Semi-solid Heat Research Articles

The effect of Cu–8%P master alloy, SIMA process, thixoforming, and heat treatment on the microstructure and mechanical properties of Al–20%Mg2Si composite

In the present research, the effect of thixoforming and age-hardening heat treatment on the microstructure and mechanical properties of un-modified and modified Al–20%Mg2Si composites produced through the strain-induced melt activation (SIMA) process has been investigated. This method showed promising potential for manufacturing the ideal microstructure, achieving globular grains, and enhancing mechanical properties. Quantitative analysis has revealed that adjusting the semi-solid heat treatment parameters to a temperature of 570 °C and a holding time of 25 min optimizes the globular α-Al grains and Mg2Si particles, achieving the size of 20.62 μm and 66.71 μm and the shape factor of 0.81 and 0.83 for Mg2Si particles and α-Al grains, respectively. The yield strength of the modified Al–20%Mg2Si composite increased by 23.67% and the ultimate tensile strength increased by 21.20%, compared to the un-modified specimen. Both composites underwent the thixoforming process, illustrating the ultimate tensile strength and elongation of 93.68 MPa and 4.32% for the original composite and 138.86 MPa and 7.29% for the modified composite. The modified Al–20%Mg2Si composite was subjected to the effective combination of thixoforming and age-hardening. According to the tensile test results, the yield strength, ultimate tensile strength, elongation, and toughness raised by 138.72%, 140.92%, 267.70%, and 471.12%, respectively, compared to the as-cast specimen. Moreover, its impressive hardness of 125.1HB contributes to its overall performance.

Utilizing powder thixoforming to fabricate heterogeneous lamella structure in aluminum alloy: Formation, strengthening and toughening mechanisms

Powder thixoforming is a promising technique to synthesize heterogeneous lamella (HL) structure in aluminum alloy to promote strength-ductility synergy by mixing softer alloy powders (Al-1.04Mg-0.56Si-0.28Cu-0.04Zn, weight fraction) and harder alloy powders (Al-13.32Zn-3.70Si-2.44Mg-1.48Cu, weight fraction) together. However, the formation mechanism of coarse grain lamella and fine grain lamella is still unclear, and the strengthening and toughening mechanisms of HL structure have not been clarified thoroughly. This work shows that coarse grain lamella results from rapid coarsening of softer alloy powders driven by reduction in surface energy, fine grain lamella comes from both solidified grains from partially remelted softer/harder alloy melts and unmelted harder alloy powders surrounded by abundant eutectics. Besides, controlling semi-solid heating duration can regulate HL structure of Al alloys effectively, and the best comprehensive properties with ultimate tensile strength of 400 MPa, yield strength of 260 MPa and elongation of 7.1% are achieved for the HL-structured Al alloy heated for 50 min. The high strength of HL-structured Al alloy is ascribed to high back stress resulting from progressive yielding from coarse grain lamella to fine grain lamella, the high toughness is due to pulling out of fine grains and bifurcation of main cracks in coarse grains.

A novel research on the solute redistribution phenomenon of sub-rapid twin-roll cast Al-50 wt.% alloy treated by semi-solid heat treatment

In this study, a vertical twin-roll cast (TRC) thermal-fluid coupling model was established to determine a suitable roll-casting velocity. A slab of 0.9 mm thick roll-cast Al-50 wt.% Si alloy with a two-phase zone width of 473 °C was prepared at a velocity of 20 m/min for the first time. In addition, the finite element simulation showed that the average heat transfer rate had a positive correlation with the roll-casting velocity. When the roll-casting velocity increased to 20 m/min, the average heat transfer rate reached 1260 K/s, at which point, the average size of the primary Si was merely 16.5 μm. This was a decrease of approximately 90% when compared with the size of alloys cast with conventional methods. After carrying out semi-solid heating, an observation on the distribution of alloy elements identified a novel phenomenon of solute redistribution, i.e., inverse segregation. This inverse segregation process was highly significant in further controlling solute redistribution in the alloy. Additionally, it provided a new concept for improving roll-cast segregation.

