Semi-solid Metal Research Articles

Ultrafast synchrotron X-ray imaging studies of microstructure fragmentation in solidification under ultrasound

Ultrasound processing of metal alloys is an environmental friendly and promising green technology for liquid metal degassing and microstructural refinement. However many fundamental issues in this field are still not fully understood, because of the difficulties in direct observation of the dynamic behaviours caused by ultrasound inside liquid metal and semisolid metals during the solidification processes. In this paper, we report a systematic study using the ultrafast synchrotron X-ray imaging (up to 271,554 frame per second) technique available at the Advanced Photon Source, USA and Diamond Light Source, UK to investigate the dynamic interactions between the ultrasonic bubbles/acoustic flow and the solidifying phases in a Bi-8%Zn alloy. The experimental results were complimented by numerical modelling. The chaotic bubble implosion and dynamic bubble oscillations were revealed in-situ for the first time in liquid metal and semisolid metal. The fragmentation of the solidifying Zn phases and breaking up of the liquid-solid interface by ultrasonic bubbles and enhanced acoustic flow were clearly demonstrated and agreed very well with the theoretical calculations. The research provides unambiguous experimental evidence and robust theoretical interpretation in elucidating the dominant mechanisms of microstructure fragmentation and refinement in solidification under ultrasound.

Semi-Solid Metal Forming - A Process for Manufacturing Composite and Hybrid Materials

The following paper deals with the manufacturing of composite and hybrid materials by using semi-solid forming technologies. On the one hand, there is presented a process for producing Interpenetrating Phase Composites (IPC) consisting of ceramic open-cell structures which are infiltrated by a semi-solid aluminium alloy. On the other hand, a new process for manufacturing hybrid metallic materials is described. Thereby, semi-finished materials made of two different aluminium alloys are simultaneously heated using an induction system and subsequently formed in the semi-solid state. The paper shows numerical investigations as well as experimental results for both processes.

Investigation of pouring temperature and holding time for semisolid metal feedstock production

Semisolid metal (SSM) processing, as a kind of new technology that exploits forming of alloys between solidus and liquidus temperatures, has attracted great attention from investigators for its thixotropic behaviour as well as having advantages in reducing porosity, macrosegregation, and forming forces during shaping process. Various techniques are employed to produce feedstock with fine globular microstructures, and direct thermal method is one of them. In this paper, the effect from different pouring temperatures and holding times using a direct thermal method on microstructure and hardness of aluminium alloy 6061 is presented. Molten aluminium alloy 6061 was poured into a cylindrical copper mould and cooled down to the semisolid temperature before being quenched in water at room temperature. The effect of different pouring temperatures of 660 °C, 680 °C, 700 °C, and holding time of 20 s, and 60 s on the microstructure of aluminium alloy 6061 were investigated. From the micrographs, it was found that the most globular structures were achieved at processing parameters of 660 °C pouring temperature and 60 s holding time. The highest density and hardness of the samples were found at the same processing parameters. It can be concluded that the most spheroidal microstructure, the highest density, and the hardness were recorded at lower pouring temperature and longer holding time.

A metal additive manufacturing method: semi-solid metal extrusion and deposition

Different existing metal additive manufacturing (AM) processes have many shortcomings and a lot of activities are centered on improving the final printed metal product. A new method of metal additive manufacturing has been proposed in this paper. Unlike most of the metal additive manufacturing processes, it does not use metal powder. The proposed method utilizes semi-solid metal (SSM) forming combined with deposition process as used in the most common additive manufacturing process for polymers. SSM forming is a promising near net shape technology with several advantages, such as being a porosity-free product and having reduced shrinkage, controlled microstructure, and excellent mechanical performance. Implementation of this technology in metallic additive manufacturing improves the mechanical properties and cost savings. Complicated time-dependent behavior of SSM makes it a challenging issue to utilize in additive manufacturing. A wire feedstock as a metallic filament was employed for the proposed process. However, some material preparations were necessary to get the desired rheological properties at the deposition head. In this study, the strain-induced melt-activated (SIMA) process was applied on a low-melting-temperature Sn-Pb alloy to obtain the desired globular feedstock microstructure. Then, preconditioned wire was fed in a thixo-extruder, which was designed and built for the proposed method in this research. A semi-melted alloy was deposited on the moving substrate to build a metallic part layer by layer. Various parameters of SIMA and thixo-extrusion processes, including wire thermomechanical cycle, feed rate, solid fraction, nozzle and chamber geometry, and others, were examined experimentally, and a sustainable semi-solid metal extrusion and deposition (SSMED) process was achieved. Finally, an acceptable metallurgical layer bonding was obtained at the interface of the deposited layers, with good mechanical properties of the fabricated parts. The new proposed method seems to have great potentials for metal additive manufacturing parts.

