Thixoformed Samples Research Articles

Effect of Semisolid and Heat Treatment Process on Microstructural Refinement of Al-7Si Alloy.

Improving the engineering properties of Al-7Si cast alloys (300 series) provides an attractive alternative to automotive and aircraft engine industries. The solubility limit of silicon (Si) in Al contributes to the precipitation of flake-shaped Si particles with sharp edges, which function as a stress riser and promote crack propagation during the eutectic phase while also weakening the protective layer's durability. In this study, the impact of microstructure refinement of Al-7Si alloys by using cooling slope, thixoforming and the T6 heat treatment process on hardness and corrosion resistance behavior was investigated. Results showed that the microstructures of the as-cast alloy had a very coarse dendritic shape, whereas the dendritic transferred to the globular α-Al phase, and the Si particles were replaced into a lamellar- or acicular-like shape after the cooling slope and thixoforming process, respectively. The as-cast, cooling slope and thixoformed samples were subjected to the T6 heat treatment process, which enhanced the hardness to 79, 99 and 104 HV, respectively, due to Si particle refinement. The potentiodynamic test revealed that the corrosion rate dropped to 0.00790 and 0.00736 mmpy-1 in the heat-treated cooling slope and thixoforming samples. This finding can be attributed to the substantially refined Si particles and reduced eutectic phase area due to the smaller cathodic to anodic area ratio.

Strength of Thixoformed A319 Alloy at Elevated Temperature

When alloys are exposed to elevated temperatures they experience a decrement in their mechanical properties that leads to material failure. However, the use of thixoforming, an alternative metal processing method, could enhance mechanical properties by minimising the defects that exist in as-received alloys. Therefore, this study aimed to determine the tensile strength of thixoformed A319 under elevated temperatures by taking into account its intended use in vehicle cylinder head components. Thixoformed A319 was compared with as-received alloy manufactured by permanent mould casting. The cooling slope method was used to prepare the feedstock for thixoforming. The feedstock was reheated by induction heating until it reached 574 °C and was then formed in a mould. Afterwards, the as-received and thixoformed samples underwent T6 heat treatment. The resulting samples were characterised by using optical microscopy, scanning electron microscopy equipped with energy dispersive X-ray, X-ray diffraction analysis and a tensile test. Elevated temperature tensile tests were performed at 250 °C, in line with the temperature condition experienced by cylinder head components during operation. The ultimate tensile strength of the thixoformed samples was 30% higher than that of the as-received samples under elevated temperatures. Also, the analyses of the fracture surfaces showed that porosity, intermetallic compounds and impurities were amongst the failure factors for both alloys.

Effects of forming parameters on microstructure and mechanical properties of a cup-shaped Mg–8.20Gd–4.48Y–3.34Zn–0.36Zr alloy sample manufactured by thixoforming

Thixoforming experiments of a cup-shaped Mg–8.20Gd–4.48Y–3.34Zn–0.36Zr alloy sample were carried out on a hydraulic press at different temperatures under different forming loads. Metallographic analysis, Vickers hardness test, and tensile tests were conducted to investigate the microstructure and mechanical properties of the samples formed under different forming parameters. Samples without macro- and micro-defects formed successfully at 580 °C under different forming loads were attributed to the excellent formability of semisolid slurry with homogeneous globular microstructure. Because solid and liquid phases in the semisolid slurries exhibited different forming behaviors during thixoforming, liquid segregation occurred and resulted in the inhomogeneous microstructural distribution in thixoformed samples. Owing to the different crystal structures of α-Mg and (Mg, Zn)3RE eutectic compounds transformed from liquid phase, their volume fraction, morphologies, and distributions affected the mechanical properties of the thixoformed samples significantly. Neither formability of semisolid slurry nor the microstructure of thixoformed sample was affected significantly by the forming load. Higher values of Vickers hardness measured in the bottom regions of the sample formed under higher forming loads were attributed to the higher residual stress caused by the excess forming energy.

EFFECT OF RAPID COOLING FROM THE SEMISOLID OR LIQUID RANGE ON STRUCTURE AND PROPERTIES OF M2 TOOL STEEL

M2 tool steel was used as a feedstock material for casting from the the semisolid range between 1310-1350oC corresponding to 25 - 50 % of the liquid phase. The chemical composition of the investigated steel is following: 0.85%C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. A specially constructed machine allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. After heating of the feedstock using induction heating by the stamping of semi-liquid sample to a graphite mould pre-heated by resistance heating. The material after thixoforming from 1315oC shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness significantly increases up to 780 HV after thixo-forming due to formation of martensite. The other part was cast from the liquid phase at 1450o C using melt spinning method allowing high cooling rate. The microstructure of melt spun ribbon shows a cellular microstructure of austenitic and martensitic structure without the eutectic. The WC and M23C6 carbides were located at the cell’s boundariers. The microhardness near 1300 HV of the ribbon was much higher than that of semisolid formed samples.