Effect of thixoforming and precipitation hardening on microstructure and mechanical properties of Al-10.5Si–3Cu-0.2Mg alloy produced by strain induced melt activation process

The effects of the thixoforming and the precipitation hardening heat treatment on the microstructure and mechanical properties of Al-10.5Si–3Cu-0.2Mg alloy produced by the strain-induced melt activation (SIMA) process have been investigated. According to previous researches of authors, new introduced alloy has superior mechanical properties in addition to good response to aging process compared with the casting aluminum alloys. The results of the image analysis showed that by increasing the deformation from 10% to 20% and the time of semi-solid heat treatment from 10 to 20 min, the best globular structure of α(Al) grains with the grain size and shape factor of 65 μm and 0.81 were obtained at 15 min for 20% pre-deformation, respectively. The thixoforming process increased the ultimate tensile strength of the alloy about 29%, the yield strength about 37% and the elongation about 6%, compared to the as cast condition. The 20% pre-deformed sample experienced precipitation hardening heat treatment in addition to thixoforming had the best mechanical properties having UTS, YS, elongation and hardness of 423 MPa, 331 MPa, 4.41%, and 144.24 BHN, respectively. Thixoforming plus precipitation hardening is an effective procedure to produce a particular microstructure in Al–Si–Cu–Mg system exhibiting the excellent castability and high mechanical properties.

Semisolid heat treatment processing window of Pb-40% Sn alloy for feedstock in the 3D printing thixo-forming process

The 3D printing thixo-forming process is a method that utilizes semisolid thixotropic feedstock for layer extrusion by controlling feedstock viscosity below the complete melting temperature of metallic alloys to form a 3D structure. The process of producing metallic feedstock has a significant impact on alloy microstructure, which is critical for the material’s printability. This study aims to propose a semisolid heat treatment processing window of Pb-40% Sn alloy for use as feedstock billets for a variety of extruder nozzles (sizes 1–3 mm) during the 3D printing thixo-forming process. Firstly, Pb-40% Sn alloys were cold deformed to reduce the specimens’ diameters. Semisolid treatments for deformed specimens were performed at a variety of semisolid heating temperatures and three separate holding times (2, 5, and 10 min). The microstructure of semisolid feedstock specimens was then investigated. The temperature and time of the semisolid heating processing window of Pb- 40% Sn alloy were optimized to feedstock specimens and thixo-extruding processes with different printed output sizes. Therefore, the proposed semisolid heat treatment processing window was a promising approach to pave the way for the low-cost manufacturing of AM, which yields higher printing rates and improved scalability of printed lines. Additionally, this approach allows for a versatile manufacturing route for producing 3D metal parts with maximum density and intricate geometries as a result of the ability to tailor the microstructure.

Roles of the semisolid heating on the microstructure of a hypereutectic Al\u2013Fe\u2013Cu alloy

A hypereutectic Al–Fe–Cu alloy with a high-volume fraction ferro-aluminum second phase (AlFe phases for short) was reheated in the solid–liquid region and the microstructure evolution was investigated. During semisolid heating, the high-melting AlFe phases in the Al–Fe–Cu alloy were demonstrated to stunt the grain growth and to block the liquid coalescing and the solid moving. Consequently, the grain sizes in the alloy increased rapidly and then slowly with increasing holding time, and the grains increased gradually with increasing temperature. Smaller grain grew into the large grain but it did not continually grow into the larger grain with increasing temperature or holding time. The shape factor (SF) of the alloy increased gradually and then decreased quickly with increasing temperature or holding time. The major alloying elements in addition to magnesium in the hypereutectic Al–Fe–Cu alloy were finally enriched at the grain boundaries or around the AlFe phases. Besides dissolving in the grains or AlFe phases, copper also diffused between the grains or around AlFe phases, resulting in the formation of diverse Cu-enriched zones. Cu constituents in the inter-grains are outnumbered in the intra-grains. The coarsening kinetics of the alloy was controlled by grain boundary diffusion. The coarsening rate constants K in the initial stage of heating (5–20 min) were several times larger than that in the later stage of heating (20–60 min), indicating the blocking effect of AlFe phases on coarsened grain being obvious.