Effect of Semisolid Metal Casting of Large Forging Ingots on Quality of Large-Sized Forgings Produced

The paper reports the findings on the effect of various casting methods on the quality and development of primary dendritic structure in large-sized forging ingots of steel 38ХН3МФА. One ingot was teemed as per a conventional teeming method while the other was teemed with an inoculated metal stream. It was established that the dendritic parameter value in the inoculated ingot is much smaller than that in the conventional ingot. Consequently, the solidification process occurs at a higher rate in the inoculated ingot compared with that in the conventional ingot, and this assumption is supported by a more homogeneous dendritic structure. It is demonstrated that disperse inoculants positively affect the structure, physical and chemical homogeneity as well as the mechanical properties of cast metal. This finding is clearly supported by the examination of the forgings made of the conventionally teemed ingot and the one teemed with an inoculated stream. When inoculants were introduced in the metal stream, total chemical heterogeneity increased on the average by 1.2-2 times. It is established that the best results for the inoculation casting method are achieved when 2.4-2.6% inoculants are introduced in the stream. Such quantity of inoculants forms at a distance of 5 meters between the guiding pipe and the hot top.

Optimization of Casting Design Parameters on Fabrication of Reliable Semi-Solid Aluminum Suspension Control Arm

The semi-solid casting process has the advantage of providing reliable mechanical aluminum parts that work continuously in dynamic as control arm of the suspension system in automotive vehicles. The quality performance of dynamic control arm is related to casting mold and gating system designs that affect the fluidity of semi-solid metal during filling the mold. Therefore, this study focuses on improvement in mechanical performance, depending on material characterization, and casting design optimization, of suspension control arms made of A357 aluminum semi-solid alloys. Mechanical and design analyses, applied on the suspension arm, showed the occurrence of mechanical failures at unexpected weak points. Metallurgical analysis showed that the main reason lies in the difficult flow of semi-solid paste through the thin thicknesses of a complex geometry. A design modification procedure is applied to the geometry of the suspension arm to avoid this problem and to improve its quality performance. The design modification of parts was carried out by using SolidWorks design software, evaluation of constraints with ABAQUS, and simulation of flow with ProCast software. The proposed designs showed that the modified suspension arm, without ribs and with a central canvas designed as Z, is considered as a perfect casting design showing an increase in the structural strength of the component. In this case, maximum von Mises stress is 199 MPa that is below the yield strength of the material. The modified casting mold design shows a high uniformity and minim turbulence of molten metal flow during semi-solid casting process.

Corrosion Behavior of Al-Zn-In Sacrificial Anode Alloys Produced by Conventional Casting and Semi-Solid Metal Casting Processes

A challenge in producing a sacrificial anode through conventional casting method is that the alloying elements in casting segregate during solidification, which further causes non-uniform anode corrosion reducing anode performance. In this paper, we investigated the performance of Al-5Zn-0.02In anode produced by conventional casting compared with by semi-solid casting technique. The performance of produced anodes were measured in terms of anode potential, current capacity, consumption rate and anode efficiency in 3.5% NaCl solution for 14 days. We found that the microstructure of the conventional cast anode had dendrites and coarse grains and the corrosion caused pitting corrosion. In contrast, the semi-solid cast anode had fine grains without any dendrites. The corrosion attacked mainly the grain boundaries and less on the matrix. Surprisingly, the conventional cast anode has about 10% higher efficiency than that of semi-solid cast anode.

