Comparison between mechanochemical effect on die cast and extruded magnesium alloys

Abstract

The mechanochemical effect (MCE) of several magnesium based alloys, obtained by both die casting and extrusion methods, was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The mechanochemical behaviour of each alloy was evaluated as a function of die cast parameter, environment, and alloy composition. Electrochemical tests were performed in a buffer solution of sodium tetraborate, (Na₂B₄O₇) with and without magnesium hydroxide (Mg(OH)₂). The MCE was correlated with the microstructure of the Mg alloys.     

Effect of magnesium content on microstructure and properties of Al-Cu-Mg alloys

Abstract

The effect of magnesium content, in the range 1 - 5%, on the microstructure of aluminium - copper - magnesium alloys was examined. Using X-ray powder diffraction it was established that the tetragonal intermetallic compound Al₂Cu with lattice parameters a = 6.034 Å, c = 4.870 Å, and V = 177.31 Å3 and the orthorhombic intermetallic compound Al₂CuMg with lattice parameters a = 4.011 Å, b = 9.251 A?, c = 7.007 Å, and V = 264.74 ų are formed across the whole range of magnesium additions. The effect of magnesium content on the microstructure was monitored quantitatively. Using automatic image analysis the linear intercept grain size, secondary dendrite arm spacing (DAS), size of eutectic cells (L _e), and the size distribution and volume fractions of α solid solution and eutectic were measured. In alloys containing high magnesium content the average values of DAS and grain size were found to decrease.     

Microstructural characterisation of several magnesium alloys in AM series

Abstract

The as cast microstructures of several magnesium alloys in the AM series have been characterised, including the dendritic structure, Al segregation and the Mn containing phases. Two types of Mn containing particles were found and studied in the alloys mainly by using TEM. Type I is of equiaxed morphologies and a higher Mn/Al ratio in the composition. The flower shaped and dendritic type II phase has a lower Mn/Al ratio. The equiaxed type I could originate in the particle residues in the melts retained in the as cast alloy and the type II might form around a small type I particle. Electron diffraction has determined that both type I and type II have bcc crystal structure with lattice parameter a=0·91 nm. The higher Mn content in an alloy may lead to a higher amount of Mn containing particles.     

Creep behaviour of AZ61 magnesium alloy at low stresses and intermediate temperatures

Abstract

Tensile creep response was investigated for AZ61 alloy (Mg - 6.4Al - 0.9Zn - 0.2Mn, wt-%) of mean linear intercept grain size ~ 25 μm at stresses in the range 0.9 - 4 MPa over the temperature range 250 - 346°C. Bingham behaviour is obtained with strain rate ? under stress σ given by ?∝σ - σ_o with a threshold stress σ_o decreasing from 1.25 MPa at 210°C to ~ 0.5 MPa at 346°C, which is similar to earlier work on pure magnesium. The corresponding Arrhenius plot of log (Td?/d σ) versus T^-1 indicates an activation energy comparable with that expected for the grain boundary self-diffusion coefficient D _B, and values of D _Bδ (where δ is the effective grain boundary thickness) derived from the Coble equation are also similar to those for pure magnesium. Grain elongation in the direction of the tensile stress is also consistent with the key indicative feature of diffusional creep: deposition of material at grain boundaries nearly transverse to the axis of tensile stressing. Strain rates versus stress are shown to be continuous with published results for superplastic flow of AZ61 at comparable temperatures but higher stresses.     

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Abstract

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10^?3 to 100 s^?1. When compressed at 10^?3 s^?1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10^?2 s^?1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10^?1 s^?1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s^?1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.     

Effect of some alloying elements on boiling point of magnesium

Abstract

The evaporation capacity of alloys differs with temperature, and this is the basis of a new experimental method to measure the boiling points of various kinds of alloys. In the present work, the effects of Al, Zn, Mn and La additions on the boiling point of magnesium have been studied. It is shown that various elemental additions and their varying contents in magnesium alloys have different influences on the boiling point of the alloys. Among these additions, Zn affected the boiling point of magnesium alloys most obviously, followed by Mn, Al and La. The boiling point of Mg–6 wt-%Zn alloy was the highest in the present study, up to 1715 K.     

The texture and anisotropy of hot extruded magnesium alloys fabricated via rapid solidification powder metallurgy

Rapid solidification magnesium alloy powders produced by spinning water atomization process were hot extruded into rectangular bars, from which tensile and compression samples have been cut at 0°, 45° and 90° angles from the extrusion direction to study their anisotropy. Electron back-scattered diffraction analysis has been used to investigate the texture evolution during the extrusion process. Texture parameters like the Schmid factor and the intensity of (0 0 0 1) basal plane in the pole figure have been evaluated and correlated to the mechanical properties. Results have shown that the extruded rods exhibited high strength and relatively less anisotropy compared to other previously reported values for wrought magnesium alloys. Tensile and compression yield stresses have shown very similar values to each other at all loading directions. This limited anisotropy could be linked to both the fine grained and inter-metallic-compound-dispersed microstructure of the extruded alloys. Dynamic recrystallization behavior during hot extrusion has also been investigated in the present study.     