Effect of Reheating in the Solid–Liquid Region on Al-5Fe-4Cu-Based Alloys

Two Al-5Fe-4Cu-based alloys, AF1 alloy and AF15 alloy, with a high-volume fraction of second-phase iron-bearing intermetallics (AlFe phases for short) were reheated to a solid–liquid region, and the microstructural evolution of the alloys was investigated. During heating in the solid–liquid region, the high-melting-point solid AlFe phases were demonstrated to stunt grain growth, block liquid convection, restrict diffusion and coalescence, and reduce movement of solids. This mechanical barrier affected semisolid heating and prompted the formation of grains with increased high-angle grain boundaries (HAGBs), resulting in alloys with smaller grain sizes and lower coarsening rate constants. Consequently, the grains from the earlier stage (5–20 min) of isothermal heating were several times larger than those from the later stage (20–60 min), whereas average diameter of the grains increased with elevated heating temperatures and prolonged holding time. The coarsening of Al-5Fe-4Cu-based alloys is predominantly controlled by grain boundary diffusion. The AF15 alloy had a higher fraction of AlFe phases and more complex phase morphologies than the AF1 alloy. Furthermore, it had coarsening rate constants only one-fifth to one tenth those of the AF1 alloy.

Influence of Heat Treatment on the Microstructure and Corrosion Behavior of Thixo-cast Mg-Y-Nd-Zr

The influence of semisolid metal processing (SSM, also called thixoforming) and T6 heat treatment (HT) on the microstructure and corrosion behavior in chlorides of Mg-Y-Nd-Zr (WE43B) magnesium alloy was investigated. The as-cast microstructure is composed of α-Mg grains with the size of 52.8 ± 1.9 μm surrounded by eutectic precipitations enriched in rare-earth elements (Y, Nd). The thixo-cast microstructure contained α-Mg globular grains with the size of 65.5 ± 2.1 μm surrounded by a fine eutectic mixture in the volume of 26.6%. The T6 HT (heat treatment and saturation at 525°C/5 h, cooling in H2O and aging at 190°C/48 h) caused an increase of yield strength to 180 MPa and tensile strength to 280 MPa at the hardness 105 ± 4 HV5. Next, the electrochemical response was investigated in 0.1 M NaCl using the global and local LSV (linear sweep voltammetry) and EIS (electrochemical impedance spectroscopy) methods. The EIS method suggests the same mechanism for the processes occurring at the electrode/electrolyte interface and shows higher values of the polarization resistances of treated samples after 24-h immersion tests. In particular, better corrosion resistance in chlorides is observed in the alloy after SSM compared to the SSM/HT specimen, which has also been confirmed by the LSV tests performed after 24-h immersion. By using a local technique, a higher susceptibility of the matrix of SSM and SSM/HT samples to pitting corrosion has been revealed.

Microstructural and ageing response assessment of eutectic Al-Cu alloy due to isothermal heat treatment in semisolid state

