J. Zhou

In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys.

Zinc and zirconium were selected as the alloying elements in biodegradable magnesium alloys, considering their strengthening effect and good biocompatibility. The degradation rate, hydrogen evolution, ion release, surface layer and in vitro cytotoxicity of two Mg–Zn–Zr alloys, i.e. ZK30 and ZK60, and a WE-type alloy (Mg–Y–RE–Zr) were investigated by means of long-term static immersion testing in Hank’s solution, non-static immersion testing in Hank’s solution and cell-material interaction analysis. It was found that, among these three magnesium alloys, ZK30 had the lowest degradation rate and the least hydrogen evolution. A magnesium calcium phosphate layer was formed on the surface of ZK30 sample during non-static immersion and its degradation caused minute changes in the ion concentrations and pH value of Hank’s solution. In addition, the ZK30 alloy showed insignificant cytotoxicity against bone marrow stromal cells as compared with biocompatible hydroxyapatite (HA) and the WE-type alloy. After prolonged incubation for 7xa0days, a stimulatory effect on cell proliferation was observed. The results of the present study suggested that ZK30 could be a promising material for biodegradable orthopedic implants and worth further investigation to evaluate its in vitro and in vivo degradation behavior.

3D FEM simulation of the whole cycle of aluminium extrusion throughout the transient state and the steady state using the updated Lagrangian approach

Abstract Aluminium extrusion involves the generation of free surface, thermal effects, large deformations and complex geometries. The established finite element method (FEM)-based 3D simulation tools using the updated Lagrangian approach, or the Eulerian approach or the arbitrary Lagrangian Eulerian approach all have limitations in describing the process that develops from the transient state to the steady state before reaching the end when the steady state is disturbed. As a result, the simulation of aluminium extrusion performed so far has been restricted to simple geometries, small length-to-diameter (L/D) ratios, the beginning stage or steady-state conditions. This paper reports on an unprecedented attempt to simulate an entire cycle of aluminium extrusion from a billet with an L/D ratio of 4 to a solid cross-shaped profile, using the DEFORM 3D software based on the updated Lagrangian approach. Simulation successfully predicts a complete extrusion pressure/ram displacement diagram that begins with a pressure breakthrough and ends with another pressure rise due to the inhibition of metal flow by the rigid dummy block. The developments of velocity, effective strain and temperature inside the deforming billet indicate that the process is non-steady, even in the steady state, as a result of continuous heat generation and sticking condition at the billet–container interface. The non-steady characteristics are reflected in the expanding deformation zone and shrinking dead metal zone. Simulation also reveals the patterns of the maximum temperature variations in the workpiece and in the tooling, due to heat generation and exchange. Even at a relatively low ram speed of 2xa0mm/s, the maximum temperature of the workpiece, after an initial steep rise, increases gradually till the end of the process, which may well lead to the occurrence of hot shortness. On the basis of these results, a change of the conventional mode of aluminium extrusion is recommended, which at present operates almost all at a constant ram speed and often begins with a uniform billet temperature across the aluminium extrusion industry in the world.

FEM analysis of aluminium extrusion through square and round dies

Abstract Understanding the state of stress, strain and the temperature of an aluminium alloy going through a die during extrusion is of great importance for running the aluminium extrusion process, because they are closely related to the surface quality of the extruded products, throughput and scrap rate. It has been made clear that surface tearing is mainly caused by excessive local tensile stresses at the surface of the extrudate and hot shortness is due to the heat generated during the process that brings the extrudate temperature above the incipient melting point of the billet material. Both the state of stress and the temperature are complicatedly related to the extrusion conditions including initial billet temperature, ram speed, reduction ratio, friction at the interfaces, deformation resistance of the billet material, die geometry, as well as thermal characteristics of the billet material and the tooling. The practical means to accurately measure the stress, strain and temperature is yet quite limited. In the present work, 3D FEM simulation of the aluminium extrusion process was performed to determine the state of stress, strain and the temperature of a commercial aluminium alloy going through square and round dies. It has been found that at the same process conditions, the state of stress in the aluminium alloy going through a round die is more favourable than going through a square die, especially at a high reduction ratio. The magnitude of the tensile stress component at the corners of the square extrudate is much higher than at the surface of the round extrudate, which makes the square extrudate more tearing prone. Simulation also reveals that while temperature evolution during the process is similar for both of the die shapes, temperature rise across the section is prominent, especially at sharp corners of the square extrudate. This has been ascribed to the non-uniform metal flow through the square die.

A 3D FEM simulation study on the isothermal extrusion of a 7075 aluminium billet with a predetermined non-linear temperature distribution

In this paper, computer simulations were performed on the extrusion of 7075 aluminium billets with non-uniform temperature distributions in order to inhibit excessive temperature rise that tends to occur during the conventional extrusion of a uniformly preheated billet. The simulations showed that when a linear temperature distribution was imposed on a billet with its rear end 150°C colder than its front end, the maximum temperature of the workpiece would still increase from 450°C to over 520°C at the end of an extrusion cycle, giving rise to hot shortness. Assigning a non-linear temperature distribution during preheating the billet could however significantly lessen this undesirable temperature increase, leading to isothermal extrusion. Such a non-linear temperature distribution was determined on the basis of the results obtained from the simulation of the extrusion of a billet with a linear temperature distribution. With this predetermined non-linear temperature distribution, the maximum temperature of the workpiece remained stable near the die entrance. The radial temperature variation on the cross-section of the extrudate became less significant as the material flowed through the die. In addition, the simulations showed another advantage of isothermal extrusion, i.e. an invariable die face pressure throughout an extrusion cycle. The maximum positive (tensile) principle stress was revealed at the corner of the die orifice, indicating that tearing tended to occur there.

