Abstract: Microstructures with fine globular grains and refined eutectic structures are important to enhance the mechanical properties of A356 alloys processed by semi-solid and gravity die casting. Rare earth (RE) additions have been shown to be capable of refining both the α-Al particles as well as modify the eutectic phase of alloys. In semi-solid die casting, Al7Si0.3Mg alloys with RE concentrations (0, 0.1 and 0.4 wt.%) were used to prepare semi-solid slurries using the SEED (Swirling Enthalpy Equilibrium Device) method, and subsequently semi-solid die cast. The same compositions of alloys were also applied to gravity die casting. The microstructure and mechanical properties of castings in two processes have been characterized. Compared to the grains produced in gravity die casting, globular grains with small size (260 μm) in the semi-solid die casting significantly enhance the UTS and elongation of alloys. Although the size of grains had no change with increasing RE concentrations in alloys. The Al-Si eutectics were changed to refined morphology with the 0.1 wt.% RE addition, which enhanced the ductility of alloys in two processes. When increasing the RE addition to 0.4 wt.%, the RE-rich phases precipitated at grain boundaries, which decreased the UTS and elongation of alloys.
Abstract: Al-Si-Mg casting alloys, such as Al-7Si-0.3Mg alloy A356, are heat treatable and can be precipitation hardened to the T6 temper condition. However, Al-Mg-Si casting alloys (5xx series) are generally not considered to be heat treatable. These 5xx series castings are known for good castability and good resistance to corrosion, especially in marine environments. This paper investigates the extent to which 5xx series alloys could possibly be artificially aged. The influences of artificial ageing time on the overageing characteristics of both Al-Mg-Si and A356 casting alloys have been studied. A356 aluminium alloy castings were produced using the CSIR rheo-high pressure die casting process (R-HPDC). Al-Mg-Si alloys were cast using permanent mould casting. The rate of overageing of these alloys is of importance for potential higher temperature applications. The overageing characteristics of Al-Mg-Si and A356 aluminium alloys have been investigated at an artificial ageing temperature of 190°C for ageing times up to 128 hours. It is shown that the rate of overageing of Al-Mg-Si casting alloys is lower than for alloy A356. This could possibly result in the use of these alloys in applications at temperatures that are higher than where alloy A356 can be employed. It also allows the possibility of using the 5xx series alloys as an alternative to other Al-alloys for R-HPDC applications.
Abstract: In semi-solid casting, a slurry consisting of primary α-Al crystals and liquid is injected into the die cavity. The solidification in the die-cavity occurs by the growth of the primary α-Al crystals formed during slurry preparation and in the shot sleeve, nucleation and growth of in-cavity solidified crystals and ends with the eutectic reaction. During solidification in the die cavity, the cooling rate near the die wall is higher in comparison to the centre of the casting, particularly for thick-walled castings. The solidification conditions for the slurry α-Al crystals that are closer to the die wall can be very different compared to the slurry α-Al crystals located at the casting centre. This can result in different solute concentration in the interior of the α-Al globules in different regions of the semi-solid casting cross-section and consequently, different response to heat treatament. The RheoMetal™ process was used to produce thick-walled semi-solid castings. Semi-solid castings in the as-cast and T6 conditions were investigated. Indentation tests for hardness measurements in the nano-range were performed in the interior of α-Al globules near the surface and at the casting cross-section centre. The hardness variation across the casting cross-section was evaluated by low-force Vickers hardness. The castings in the as-cast condition showed more uniform properties in the cross-section compared to the T6 condition. Additionally, the results suggest that microsegregation in the interior of α-Al globules is very low in castings in the as-cast and T6 conditions.
Abstract: Forging processes, such as liquid forging, rheoforging and thixoforging process, were used in this study as an effective method for manufacturing high-strength, finely-dispersed and highly-uniform AZ91D alloy parts. It has been found that although the microstructures of the three forging parts are all composed of α-Mg and β-Mg17Al12 phases, their shapes, sizes, numbers and distributions are all different, which also influences their mechanical properties. The tensile fracture morphologies were observed and their energy spectrums were analyzed by scanning electron microscopy. The results show that the cracks are mainly originated from the brittle fracture of the β-Mg17Al12 eutectic phases, whereas their fracture morphologies are greatly different, which indicate the direction for providing a reasonable forging process.
Abstract: A kind of 9Cr18Mo multidiameter shaft part was fabricated through a designed thixoforging set-up. As compared with conventional heat treatment specimen, wear and corrosion behavior of 9Cr18Mo thixoforging specimen were studied. The results showed the surface of 9Cr18Mo thixoforging specimen demonstrated fine wear and corrosion resistance. Thixoforging specimen exhibited functionally graded property with inner austenite particles and fine dendrite eutectic skin layer. At the skin layer, the former solidified liquid led to the concentration of alloying elements such as Cr and Mo. Besides, eutectic structure of secondary austenite and M7C3 carbide helped to improve the hardness. The high hardness of M7C3 carbide and high concentration of Cr, Mo elements at the edge skin layer contributed to reducing the delamination wear, abrasive wear and the appearance of oxidation. Even at higher wear load, relative sliding occurred at the wear surface, which avoided crack formation and chunk spalling. As compared with conventional heat treatment specimen, corrosion current density of thixoforging surface reduced, while corrosion potential increased. Thixoforging surface also exhibited passivation behavior and the corrosion resistance increased slightly. High concentration of Cr and Mo at the skin layer avoided the formation of chromium depleted zone. In view that wear and corrosion failure tend to occur at the surface of specimen, the thixoforging specimen with a wear and corrosion skin layer might provide a possibility for expanding the application fields of 9Cr18Mo steel.