An as cast Al-33%Cu eutectic alloy has been processed by semisolid heat treatment, with the ultimate objective of developing a ductile and strong eutectic composite material for probable application as bearing materials and other sliding wear components. In this phase of work only microstructural characterises has been attempted. The as cast sample of Al–Cu alloy is subjected to isothermal heat treatment at a temperature of 560 °C for varying periods of time and followed by water quenched. The microstructures of the semisolid heat treated (SSH) samples are analysed using an optical microscope and scanning electron microscopy (SEM). The microstructures of the quenched Al-33%Cu alloy samples show lamellar to lamellar + dendritic transition after SSH treatment. The quenched microstructure after 20 min of holding time primarily consists of copper aluminide dendrites + supersaturated α–Al grains. A massive copper-aluminide (>80%Cu) intermediate phase is observed after 3 to 6 min SSH. The sample configuration did not change after semisolid heat treatment and water quenching. This suggest that complete remelting and overburning did not occur during the semisolid heat treatment. The SSH sample shows maximum hardness, after 10 min holding time compared to solution treated (at 560 °C) and aged (at 150 °C) sample of a standard Al-4.5%Cu cast alloy. The Transmission electron microscopy (TEM) micrograph of the as quenched 10 min. SSH sample shows rod shaped CuAl2 (θ) precipitates, a dislocation network, cellular subgrains and an extremely fine lamellar eutectic. Solute cluster probably Guinier–Preston (GP) zones, precipitates and fine α –Al grains was observed in the peak aged sample. The x-ray diffraction (XRD) analysis of aged SSH treated sample shows several intermetallic copper-aluminide phases in each sample.

Effect of Semi-Solid Heat Treatment on the Microstructure and Dry Sliding Wear Behavior of Al–20Si Alloy at Optimized Parametric Conditions

Hypereutectic Al–20Si alloy needs optimization of different process parameters in order to obtain maximum wear resistance for tribological applications aerospace and automobile industries. The present study attempts to find the best possible combination of the process parameters for the semi-solid heat treatment of the alloy in Taguchi Method. The effects of three different control variables i.e. sliding velocity, sliding distance, and load on the wear characteristics were investigated. The optimal values of control factors were determined on the basis of ‘smaller-is-better’ approach from Taguchi optimization technique. The optimized conditions for wear tests were then applied to the isothermally heat-treated samples (10–30 min) at 600 °C. The quantitative analysis shows the significant change in the size, morphology, and distribution of primary Si and eutectic Si with heat treatment. Moreover, the worn surfaces were analyzed from the microstructures obtained in Field Emission Scanning Electron Microscopy (FESEM). The improved wear resistance observed in the case of 20 min of holding time can be attributed to the combined effect of refinement of primary Si, modification of eutectic Si and equiaxed α-Al during semi-solid heat treatment.

Thixoforming of Aluminum A201- Expectations and Fulfilment

Thixoforming as a Semi-Solid Metal Processing (SSM) route is a near net shape forming of metals in the semi-solid state, i.e. within the freezing or melting range between the fully solid and fully liquid states. Aluminum A201 is a copper containing casting alloy with additional small quantities of magnesium, silicon and silver. Although this alloy is difficult to cast, it has a particularly high response to age-hardening and therefore offers mechanical properties close to the wrought 2014 alloy. Alloy development experiments carried out at the University of Sheffield on A201 alloy have shown that feedstock of this alloy can be produced having uniform non-dendritic microstructures amenable to thixoforming, exhibits thixotropic properties under these conditions, can be shaped into complex near net-shape parts and develops impressive mechanical properties after appropriate heat treatment. A201 aluminum alloy billets when thixoformed in the semi-solid state and heat treated with a T7 followed by T6 treatment before being mechanically tested, have exhibited properties approaching those of wrought 2000 series aluminum alloys. Here follows a review of work done to date on shaping the A201 aluminum alloy in the semi-solid state, describing the hopes, expectations and fulfilment of researchers in the field.

Metallurgical Interface of 7075/6061 Bimetallic Composite Ingot Casted by Semi-Solid Slurry and Molten Metal Pouring

The 7075/6061 bimetallic composite ingots were prepared by semi-solid/liquid method. The microstructure characteristics of composite ingots were studied, and the diffusion law of interface components was analyzed. The results show that the semi-solid/liquid composite of 7075 and 6061 can be realized under the conditions of casting temperature of 983 K, semi-solid heating temperature of 918 K, holding time of 1h, and rapid water cooling. The obtained composite ingot has a layered distribution characteristic of "semi-solid microstructure/dendritic structure". The interface is clear, continuous, and free of impurities. From the interface to the semi-solid side, it shows such a pattern of change: “Dendritic structure” →”Semidendritic and hemispherical” → “Globularization”. Through the observation of the microstructure and the analysis of the components, the semi-solid/liquid composite mechanism is discovered to be a combination of fusion bonding and diffusion bonding.

