Fabrication of graphene nanoplatelets reinforced Mg matrix composites via powder thixoforging

A powder thixoforging route combined with slurry based mixing process was proposed to fabricate graphene nanoplatelets(GNPs) reinforced magnesium matrix composites(MgMCs). The originally spherical and ball-milled ZK60 powders were used as matrices, respectively.The mixing of 0.05 wt.% GNPs with the spherical powder led to GNPs clusters and degraded the mechanical properties of the composite.In contrast, with the addition of an optimal content(0.1 wt.%) of GNPs, the composite fabricated from ball-milled powder achieved a joint enhancement in tensile yield strength(52%) and fracture toughness(19%), demonstrating a pronounced strengthening efficiency of 650% and a good balance between strength and toughness. The ball-milled powder endowed the composite with a homogenous distribution of GNPs and a denser microstructure with reduced Mg-Zn eutectics, and the thixoforging process offered a well-bonded Mg/GNP interface, making full use of the strengthening and toughening potential of GNPs. Theoretical predication based on a modified shear-lag model suggested that load transfer dominated the strengthening mechanisms. In-situ tensile tests verified that crack deflection, secondary cracks and GNPs bridging mainly accounted for the toughening mechanisms. A numerical model with consideration of GNPs orientations was also established to understand the toughening effect from GNPs bridging.

X38CrMoV5 hot-work tool steel as tool material for thixoforging of steel:Numerical and experimental evaluation

Although thixoforming of low melting point alloys as aluminum or magnesium is now an industrial reality,thixoforming of high melting point alloys as steel is still at the research level.High working temperature,die wearing and production rate are problems that must be solved and are under investigation.The aim of this work is to evaluate the thermal and mechanical loadings applied to the tools during the steel thixoforging process in order to determine whether classical hot-work tool steel can be an appropriate tool material.This evaluation has been realized thanks to experimental trials and to simulations on the finite elements code Forge2008.The effect of the loadings on the tool's failure modes are highlighted and compared with the ones observed in classical hot forging.Beyond this,the failure modes of hot-work tool steel,the X38CrMoV5 or H11,were presented.

Investigation of semi-solid aluminum alloy filling-plastic flowing in thixoforging

The semi-solid filling-plastic flowing integrated forging process of semi-solid 6061 Al alloy was simulated by commercial finite element software DEFORM-3D.Temperature,fluid and stress-strain fields were considered in numerical simulation.The simulation results show that the plastic deformation of billet of the ends is higher than that of billet in the straight cylinder.The value of plastic deformation varies with loading mode and plastic deformation fields at the stage of increasing pressure to constant value.When the thixoforging experiments were performed at 590 ℃,15 mm/s of punch velocity and 46 MPa of pressure side urn,it gets the filling wholly and dense internal organization of semi-solid thixoforging parts is gotten.Finite element analysis results are compatible with experimental ones.

Effects of Temperature and Strain Rate on Solid-/Liquid-Phase Flow Behavior of 9Cr18 Steel During Thixoforging

A thixoforging process of the 9Cr18 steel was conducted in a designed setup, and a kind of multi-diameter component was fabricated. The effects of the forming temperature and the strain rate on the solid-/liquid-phase flow behavior were discussed. The results showed that functional gradient properties of the 9Cr18 steel could be obtained after thixoforging. Changes of microstructure along radial direction could be obtained. Solid austenite was retained after fast cooling, and the liquid film enriched in alloying elements was extruded outside to form a dendrite skin layer. As temperature increased, more molten liquid formed during thixoforging. A heterogeneous flow phenomenon was activated as free liquid channels were formed. The macro-separation of solid and liquid phases was critical for the formation of functional gradient properties. Above 1300 ℃, full dendrite skin layer could be formed. The strain rate affected the thixotropic property via influencing the deformation time of thixoforging. In the presence of lower strain rates, there was more time for the flow of liquid metal, which was the key to the extension of the thixotropic stage. High temperatures and low strain rates contributed to the formation of full skin layer for the designed specimen. The average thickness of skin layer for current specimen could be over 1000 ktm when thixoforged at 1340 ℃ and under a strain rate of 0.02 s^-1.

Microstructural characteristics of thixoforged A356 alloy in mushy state

Tensile properties of a thixoforged A356 alloy were measured and compared with those of rheocast and gravity cast alloys with the same composition.In the thixoforging process,A356 rheocast alloy produced by stirring method was reheated to 600 °C or 610 °C,held for 10 min,and then 50% deformation was applied.Microstructure,tensile properties and fracture surfaces of produced specimens were then investigated.Results obtained show that tensile strength,yield strength and elongation-to-failure of thixoforged samples formed at lower temperature are higher than those of samples formed at higher temperature.By increasing temperature,in the case of thixoforged samples,tensile fracture path is changed from trans-primary alpha phase to inter-primary alpha phase.Differences observed in the tensile fracture path and improvement in the mechanical properties due to thixoforging process is attributed to microstructural changes as well as morphological aspects of silicon phase.

Metal flow and die wear in semi-solid forging of steel using coated dies

Thixoforging of steels is a potential forming technology,which aims at producing near-net-shaped components with good quality from high strength steels in one forging step.The thixoforging process parameters such as billet temperature,temperature distribution after reheating,argon gas pressure,transportation time and forging load were investigated on the thixoforging of non axis-symmetric parts of steel grade X210CrW12.The experimental and numerical study of the material flow and tool temperature load reveal the areas of intensive tool wear,thus being useful for further tool design.Hardened hot working steel X38CrMoV5-1 as a tool bulk material with protecting thin films of TiAlN/γ-Al2O3 shows good experimental results at 170 forging cycles.