Semisolid Strip Casting of Aluminum Alloy by a Twin Roll Caster Equipped with a Channel Scraper

An unequal diameter twin-roll caster equipped with a channel scraper is proposed to improve center-line segregation. A semisolid metal layer was made by the channel scraper on the upper side of the strip and solidified by an upper roll. The twin-roll caster cast a strip of 5182 with equiaxed grain and without center-line segregation at a speed of 30 m/min and a unit roll load of 104 N/mm. The tensile stress and elongation after cold rolling and annealing were 294 MPa and 28%, respectively.

EFFECT OF THIXOFORMING ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AL-6%SI-3%CU ALLOY

Thixoforming is a type of semisolid metal (SSM) processing for forming alloys in the semisolid state to near net-shaped products. In the present study, the effect of a thixoforming process on the microstructure and mechanical properties of Al-6Si-3Cu aluminium alloy was investigated. Melt was poured on a cooling slope at 630oC and the samples were obtained through permanent mold casting. They were thixoformed using a hydraulic press after holding at 571oC for 5min to yield a microstructure predominantly composed of α-Al globules and inter-globular Si particles. As-cast and thixoformed samples were characterized using optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) as well as hardness measurements and tensile tests. The results indicate that the mechanical properties of thixoformed alloy have improved as compared to permanent mold cast alloy. The ultimate tensile strength of as-cast sample was 210 ± 3.5 MPa and increased to 241 ±3.1 MPa in the thixoformed sample while the yield strength of as-cast alloy was 140 ±4.5 MPa and increased to 176±3.3 MPa in the thixoformed sample. The thixoformed alloy also showed an improvement in elongation to fracture as it increased from 2% in as-cast sample to 3.2% in thixoformed alloy. The fracture of as-cast sample showed a cleavage fracture, whereas in the thixoformed alloy, a combination of dimple and cleavage was observed.

Floating Behavior of Entrapped Gas in Semi-Solid Metal

Semi-solid die casting technology has great advantages at defects control and has been successfully used to produce high quality aluminum alloy components for several years. In this process, semi-solid metal with high apparent viscosity and low plunger velocity are used to avoid surface turbulence which is the main source of entrapped gas in conventional die casting processes. But, entrapped gas still has other sources, such as melting, pouring, surface flooding and confluence weld. Solution heat treatment is always used to strengthen semi-solid die castings. The entrapped gas leads to blister defects, which directly decreases the acceptance rate of semi-solid die castings. So, the entrapped gas is still a serious issue in semi-solid die casting process. We studied the floating behavior of entrapped gas bubble in semi-solid metal. Two floating models were established for gas bubbles with different sizes. These models were used to analyze the possibility of entrapped gas escaping from semi-solid metal in casting practice. The results showed that entrapped gas from feed billet could not escape from the semi-solid metal in the casting process of impeller, which was proved by experiment results. These results emphasized the importance of clean melt and semi-solid metal. Some advices were given at last for avoiding or removing the entrapped gas in semi-solid die casting process.

Study on the effects of forming conditions on microstructural evolution and forming behaviors of Cr-V-Mo tool steel during multi-stage thixoforging by physical simulation

The inhomogeneous quality of products manufactured by semisolid forming is mainly attributed from the phase segregation of semisolid metal slurry during forming. Phase segregation can be inhibited by lower forming temperature, higher strain rate, and lower stroke. However, the realization of net-shape forming with above parameters is quite difficult. In this study, a multi-stage thixoforging strategy including a partial melting, a fast first compression, a partial solidification, and a secondary compression was proposed to weaken phase segregation of thixoformed samples. A physical simulation of multi-stage thixoforging was conducted on commercial rolled Cr-V-Mo tool steel SKD11, using a multistage hot compression test machine. The microstructural evolution and forming behaviors of the SKD11 steel slurries in different stages of multi-stage thixoforging with various forming parameters were investigated experimentally. Out flow of liquid phase does not occur throughout the thixoforming, and can be inhibited effectively by partial solidification and subsequent secondary compression. SKD11 steel sample more homogenous microstructure and smoother surface was manufactured by multi-stage thixoforging including a fast first compression with a stroke of 4mm and a strain rate of 2.0/s at 1280°C, a partial solidification to 1240°C with a cooling rate of 2°C/s, and a secondary compression with a stroke of 2mm and a strain rate of 0.1/s at 1240°C.