Microstructure and homogeneity of semi-solid 7075 aluminum tubes processed by parallel tubular channel angular pressing

Semi-solid processing is a new developing technology for the fabrication of intricate parts at low temperatures in comparison with conventional casting routes. In this research, a parallel tubular channel angular pressing (PTCAP) was used as a pre strain-inducing process, and the influence of the number of PTCAP passes and semi-solid heating parameters on the microstructural characteristics of Al7075 tubes was investigated. The results demonstrated that PTCAP process could successfully be used as a best pre-straining method for tubular samples showing semi-solid microstructures. As the temperature is increased, the solid particle size first decreased, then increased, and gradually became more spherical. The appropriate condition for the subsequent semi-solid forming process was gained at reheating temperature of 620 oC. The suitability of this temperature was confirmed by the uniform distribution of grains. The grain size and shape factor both increased along with increasing the holding time. The distribution of the solid particles was strongly dependent upon the holding time and became uniform as the holding time increased. Comparison of the grain size and shape factor between ECAPed and PTCAPed samples revealed a high capacity of the PTCAP process as a strain-inducing stage. Also the two-pass PTCAP samples exhibited higher hardness than one-pass treated samples in the semi-solid state.

Porosity Defects Induced by Semi-Solid Isothermal Heat Treatment in Cast Hypereutectic Al-18% Si Alloy

The Present study was undertaken to investigate the porosity defects induced by semi-solid isothermal heat treatment in cast hypereutectic Al-18% Si alloy for different holding time. All specimens were heated up to 585 °C then hold for 10, 20, 30 and 40 min. in an electrically heated resistance furnace with heating rate of 10 °C.min-1. After the semi-solid heat treatment, the samples were taken out immediately for water quenching. Bulk density, total porosity and pore sizes at center and edge of each treated sample were investigated. It was found that the porosity in final treated samples increases by increasing the holding time. The pore size increases and the density decreases steeply with increasing the semi-solid isothermal holding time. Above 20 min., holding time the pore size at the edge shows higher value compared with that measured at the center of sample. It might be recommended that shorter holding time during semi-solid isothermal heat treatment should be selected in order to obtain minimum porosity percentage and higher density Al-18% Si alloy castings.

Properties of „In Situ” 7475 Alloy Matrix Composites Reinforced Al3Ti Intermetalic Compound Extruded in Semi-Solid State

Studies were conducted to improve the mechanical properties of composites based on 7475 aluminium alloy reinforced with Al3Ti particles fabricated by the “in situ” process. The first step involved “dissolving” of titanium in the liquid aluminium alloy and fabricating in this way composite materials with different content of the reinforcing phase (15-45wt%). A relationship between the composite hardness and content of the reinforcing phase was confirmed. The second step involved the improvement of cohesion between the reinforcing particles and composite matrix. By extrusion of samples in semi-solid state, an average increase in hardness by 15-20% relative to the unextruded composite was obtained. In the third step, the fabricated composite was subjected to a heat treatment corresponding to the state T6 in 7475 alloy, which raised the hardness by about 30%. Structure examinations carried out by means of optical microscopy and scanning electron microscopies as well as the results of hardness measurements were described. They enabled estimating the effect of the content of produced Al3Ti particles, and of the extrusion process in semi-solid state and heat treatment parameters on the composite properties. In compression test, the yield strength and compressive strength of the heat-treated composites were determined.