Corrosion fatigue of extruded magnesium alloys

Corrosion fatigue tests were carried out on extruded AZ31 (3% Al, 1% Zn, 0.3% Mn, Mg—the rest), AM50 (5% Al, 0.4% Mn, Mg—the rest) and ZK60 (5% Zn, 0.5% Zr, Mg—the rest) Mg alloys in air, NaCl-based and borate solutions. N_sol/N_air ratios (the relative fatigue life) were used for the analysis of the corrosion fatigue behavior of Mg alloys in various environments, where N_sol and N_air are the numbers of cycles to failure in the solution and in air, respectively. Extruded ZK60 alloy reveals very high fatigue and corrosion fatigue properties in comparison with other alloys. However, it has the lowest relative fatigue life (N_sol/N_air 10⁻³–10⁻²) or the highest sensitivity to the action of NaCl-based solutions in comparison with that of AM50 and AZ31 alloys (N_sol/N_air 10⁻²–10⁻¹). Under the same stress, the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys (N_sol for extruded AM50 in NaCl is two to three times longer than that of die-cast AM50).     

Deformation twinning in AISI 316L austenitic stainless steel: role of strain and strain path

Abstract

AISI 316L austenitic stainless steel was deformed at different strain and strain paths. The twin boundaries in the deformed microstructure had two possible origins: decay of original annealing twins and generation of deformation twins. Assuming that rotations of grains, specifically grains on both sides of a twin boundary, are responsible for the twin decay, a simple model was proposed to bring out the domain of relative twin generation. A biaxial strain path, in general, was associated with strong twin generation – an association or dependency linked to the texture estimated values of Taylor factor. Formation of strain induced martensite was also observed to be strain and strain path dependent and was more in biaxial strain path.     

Development of mechanical properties during secondary aging in aluminium alloys

Abstract

Earlier work has shown that, if the artificial aging of aluminium alloys is interrupted by a dwell period at lower temperature, higher values of tensile properties and fracture toughness may be achieved than are possible with single stage T6 tempers. A second interrupted aging cycle has now been developed that involves underaging at the elevated temperature, quenching, and then allowing secondary precipitation to occur at, or just above, room temperature. Designated a T6I4 (I=interrupted) temper by the authors, this simpler aging cycle may reduce heat treatment costs. Tests on some 30 cast and wrought alloys have resulted in tensile properties close to those for a T6 temper, with higher values of fracture toughness being recorded for some cases. Such an aging treatment can be incorporated into a paint bake cycle to simplify the heat treatment of coated automotive components.     

Characterisation and formation of boundary in laser cladding AZ91D with Al + Al2O3

Abstract

The laser surface cladding of AZ91D magnesium alloy with Al + Al₂O₃ powders was investigated. The growth morphology of the boundary zone between the laser surface cladding layer and AZ91D substrate, and the distribution of Al₂O₃ particles in the laser surface cladding layer, were observed by optical microscope and scanning electron microscope. Furthermore, the element line and mapping scans analyses on the boundary zone were carried out with energy dispersive spectrum. The phases in the laser cladding layer and the substrate of AZ91D magnesium alloy were identified by X-ray diffraction. The results showed that the distribution of the Al₂O₃ particles was homogeneous in the laser surface cladding layer and the growth morphology of the boundary zone was in a unique parallel branching feature. The formation of the parallel branching dendrite is mainly related to the temperature gradient, the ultimate rate of dendrite growth and the rate of pool solidification on the growing fronts.     

Mixed mode I/III fracture toughness in extruded magnesium alloys

Fracture toughness under mode I and mixed mode I/III loading were determined for magnesium (Mg) as well as binary Mg-Al and Mg-Zn alloys in as-extruded condition. It was found that in Mg and in Mg-1.25Zn alloy the fracture toughness under mixed mode I/III loading was higher than that under mode I loading whereas in binary Mg-1Al and Mg-3Al alloys it was lower than that under mode I loading. The results have been explained on the basis of the fracture mechanism and the nature of the stress fields ahead of the crack tip under mixed mode I/III loading.     

大挤压形变AZ91 D镁合金半固态等温组织演变

通过液淬法获得挤压比为40的AZ91D镁合金等温处理过程中的半固态组织,研究其球化机制.结果表明:密集位错促使再结晶速度加快,球化时间缩短.球化机制为等温过程中再结晶导致形变组织粒化,晶界低熔点组份聚集处以及位错密度相对较高的区域熔化,形成蜂巢状组织;固液界面张力和曲率过热促使多边形晶粒进一步球化.     