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.

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.

Investigation of wear behaviour of thixoformed and conventional gravity cast AlSi8Cu3Fe alloys

Purpose – In the present work, thixoformed AlSi8Cu3Fe alloy was investigated for its tribological properties. The tribological analysis reveals good sliding wear behaviour for thixoformed alloy due to thixotropic microstructural character. The results are compared with sliding wear behaviour of conventional gravity cast alloy. This paper aims to discuss these subjects. Design/methodology/approach – In this study, thixoforming and gravity casting methods are used to produce materials. The ball-on-disc CSM tribometer was used to carry out the tribological tests that were performed under dry sliding conditions, under two different loads of 1N and 5N. Wear tests were performed first on the conventional gravity cast samples and then on the thixoformed counterparts in exactly the same fashion to facilitate a comparison. The microstructural characterizations are performed in optical microscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. Findings – Thixoformed samples have better wear resistance than conventional gravity cast samples. The wear performance improves in thixoformed AlSi8Cu3Fe alloy thanks to changing the microstructure. The superiority of the former is attributed to its structural features. Originality/value – In this study, the thixoforming of AlSi8Cu3Fe alloy can provide important wear performance in comparison with conventional casting of AlSi8Cu3Fe alloy. This alloy is known to be used widely in automotive industry. Therefore, the alloy is highly exposed to wear and it is thought that thixoforming method can increase the wear resistant.

Effect of annealing on microstructure, phase composition and mechanical properties of thixo-cast 100Cr6 steel

Abstract Steel is a particularly challenging material to semi-solid process because of the high temperatures involved and the potential for surface oxidation. Hot forged and annealed 100Cr6 steel was used as feedstock for the thixoforming process. Samples were heated in an argon atmosphere up to 1425 °C which corresponded to about 25% of liquid fraction. Specimens after heating were transported into a modified shot sleeve of a high pressure die casting machine and pressed into a die preheated up to 150 °C. After thixoforming the microstructure of thixo-casts consisted of large globular grains (average grain size about 300 μm). Martensitic plates in the form of globules were found inside austenitic matrix. Among large globules, a fine grain structure was identified to appear from the liquid phase (average size 20 μm). A mixture of ferrite and carbides was identified within the region of fine grains. Average hardness of thixoformed samples was 759 HV5, while the compression strength attained 3850 MPa. Dilatometric and calorimetric studies carried out directly after thixoforming allowed the determination of characteristic temperatures of retained austenite decomposition and carbide formation. They were used to choose temperatures of heat treatment conditions: 150 °C, 180 °C, 250 °C, 320 °C and 400 °C. After tempering at 150 °C, precipitates of e carbide in martensite needles were found and at the same time the retained austenite transformed into martensite. X-ray phase analysis showed that thixo-cast contained: 78.7% – α′-Fe, 15.3% – γ-Fe and 6% – e carbides. Hardness increased after annealing at 150 °C up to 808 HV5, while after tempering at 180 °C and 250 °C the hardness decreased to 693 HV5 and 665 HV5, respectively. This effect was connected with the decomposition of austenite into ferrite and transformation of martensite into ferrite. After treatment at 320 °C the hardness decreased to 556 HV5, which, according to quantitative phase analysis was due to the presence of α-Fe and Fe3C. After the treatment at 400 °C for 2 h the microstructure consisted of ferrite and coarse cementite. Its average hardness was 508 HV5. The increase of annealing temperature caused a gradual decrease of compression strength to 2420 MPa at 400 °C with the increase of plastic contraction strain to 22%.

Microstructure and mechanical properties of A356 thixoformed alloys in comparison with gravity cast ones using new criterion

The effect of thixoforming process on morphologies of silicon particles that affect fracture mode of A356 alloy was investigated. Microstructure and fracture surfaces of thixoformed samples were investigated by image analyzing technique and scanning electron microscopy. A new combination parameter, called silicon density ratio (SDR) index, was introduced. SDR index approximates the collective effects of morphological characteristics of silicon particles on microstructure transparency of alloy in crossing the dislocation. It is suggested that samples with lower SDR index have superior mechanical properties, especially elongation, and consequently intergranular fracture mode. On the contrary, samples with higher SDR index have inferior mechanical properties and fracture path tends to propagate along the cell boundaries leading to transgranular fracture.