Effect of semi-solid heat treatment on elevated temperature plasticity of 304L stainless steel

The present work explores the potential of semi-solid heat treatment technique by elucidating its effect on the plastic behavior of 304L SS in hot working domain. To accomplish this objective, hot isothermal compression tests on 304L SS specimens with semi-solid heat treatment and conventional annealing heat treatment have been carried out within a temperature range of 1273-1473 K and strain rates ranging from 0.01 to 1 s(-1). The dynamic flow behavior of this steel in its conventional heat-treated condition and semi-solid heat-treated condition has been characterized in terms of strain hardening, temperature softening, strain rate hardening, and dynamic flow softening. Extensive microstructural investigation has been carried out to corroborate the results obtained from the analysis of flow behavior. Detailed analysis of the results demonstrates that semi-solid heat treatment moderates work hardening, strain rate hardening, and temperature sensitivity of 304L SS, which is favorable for hot deformation. The post-deformation hardness values of semi-solid heat-treated steel and conventionally heat-treated steel were found to remain similar despite the pre-deformation heat treatment conditions. The results obtained demonstrate the potential of semi-solid heat treatment as a pre-deformation heat treatment step to effectively reduce the strength of the material to facilitate easier deformation without affecting the post-deformation properties of the steel.

Microstructure and Induced Defects of 6061 Al Alloy after Short Times Cyclic Semi-Solid Heat Treatment

Short times (5 min. /cycle) cyclic semi-solid heat treating process up to four cycles for 6061 aluminum alloy was studied. The microstructures of as-received and cyclic semi-solid heat treatment at the edge of samples at 630◦C for one to four cycles were investigated.Short time cyclic heat treatment up to two cycles improves the microstructure to be finer and more globular compared with asreceived one. Short time cyclic heat treatment significantly affects the microstructure at the edge of samples only due to short time heating.Optimum values of grain size and grain sphericity at the edge of samples for short cyclic heating time (5 min./cycle) of about 160 μm and 0.57 respectively. Increasing the number of heat treatment cycles above three increases both shrinkage porosity and micro-cracks formation.

Modification of the Cast Structure in a Cu-12.6 Wt Pct Sn Alloy by Heat Treatment in the Semisolid State

The dendritic structure in a cast Cu-12.6 wt pct Sn alloy can be transformed to a globular structure if the alloy is heat treated in the semisolid range. A Cu-12.6 wt pct Sn alloy has a wide solidification range which encourages the development of a cored dendritic structure during solidification. In order to transform it into a globular structure, heat treatment was carried out separately at three temperatures in the semisolid range, namely 1183 K, 1153 K, and 1123 K (910 °C, 880 °C, and 850 °C) with corresponding liquid volume fractions of 50, 40, and 20 pct, respectively. An attempt was made to optimize the temperature and vol pct of liquid combination. It was observed that microstructural modification proceeded mainly through coarsening, fragmentation, and spheroidization of the dendrite arms. Semisolid heat treatment at 1123 K (850 °C) for 2 hours with 20 vol pct liquid yielded the best result. This microstructure was compared with the microstructure developed by solid-state heat treatment at 1023 K (750 °C) for 2 hours. Assessment of the structure–property co-relation demonstrated that hardness, yield stress, ultimate tensile stress, and toughness of the semisolid heat-treated samples were superior to those of subcritically annealed samples.

Influence of semisolid heat treatment on aging behavior of Al- Cu alloy

The present study focuses on the aging response of semisolid heat treated Al-5 wt.% Cu alloy. Semisolid heat treatment has been employed to modify cast dendritic structure. Globular structure with grain size of 90±25 μm was obtained after semisolid heat treatment. Subsequently T6 heat treatment has been carried out on both as cast and semisolid heat treated conditions. The semisolid heat treated alloy is found to peak-age in 4 h at 170 °C after solution treatment at 500 °C whereas, as cast sample was found to peak age in 18 h. Semisolid heat treatment leads to higher Cu dissolution in Al-α and these accelerated precipitation kinetics during ageing.

Material Characteristics, Manufacturing Processes and Product Performance: A Review of Some Experimental Studies

The paper reviews the influence of material characteristics on the manufacturing process and product performance. Results of some previous experimental studies on hot machining, plasma surface coating and laser surface treatment, semi solid heat treatment and clean steel production are compiled, analyzed and discussed. In each case, a correlation is observed between the material characteristics and its response to the manufacturing process.