Wei Min Gan

Equal Channel Angular Pressing of Magnesium Alloy Containing Quasicrystal Phase

ZWK510 (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) magnesium alloy containing Mg3YZn6 quasicrystal phase was prepared by conventional permanent mold casting. Part of the cast ingot was subjected to equal channel angular pressing (ECAP) directly; another part of the cast ingot was extruded initially, then ECAP was applied to the extruded alloy. After 4-pass ECAP, the fraction of coarse grains of the as-cast alloy was decreased to about 30%, and the grain size of fine grain was decreased to about 2 μm. Both strength and ductility of the as-cast ZWK510 alloy were significantly improved with increasing ECAP passes, which was resulted from broken and dispersed I-phase, and fine grains formed due to recrystallization. The as-extruded ZWK510 had an initial grain size of about 2 μm and bands of quasicrystal phase parallel to the extrusion direction. After the extruded alloy was subjected to ECAP, the grain size of the extruded alloy was further refined, the grain size was refined to below 0.5 um after 8-pass ECAP; and the quasicrystal phase was further broken and dispersed in the matrix. After ECAP, the elongation to failure of the extruded alloy was improved. However, both yield strength and ultimate tensile strength were decreased, which is considered to be resulted from the texture modification during ECAP.

The Robot Concept at STRESS-SPEC for the Characterisation or Semi-Finished Products

The materials science diffractometer STRESS-SPEC located at Forschungsneutronen¬quelle Heinz Maier-Leibnitz (FRM-II) in Garching/Germany is a dedicated instrument for residual strain, texture and microstrain analysis. Comparably low gauge volumes of 1x1x1mm³ for gradient investigations in many types of materials can be investigated. A robot system consisting of a robot type Stäubli RX160, a laser tracker and a heavy vibration free basement was installed to overcome limitations in gradient investigations of residual stresses and crystallographic textures in semi-finished and finished products. The robot can carry up to 30kg with precise positioning. Al7020, an Al-alloy for airspace application, was used to perform a first global texture measurement using the robot.

Influence of Extrusion Temperature on Microstructure, Texture and Fatigue Performance of AZ80 and ZK60 Magnesium Alloys

The wrought magnesium alloys AZ80 and ZK60 were extruded at 175°C ≤ T ≤ 350°C at an extrusion ratio of ER = 12. With decreasing extrusion temperatures a marked refinement in grain size was found for both alloys resulting in higher values of yield stress while UTS values were hardly affected. As opposed to AZ80, a marked yield stress differential between loading in tension and compression was observed in ZK60, this effect being explained by the differences in crystallographic texture.

Microstructure and Tensile Properties of a Mg-Zn-Y-Zr Alloy Containing Quasicrystal Phase Processed by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) was performed on extruded Mg-Zn-Y-Zr (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) alloy at 300 oC. After 8 ECAP passes, average grain size of the alloy was reduced to about 1.4 μm, and the quasicrystalline phases were broken and dispersed in the matrix. In addition, nano- quasicrystallines were precipitated from the matrix during ECAP processing. After ECAP, the elongation to failure of the extruded material was significantly improved. Only after 2 ECAP passes, the elongation to failure was 29%, and after 8 ECAP passes, it reached 35%, which was three times larger than that of the as-extruded alloy. However, both yield strength and ultimate tensile strength were decreased with the increasing ECAP passes, which was considered to be resulted from the {0002} basal plane texture modification during ECAP.

Investigation of Texture Gradients of Semi-Finished Products by Neutrons and Photons

