that the distinction arises from predominant planar growth of the α-primary globular grains during the SSM conditioning stage. Examination of the in-grain structure using electron backscattered diffraction revealed a quasi-periodic fluctuation in misorientation that may have arisen from local perturbations on the planar solid interface as solidification progressed. Homogenisation prior to artificial ageing is severely compromised by the undesirable segregation pattern.

Single Roll Caster Equipped with a Scraper

A scraper was developed and attached to the single roll caster in order to improve the free solidified surface of as-cast strip cast. Semisolid metal on the free solidified surface was flattened by the scraper and the surface became flat. The AA5182 aluminum alloy could be cast at speeds up to 40m/min. Thickness was about 3mm. Pressure of the unit width of the scraper was ranging from 0.1N/mm to 1.0N/mm, and these pressure was enough to make the free solidified surface flat. Center line segregation did not occur as the strip was solidified from single side at the strip cast by the single roll caster. Roll-cast strip could be cold rolled down to 1mm. There was not difference between the roll contact surface and the free solidified surface of the strip after cold rolling judging by a visual examination. Result of tension test of the roll cast and cold rolled strip was as same as that of D.C. cast and rolled strip. Deep drawing test was operated at the conditions both of the roll contact surface and the free solidified surface were outer side. LDR (limited drawing ratio) was same at both conditions and they were 1.8. Thickness of the strip was controlled by the roll speed, solidification length (length of the melt pool) and pressure of the scraper. The single roll caster is simpler than a twin roll caster. Rigidity for rolling was not required for the single roll caster. Cost of the roll is half comparing a twin roll caster. The equipment cost of the single roll caster is more economy than that of the twin roll caster.

Roll Casting of Aluminum Alloy Strip by a Single Roll Caster Equipped with a Scraper

A single roll caster equipped with a scraper was devised to cast the strip with sound free solidified surface. A property of this single caster is equipment of the scraper. Semisolid metal on the free solidified surface was scribed by a scraper and the surface became flat. Center line segregation did not occur as the strip was solidified from single side. The AA5182 aluminum alloy could be cast at speeds up to 40m/min. Thickness was about 3mm. Pressure of the unit width from the scraper was ranging from 0.1N/mm to 1.0N/mm, and these pressure was enough to make the free solidified surface flat. Roll cast strip could be cold rolled down to 1mm. There was not difference between roll contact surface and the free solidified surface after cold rolling by a visual examination. Result of tension test of the roll cast strip was as same as that of D.C. cast and rolled strip. Deep drawing test was operated at the conditions both of the roll contact surface and the free solidified surface was outside. LDR (limited drawing ratio) was same at both conditions and they were 1.8. Thickness of the strip was controlled by the roll speed, solidification length (length of the melt pool) and pressure of the scraper. The single roll caster is simpler than a twin roll caster. Rigidity for rolling was not needed for the single roll caster. Cost of the roll is half comparing a twin roll caster. The equipment cost of the single roll caster is more economy than that of the twin roll caster.