Thixoforming of spray formed M2 tool steel

Using SW7M (M2) tool steel as a feedstock material, several thixoforming tests were performed at various starting temperatures in the semisolid range between 1310-1350°C corresponding to 25 - 50 % of the liquid phase as indicated by the results of DSC studies. The chemical composition of the investigated steel is following: 0.85 %C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. Specially constructed apparatus allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. Fast heating of the feedstock was achieved using induction heating through a high frequency generator, followed by the stamping of semi liquid sample to a graphite mould pre-heated by resistance heating. Observations of the microstructure before and after semisolid processing were performed using optical, scanning and transmission electron microscopy. The initial material has a very fine grain size of ∼2-5 μm. This material after thixoforming at 1315°C shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness of spray formed steel is around 304 HV significantly increases up to 780 HV after thixo-forming due to formation of martensite. At the end of paper the basic results of numerical simulations were shown.

Effect of a novel thixoforming process on the microstructure and fracture behavior of A356 aluminum alloy

In the present study, a novel thixoforming process for semi-solid deformation of A356 aluminum alloy is introduced using a continuous hot deformation process to the temperature being lower than the eutectic temperature of the alloy. A new hypothesis was introduced and the deformation mechanism of the alloy was investigated using the presented hypothesis. Microstructure and fracture surfaces of thixoformed samples were investigated using image analyzing technique and scanning electron microscopy. Obtained results indicated that this novel thixoforming process produces fine and compact silicon particles, dispersed uniformly in the microstructure of the alloy, compared to those produced by conventional thixoforming and gravity-cast processes with large and integrated morphology for silicon particles. The production stages of these silicon particles in this process were well documented by mentioned hypothesis. In order to investigate the effect of this novel process on mechanical properties of A356 alloy, tensile tests were conducted on produced samples. It was found that morphological changes of silicon particles as well as increasing the density ratio of samples in this process have a remarkable effect on enhancing the mechanical properties of produced alloy in comparison with other production routes. A new combination parameter, i.e. silicon density ratio (SDR) index was introduced. This parameter correlates the mechanical properties of samples to morphological properties of silicon particles and density ratio of them. Results of the study also indicated that samples with low SDR index have superior mechanical properties and consequently intergranular fracture mode.

Pitting corrosion of thixoformed, rheocast and gravity cast A356-T6 alloy in chloride media

In the research work presented here, the pitting behaviour of thixoformed A356-T6 alloy, with different reheating temperatures, was evaluated and compared with the pitting behaviour of rheocast and gravity cast A356-T6 alloys with the same composition. To study the pitting behaviour, linear sweep voltammetric tests were performed on thixoformed, rheocast and gravity cast A356-T6 alloys in a 3·5%NaCl solution. A simulation method was also used to identify local galvanic corrosion current density between local galvanic couples. Results obtained show that the resistance to pitting corrosion of thixoformed samples formed at 590°C was higher than that of the samples formed at 600°C as well as rheocast and gravity cast samples. The improvement in the pitting corrosion resistance due to the thixoforming process is attributed to morphological aspects of the silicon phase as well as the area effect as related to galvanic corrosion between the silicon particles and the eutectic aluminium phase.

Fracture behavior of thixoformed A356 alloy produced by SIMA process

There are various methods for production of thixoforming stock. One of these methods is strain induced melt activation (SIMA) process. In the present study, the effect of thixoforming process on morphological properties of silicon particles that affect fracture mode of A356 alloy was investigated. Microstructure and fracture surfaces of thixoformed samples were investigated by image analyzing technique and scanning electron microscopy. A new combination parameter, called silicon particle appearance (SPA) index, was introduced. SPA index approximates the collective effects of silicon particles morphological characteristics on microstructure transparency of alloy in crossing the dislocation. It is suggested that samples with lower SPA index have superior mechanical properties, especially tensile elongation, and consequently intergranular fracture mode. On the contrary, samples with higher SPA index have inferior mechanical properties and fracture path tends to propagate along the cell boundaries leading to transgranular fracture.

Quantitative analysis of pitting corrosion behavior of thixoformed A356 alloy in chloride medium using electrochemical techniques

Pitting behavior of thixoformed A356 alloy, with different reheating temperatures, was evaluated and compared with the pitting behavior of rheocast and gravity-cast A356 alloy with the same composition. The presence of silicon particles lead to the development of pitting at the silicon–matrix interface. Results of galvanic corrosion simulation showed that the resistance to pitting corrosion of thixoformed samples formed at 590 °C was higher than the samples formed at 600 °C as well as rheocast and gravity-cast samples. These results relate pitting corrosion behavior of thixoformed, rheocast and gravity-cast A356 alloy to their microstructure characteristics using a new parameter in the form of λ/ DV′.