Texture gradients are present in most samples, which are due to materials processing. Standard methods to evaluate texture gradients are based on the cut of samples, such as the X-ray investigation of surface textures against the texture inside a sheet. Bulk textures itself averaging over the whole sheet thickness are analysed by thermal neutrons. Both thermal neutrons and photons with high energies allow investigations non-destructively. The beam port Stress-Spec at the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) at Garching/Germany is equipped with a robot system based on a RX160 Stäubli robot, a Laser Tracker and a heavy basement. Samples up to 30kg can be investigated. Main restrictions are the available neutron flux, the detector efficiency and the detector size. Thus, the gauge volume is restricted to 1x1x1mm for ideal scattering conditions to measure in acceptable time scale. Photons with up to 200keV are known as high brilliant and high intense beam with similar penetration power than thermal neutrons. A typical set up of a high energy beamline for texture gradient investigations works without an Eulerian cradle so that restrictions in handling large sample are of less importance. The HZG materials science beamlines at Doris III and Petra III (Harwi-II@DorisIII and HEMS@PetraIII) are equipped with massif units for sample rotation and x-, y- and z- scanning for samples and additional equipments up to 200kg. Compared to thermal neutrons, which work with wavelengths between 1Å-2.5Å, the wavelength of high energy photons is small (0.05Å – 0.20Å). That leads on one hand to low scattering angles (1° - 10°) and on the other hand to an anisotropic ellipsoidal gauge volume. The local resolution of the synchrotron beam is much better than for thermal neutrons. In both methods corrections for constant gauge volume during pole figure scanning and for anisotropic absorption are of great importance.

Development of Extrusion and Rolling Textures during ECAP of Mg-Alloys

Crystallographic texture includes much information on a material’s behaviour that depends on the processed material itself, the particular thermomechanical processing it has undergone, and the quality of the process. ECAP processed pure Mg and Mg alloys were characterised by their texture and are discussed in terms of texture symmetry. For all types of deformation, including ECAP, the basal plain orientation has to follow certain general rules. Particularly for larger number of passes the ECAP texture can be explained in terms of standard rolling or extrusion textures.

Compressive Behavior of Extruded SiCw/AZ91 at Temperatures close to and above the Solidus of the Matrix Alloy

Compressive deformation behaviors of extruded SiCw/AZ91 were investigated in Gleeble-1500 thermal simulator at temperatures from 743 K to 783 K and strain rates from 6.4×10-2 s-1 to 1.0×101s-1. Results showed that high strain rate sensitivity (~0.5) occurred during compression; deformation activation energy normalized by threshold stress was higher than the lattice self-diffusion activation energy of magnesium. Dynamic recovery (DRV) and dynamic recrystallization (DRX) took place during compression, which refined the grains. The increase of deformation energy was attributed to non-basal planes slip and climbing of dislocations and also the presence of liquid phase.

New Developments of the Materials Science Diffractometer STRESS-SPEC

The high flux neutron diffractometer STRESSSPEC at FRM II, Garching Germany, offers a flexible instrument setup suitable for fast and surface residual strain measurements. Likewise bulk, local or gradient texture analyses are feasible. Here improvements of the hardware (detector, slits) of the instrument as well as developments on methods for residual stress analysis are presented. A new detector system developed inhouse was recently installed and successfully commissioned. Compared to the original delay line detector the new detector provides much higher resolution and allows event mode type measurements. Results of the commissioning measurements show a performance increase of nearly a factor of 2 compared to the former detector. Moreover the new analytical model, recently developed for surface spurious strain corrections, was successfully applied at a welded austenitic steel sample. Thus nondestructive measurements from the surface (200 μm) into the bulk (several millimeters) are possible without any extra time consuming experiments for spurious strains corrections.

Microstructure and Residual Stress in Rotary Friction Welded Dissimilar Metals of AA7020 Aluminium Alloy with 316L Steel

Rotary friction welding (RFW) was used in the current study to join the dissimilar metals AA7020-T6 aluminium alloy and 316L steel. Neutron diffraction was performed to investigate the texture gradient around the weld line and to map the residual stress over the whole specimen. The texture analysis showed a weak shear component near the bond line of AA7020-T6-T6 side which indicated a plastic deformation of AA7020-T6 during welding. The shear bands were also observed in optical microstructures. Relatively high tensile residual stresses were observed near the bond line on the AA7020-T6 side, which were in-homogeneously distributed from the perimeter to the rod centre, while high compressive residual stresses were found in the sample centre at the bond line in the 316L steel.

Microstructure and Properties of Pure Mg/ZK60 Laminate Processed by Accumulative Roll Bonding

Commercial pure magnesium with excellent damping capacity and ZK60 magnesium alloy with high strength were accumulative roll bonded (ARBed) at 300oC up to 3 cycles to fabricate Mg laminate consisting of alternating layers of pure Mg and ZK60. Microstructure, tensile properties and damping capacity of the sheets were analyzed. The research suggests that Mg sheet having both high strength and excellent damping capacity can be developed by accumulative roll bonding of pure Mg and ZK60 Mg alloy.