Critical assessment: opportunities in developing semi-solid processing: aluminium, magnesium, and high-temperature alloys

Semi-solid metal processing (SSM) is a modern metal forming technology offering net-shape metal components of complex geometry in a one-step operation. The process relies on the thixotropic behaviour of metals with non-dendritic microstructures in a wide semi-solid temperature range. The beneficial effects are currently exploited in aluminium and magnesium alloys. This alternative manufacturing route to casting and forging has generated high expectations regarding high melting alloys, although no conclusive results have been achieved so far. Current studies focus on a deep understanding of the fundamental basics of alloys in the semi-solid range, e.g. rheology, microstructural evolution, and the consequences of forming, e.g. relations between process parameters, microstructure, and properties. This critical assessment aims at defining the important needs for further development of SSM, and assessing current knowledge.

Surface roughness modeling of semi solid aluminum milling by fuzzy logic

In the paper carried out was modeling of cutting parameters in face milling process of Semi Solid Metal alloys. As input parameters in the process of modeling were taken: cutting speed v, the feed per tooth and depth of cut, while for the output characteristics of the process were taken arithmetic mean surface roughness Ra and maximum roughness height Rmax. Modeling was done in two ways. The first model was made with the help of mathematical and statistical method- factorial experiment DoE, where it was used model with parameters’ interaction. The second model was made by artificial intelligence and as a tool was chosen fuzzy logic.

Microstructure-dependent mechanical properties of semi-solid copper alloys

Rotary swaging strain induced melt activation (RSSIMA) method is proposed to fabricate semi-solid copper alloys. The micro-grains size evolution of the globular particles during isothermal heat treatments is described by the Lifshize-Slorovitze-Wagner (LSW) equation and mechanical behaviors of semi-solid copper alloys and as-cast copper alloys are analyzed with the power-law Holloman work-hardening model. The effects of microstructure alteration on elasto-plastic properties of semi-solid tin copper alloys are reported. It has been discovered that the stiffness and strength of the alloy samples increase as micro-grain sizes decrease, whereas the alloys originated from various isothermal heat treatments. The strength of the semi-solid samples with globular micro-grains is also discovered to be higher than the strength of as-cast copper with dendrite micro-grains. These enhancements in mechanical properties can be understood by fine-grain mechanisms and precipitation strengthening of the second phase based on microscopy observations. These results can be essential for applications of semi-solid metal alloys.

Machinability of hypereutectic cast Al–Si alloys processed by SSM processing technique

Experimental investigation carried out on the machinability studies to determine the influence of semi-solid metal processing and modification on hypereutectic Al–20Si–0.5Mg–1.2Fe-based alloy produced by conventional cast and semi-solid metal processing technique (mechanical stirring) and modified with iron correctors (Be and Cd) has been presented in this paper. The alloys under investigation were prepared by controlling melt using an induction melting furnace. Stirring of semi-solid metal takes place at constant cooling conditions from liquidus temperature at a constant stirring speed of 400 rpm. To determine the machining performance characteristics an orthogonal array, signal-to-noise ratio and statistical tool analysis of variance were jointly used during experimentation. A CNC lathe was used to conduct experiments in dry condition and coated carbide inserts were used as tool inserts. Machining variables like cutting velocity, approaching angle, feed rate and depth of cut, which can be considered as process parameters, are taken into account. The combined effect of modification and semi-solid metal processing has a significant effect on the machining characteristics, which was concluded from study. The modified alloy processed by semi-solid metal processing technique exhibits better machinability conditions when compared with the conventional cast. The feed rate has more effect on machining behaviour.