Application of three­dimensional numerical simulation to analysis of development of deformation zone at beginning of aluminium extrusion process

Abstract Understanding the thermomechanical phenomena that occur during aluminium extrusion with respect to the variations of temperature, flow stress, strain, and strain rate is of importance for process optimisation. Conventional analytical methods are restricted to the steady state stage of the process and thus cannot provide an insight into the dynamic changes taking place during the initial stage. In the present work, threedimensional simulations using the finite element method were carried out to analyse the development of the deformation zone at the die front and the temperature evolution, before the process attains the steady state. The analysis revealed that a change in friction factor at the billet/container interface from 0.3 to 0.9 enlarges the dead metal zone. However, its size appears unaffected by a change of die orifice shape from round to square at the same reduction ratio. The increase in friction results in an increase of initial extrusion load of about 6%.

Characterisation of space holder removal through water leaching for preparation of biomedical titanium scaffolds

Abstract Scaffolds for bone tissue engineering are highly porous materials having interconnected and homogeneously distributed pores to facilitate the formation of new bone tissue. At the same time, appropriate mechanical strength is required in the scaffolds to withstand stresses in the in vivo environment. The space holder method has been used to fulfil these contradictory requirements in the fabrication of titanium scaffolds. Space holding particles are mixed with titanium particles then removed before or during sintering, to leave pores in the scaffolds. Despite its importance, the removal of space holders has rarely been studied. In the present study, removal by water leaching was investigated. Leaching was characterised using a novel real-time measurement technique adopted from ASTM B963-08 that achieved precise scaffold weight loss data reflecting the removal of space holding particles. The acquired data fit existing solvent debinding models for powder injection moulded parts, allowing the mechanism involved during water leaching to be determined.

Microstructure Prediction of Hot-Deformed Aluminium Alloys

In this paper, a deformation test method to reproduce, on a laboratory scale, the microstructure evolution of aluminium alloys occurring during industrial forming processes with a limited number of tests is presented. A hot inverse extrusion setup was designed in order to generate, inside one single specimen, a wide range of strains at a given temperature and ram speed. Two commercial aluminium alloys (AA6060 and AA6082) were investigated at different processing conditions (temperatures and forming rates). Detailed optical microstructures were examined and grain sizes were determined at different spots of each specimen. Thermo-mechanical coupled simulations of the deformation tests were performed using the DEFORM 3D FEM code. On the basis of recrystallization equations, the distributions of strains, strain rates and temperatures were correlated to the grain sizes measured through linear regression. Finally, FEM simulations were run again with the established recrystallization model, and the results were compared with the experimental data.

Characterisation of different types of dispersoids present in homogenised Al–4·5Zn–1Mg alloy containing Zr, Cr and Mn

Abstract In the present paper, various types of dispersoids possibly formed during homogenisation of AA7020 alloy containing Cr, Zr and Mn are described. It shows that, in addition to the Zr containing dispersoids, three other types of dispersoids may be present in the homogenised microstructure of AA7020 aluminium alloy. These dispersoids are Cr and Mn containing ones and the one with a mixture of different elements. The Zr and Cr containing dispersoids are formed in the grain interior at all of the homogenisation conditions. However, the Mn containing ones form only in the grain boundary regions in the vicinity of Al17(Fe3·2,Mn0·8)Si2 particles at temperatures ⩾510°C and holding times longer than 4 h.

Grain boundary versus particle stimulated nucleation in hot deformed Al–4·5Zn–1Mg alloy

Abstract In the present research, an analytical model of recrystallisation combining grain boundary (GB) and particle stimulated nucleation (PSN) in hot deformed Al–4·5Zn–1Mg alloy was developed. The effects of evolving subgrain sizes and associated stored energy in addition to the Zener drag pressure on recrystallisation were incorporated into the model. A number of constants, fitting parameters and activation energies for recrystallisation nucleation were determined for the Al–4·5Zn–1Mg alloy by investigating recrystallisation after axisymmetric hot compression testing. It was found that with increasing temperature and decreasing strain rate, the number density of the nuclei decreased. Both GB and PSN played roles in the recrystallisation of this alloy. The temperature of subsequent annealing was found to be not strongly influential on the mode of recrystallisation, i.e. recrystallisation originated by GB nucleation or PSN. However, deformation conditions (strain, strain rate and temperature) clearly affected the mode of recrystallisation. Nucleation by the GB mechanism was especially marked. However, PSN played a dominant role in recrystallisation after deformation to large strains. In addition, when strain was <1, nucleation by the GB mechanism was only dominant if the strain rate was smaller than 1 s−1 and the deformation temperature was higher than 450°C.

Subgrain growth in presence of nanosized dispersoids in Al–4·5Zn–1Mg alloy

Abstract In this paper, an analytical model for subgrain growth in the presence of nanosized dispersoids is presented. The growth rate of subgrains is correlated to the mobility of low angle grain boundaries (LAGBs) and the net driving force for growth. The driving force is considered as the difference between stored energy, being inversely proportional to the average subgrain size, and the Zener drag pressure. A material dependent constant necessary for the determination of the mobility of LAGBs is estimated by fitting the model predictions into the experimental results. Model predictions of the evolution of subgrain sizes with annealing time at different temperatures show that subgrain growth intensifies with increasing annealing temperature. The magnitude of the Zener drag pressure has a predefined effect on the subgrain growth rate. The model predicts that when the PZ/γs ratio is smaller than 1 μm−1, the Zener drag pressure has an effect on subgrain size and the subgrain growth rate tends to decrease. However, when the PZ/γs ratio is larger than 1 μm−1, there is a limit beyond which the subgrain size does not increase with increasing annealing time. The limiting subgrain size is a function of the surface boundary energy and Zener drag pressure.