Interfacial reaction mode and its influence on tensile strength in laser joining Al alloy to Ti alloy

Abstract

In order to obtain a robust dissimilar joint of Al/Ti alloys, a filler wire of Al–12 wt-%Si and 45° V shape groove on the base metal were used during CO₂ laser welding. Heat input had evident influence on the interfacial reaction mode. It was found that a dissolution mode for low heat input and a melting mode for high heat input exist at the joint interface, which was analysed from thermodynamic point of view. Tensile strength of the joints in the dissolution mode reached as high as 296 MPa, which was significantly higher than that in the melting mode.     

Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Texture control and the anisotropy of mechanical properties in titanium sheet

By means of three-dimensional crystallite orientation distribution function (CODF) analysis, the relationship between texture control processes and mechanical properties anisotropy in titanium sheet has been investigated. The formation and transition of cold-rolling texture, recrystallization texture and phase transformation texture are also discussed. The results show that, after being cold rolled unidirectionally and annealed, titanium sheet exhibits a strong anisotropy of mechanical properties due to the pyramidal textures (2 1 1 5) [0 1 1 ] and (1 0 1 3) [1 2 1 0]. After a cyclic phase transformation process coupled with cold rolling and annealing treatment, the recrystallization texture component (1 0 1 3) [1 2 1 0] is suppressed and [h k i l] 9∥ND transformation fibre texture (where ND is the sheet normal direction) are developed, which produce a well improved mechanical anisotropy. The cross-rolling process can create a basal type texture (0 0 0 2) 〈u v t w〉 or a near basal type texture, which leads to low level planar mechanical properties anisotropy but to relatively high normal plastic anisotropy (R value). This revised version was published online in November 2006 with corrections to the Cover Date.     

High performance automotive and railway components made from novel competitive aluminium composites

Abstract

The traditional materials encounter difficulties to comply with all the properties required by new components under service conditions. The automotive and the railway industries require low cost solutions to improve the final performance of components made from steel, cast iron, or even conventional aluminium alloys (e.g. clutch discs, brake discs, or pistons). Weight reduction, improvement in wear behaviour, high thermal conductivity, low coefficient of thermal expansion, and easy recycling are sought, among other characteristics, in order to obtain more efficient products leading to a reduction of pollutant emissions. The range of materials that can meet these requirements is presently very narrow and their final price is more expensive (e.g. aluminium matrix composites) than currently used materials. In this paper a new low cost metal matrix composite that has been used to produce pistons, clutch discs, and train brake discs is presented.     

Microstructural characteristics and mechanical properties of the hot extruded Mg-Zn-Y-Nd alloys

A new type of Mg-Zn-Y-Nd alloy for degradable orthopedic implants was developed.In the present study,the Zn and Y content was adjusted and their influences on the microstructures and mechanical behaviors were discussed in depth.The results showed that the as-extruded Mg-Zn-Y-Nd alloys are mainly composed of fine dynamic recrystallized grains(DRXed grains),la rge unDRXed grains and linearly distributed secondary phases.The cha nge of Zn content exerts little influence on the grain structure of the extruded Mg-Zn-Y-Nd alloy,while the increase of Y content would hinder the dynamic recrystallization process and the growth of the DRXed grains,thus the size and volume fraction of the equiaxed DRXed grains decrease.The tensile and compressive properties are very little affected by Zn content because of the similar grain structure.As Y content increases,the tensile yield strength(TYS) and ultimate strength(TUS) increase while the elongation decreases,this is caused by a combined strengthening effect of grain refinement,texture,precipitation and twinning.The compressive yield strength(CYS) and ultimate strength(CUS) of Mg-Zn-Y-Nd alloy with diffe rent Y content exhibit a similar tendency as the tensile test.     

Comparative studies on wear behaviour between as cast AZ91 and Mg97Zn1Y2 magnesium alloys

Abstract

The wear behaviour of as cast magnesium alloys, Mg₉₇Zn₁Y₂ and AZ91, was investigated under dry conditions in load ranges of 20–380 and 20–240 N respectively, using a pin on disc wear testing machine. The microstructure, thermal stability and elevated temperature tensile properties were characterised by optical microscopy, X-ray diffraction, differential thermal analysis and tensile testing respectively. The wear behaviour can be divided into three successive phases in terms of surface temperature induced by frictional heat, i.e. ambient temperature to eutectic temperature or precipitate dissolution temperature, eutectic temperature or precipitate dissolution temperature to the liquidus temperature and above the liquidus temperature. The Mg₉₇Zn₁Y₂ alloy exhibited good wear resistance compared with the AZ91 alloy for applied loads in excess of 80 N, which has been explained in terms of thermal stability of the intermetallic phase and elevated temperature mechanical properties of the two materials tested, by surface temperature analysis and subsurface observation.