Global and local investigations of the electrochemical behavior the T6 heat treated Mg–Zn–RE magnesium alloy thixo-cast

The influence of semi-solid metal processing (SSM called also as thixoforming) of ZE41A magnesium alloy on the electrochemical behavior in 0.1M NaCl solution was investigated. To describe the corrosion behavior of ZE41A alloy, the electrochemical measurements were conducted in global and local scale for two types of specimens: (1) ingot-feedstock, (2) specimen after thixoforming and T6 treatment. The heat treatment and thixoforming significantly improved mechanical properties of ZE41A alloy. The global corrosion potential is slightly higher for treated sample what is related to the presence of Zr–Zn nanoparticles distributed in solid solution. The corrosion behavior differences between feedstock and thixo-cast after T6 samples are also visible in local scale, what has been revealed by using microcapillary technique. However there is no improvement in corrosion behavior after treatment. Corrosion morphology of the treated sample indicate higher susceptibility to pitting and filiform corrosion. Corrosion rate is also slightly higher.

Variation of microstructure and mechanical property of slurry die cast Al-Si-Mg-Fe alloy

Semi-solid metal processing technologies have been intensively studied in recent years. Gas Induced Semi-Solid (GISS) is a slurry preparation technique for producing non-dendritic or globular structures for cast alloys. In the present study, GISS technique was used in conjunction with conventional die cast process for casting Al–Si–Mg–Fe alloy. The shape of die cast specimen was designed as a simple flat plate. The variation of microstructures and tensile properties of specimens from different locations of cast plates was studied. The results show that the specimens from bottom location, near the gate section and the middle location of cast plates are stronger and more ductility than those from the top location. The microstructural examination reveals that the specimens from top location of cast plates contain more defects, such as shrinkage pores than those from the other locations. These defects are resulted in lower strength and ductility of the cast product.

Semi-solid processing of the CoCrCuFeNi high entropy alloy

High entropy alloys (HEAs) are a particularly challenging material for semi-solid processing because of the high temperatures involved. In the CoCrCuFeNi HEA examined here, the globular microstructure required for the thixoforming process was obtained by the recrystallization and partial remelting method. Based on Kazakov's criteria, a detailed characterization of the solidus-liquidus temperature range, was conducted. Semi-solid metal processing of the CoCrCuFeNi HEA was carried out at 1150°C and 1340°C, which corresponded to 15% and 35% of the liquid phase, respectively. The microstructure of the thixo-cast consisted of two face-centered cubic solid solutions: one in the form of globular grains, containing near equiatomic concentrations of Co, Cr, Fe and Ni, and the other, enriched in Cu, homogeneously distributed around grains. Depending on the process temperature, significant changes were observed in the microstructure. Compression test results of the CoCrCuFeNi HEA thixo-cast showed that yield strength was around 395MPa, while the compression strength at 40% of strain reached 1810MPa at an average hardness of 201HV. The obtained values were about 30% higher than those of the ingot material after direct casting. The melting behavior, microstructure and mechanical properties, as well as the possibility of obtaining a net-shape component in a one-step operation, make the CoCrCuFeNi HEA a promising material for thixoforming.

Effect of angle, length and circulation cooling system on microstructure A360 Aluminum alloy in semi-solid metal forming by cooling slope method

Article history: Received 6 July, 2016 Accepted 30 October 2016 Available online 31 October 2016 Application of cooling slope in casting is a relatively simple process with low equipment and processing costs, which is able to produce semi-solid slurries with different parameters such as pouring temperature, length and angle of slope plate, and plate type. In this study, the effect of angle and length of copper made slope plate on the microstructure of A360-Aluminum alloy is investigated. Microstructure study of metallographic samples in different conditions revealed that for 400mm slope length and 60° slope angle the optimum sphierized and homogenized microstructure is achieved. Also, under these conditions, the most hardness (77HB) was obtained, which might be because of suitable solidification conditions namely time and rate of shear stress. After finding the suitable conditions of slope length and angle, the effect of the circulation cooling system was examined; and the method with cooling system was found to result in more homogenized microstructure compared to ordinary method. © 2017 Growing Science Ltd. All rights reserved.

Compression deformation behavior of semisolid Al2O3np reinforced 7075 aluminum matrix composites with high solid fraction

Abstract