Abstract: In the field of material production and application, strength and hardness are the two most common properties of metallic materials. It’s one familiar phenomenon that the hardness of one certain alloy has positive relationship with its strength in conventional dendritic alloys. When it comes to non-dendritic semi-solid alloys, it’s unclear that the relationship is still right or not. In this paper, the molecular mechanics, as well as finite element simulation and experimental verification were combined to study the internal deformation regularity of metallic material and the correlation between the two parameters was illustrated. Firstly, the displacement of solid atom in metallic crystal cell was well described in the view of energy cost. Secondly, the total strain amount under local indenting deformation (resistant boundary) and overall impressing deformation (free boundary) were compared to study the correlation between hardness and compression strength in semi-solid globule grain alloy. Finally, the data collected in semi-solid processed products was applied to be compared against traditional casting and wrought aluminum alloys.
Abstract: The research on semisolid metal forming of Ni-based superalloys is limited because of the difficulties in preparing semisolid billets. Traditional methods for fabricating semisolid billets are SIMA and RAP. In order to decrease the processing procedures, a new method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was used to fabricate semisolid billets in this paper. In order to get the optimal parameters to fabricate Ni-based GH4037 semisolid billets, the effects of isothermal temperature and soaking time on microstructure evolution were investigated. The isothermal temperature ranges from solidus to liquidus, and the soaking time ranges from 10min to 60min. The results show that near-globular grains can be obtained only in the semi-solid state with appropriate solid fractions. When the solid fraction was high, the shape of grains was irregular and the average size of grains was large and inhomogeneous. When the isothermal temperature reached 1350°C, spherical grains appeared, and many small intragranular liquid droplets existed. During the appropriate isothermal temperature range, from 1350°C to 1380°C, the shape of solid grains was globular and the size of solid grains was homogeneous. At the same time, the size of the grains and the roundness of the grains increased with the increasing of isothermal temperature.
Abstract: Compared with monolithic bulk-metallic glasses at room temperature, in-situ two-phase bulk metallic glass matrix composites (BMGMCs) exhibit improved toughness. However, most of in-situ BMGMCs are developed through extremely rapid cooling, during which the solidification process cannot be controlled effectively. As a consequence, the microstructures of in situ two-phase composites, even with the same composition, vary from one to another, highly depend on the cooling rate. In the present investigation, the Ti-Zr-V-Cu-Be BMGMCs was prepared by using copper mould suction process, and the evolution of microstructures at semi-solid stage was investigated. The results indicated that microstructures of billets, produced by copper mould suction and water quenching after isothermal holding at semi-solid stage, mainly contained β-Ti phase and glass matrix. Isothermal holding temperature and time interval determined the final morphology of β-Ti phase. Compression tests showed that semi-solid processing could further improve ductility of BMGMCs effectively.
Abstract: The application of hybrid structures or components made of dissimilar metal offers the potential to utilize the advantages of different materials often providing unique solutions to engineering requirements. However, the joining of materials by conventional welding techniques becomes difficult if the physical properties such as melting temperature and thermal expansion coefficients of the two materials are different. In this study, a new process of joining semi-solid AISI D2 tool steel and AISI 304 stainless steel using a partial remelting method is proposed. Moreover, the effect of the holding time on the microstructural evolution was investigated. The processing temperatures for the thixojoining was 1320°C and held for 5, 12, 20 and 30 minutes, respectively. The results obtained from investigating the basic geometries demonstrated a good joining quality that differs from the conventional process of welding. Metallographic analyses along the joint interface between semi-solid AISI D2 and 304 stainless steel showed a smooth transition from one to the other, with neither oxides nor microcracking being observed.
Abstract: CNTs-grafted carbon fiber reinforced magnesium composites (CNTs-Cf/Mg) were successfully prepared by liquid-solid infiltration extrusion process, in which the carbon fiber cross-ply preform were grafted with carbon nanotubes (CNTs) by using the injection chemical vapor deposition (ICVD) technique. The mechanical properties of AZ91D matrix alloy and magnesium matrix composite at different states were tested and compared. The results show that the dendrites of the as-cast AZ91D alloy are transformed into granular grains after liquid-solid forming. The composite reinforced by carbon fiber with grafting nanotubes is beneficial to the one reinforced with carbon fiber but without grated CNTs. The nanotubes grafted on carbon fiber improve the bonding property of the fiber-matrix interface and protect the carbon fiber from degradation more effectively. The tensile strength of the CNTS-Cf/Mg composites is 28.3% higher than the Cf/Mg composites.