Hans Jørgen Roven

Recycling of automotive aluminum

With the global warming of concern, the secondary aluminum stream is becoming an even more important component of aluminum production and is attractive because of its economic and environmental benefits. In this work, recycling of automotive aluminum is reviewed to highlight environmental benefits of aluminum recycling, use of aluminum alloys in automotive applications, automotive recycling process, and new technologies in aluminum scrap process. Literature survey shows that newly developed techniques such as laser induced breakdown spectroscopy (LIBS) and solid state recycling provide promising alternatives in aluminum scrap process. Compared with conventional remelting and subsequent refinement, solid state recycling utilizing compression and extrusion at room or moderate temperature can result in significant energy savings and higher metal yield.

OBSERVATIONS AND CALCULATIONS ON MECHANICAL ANISOTROPY AND PLASTIC FLOW OF AN AlZnMg EXTRUSION

Abstract An extruded plate of the alloy AA7108 has been characterized using texture analysis and uniaxial tensile tests. The yield surface, directional yield stresses and plastic strain ratios are calculated from Taylor-Bishop-Hill analysis, employing the measured texture data in combination with either the full constraints model or a relaxed constraints model. Three phenomenological yield functions, capable of describing yield anisotropy, are compared by means of their ability to reproduce the shape of the crystallographic yield surface and to predict directional properties, i.e. R -values and yield stress. On the basis of experiments and calculations it has been found that the Taylor relaxed constraints model describes the present material more consistently with respect to experimental findings than the Taylor full constraints model.

Application of EBSD technique to ultrafine grained and nanostructured materials processed by severe plastic deformation: Sample preparation, parameters optimization and analysis

Abstract With the help of FESEM, high resolution electron backscatter diffraction can investigate the grains/subgrains as small as a few tens of nanometers with a good angular resolution (∼0.5°). Fast development of EBSD speed (up to 1100 patterns per second) contributes that the number of published articles related to EBSD has been increasing sharply year by year. This paper reviews the sample preparation, parameters optimization and analysis of EBSD technique, emphasizing on the investigation of ultrafine grained and nanostructured materials processed by severe plastic deformation (SPD). Detailed and practical parameters of the electropolishing, silica polishing and ion milling have been summarized. It is shown that ion milling is a real universal and promising polishing method for EBSD preparation of almost all materials. There exists a maximum value of indexed points as a function of step size. The optimum step size depends on the magnification and the board resolution/electronic step size. Grains/subgrains and texture, and grain boundary structure are readily obtained by EBSD. Strain and stored energy may be analyzed by EBSD.

Microstructure and texture characteristics of ZK60 Mg alloy processed by cyclic extrusion and compression

Abstract The microstructure and crystallographic texture characteristics of an extruded ZK60 Mg alloy subjected to cyclic extrusion and compression (CEC) up to 8 passes at 503 K were investigated. The local crystallographic texture, grain size and distribution, and grain boundary character distributions were analyzed using high-resolution electron backscatter diffraction (EBSD). The results indicate that the microstructure is refined significantly by the CEC processing and the distributions of grain size tend to be more uniform with increasing CEC pass number. The fraction of low angle grain boundaries (LAGBs) decreases after CEC deformation, and a high fraction of high angle grain boundaries (HAGBs) is revealed after 8 passes of CEC. Moreover, the initial fiber texture becomes random during CEC processing and develops a new texture.

Grain refinement of magnesium alloys processed by severe plastic deformation

Abstract Grain refinement of AZ31 Mg alloy during cyclic extrusion compression (CEC) at 225–400 °C was investigated quantitatively by electron backscattering diffraction (EBSD). Results show that an ultrafine grained microstructure of AZ31 alloy is obtained only after 3 passes of CEC at 225 °C. The mean misorientation and the fraction of high angle grain boundaries (HAGBs) increase gradually by lowering extrusion temperature. Only a small fraction of twinning is observed by EBSD in AZ31 Mg alloys after 3 passes of CEC. Schmid factors calculation shows that the most active slip system is pyramidal slip and basal slip {0001} at 225–350 °C and 400 °C, respectively. Direct evidences at subgrain boundaries support the occurrence of continuous dynamic recrystallization (CDRX) mechanism in grain refinement of AZ31 Mg alloy processed by CEC.

Deformation twins in ultrafine grained commercial aluminum

Abstract Deformation twins in an ultrafine grained commercial Al – Mg – Si alloy with nonequilibrium grain boundaries processed by equal channel angular pressing at room temperature have been observed by means of a transmission electron microscope and a high-resolution transmission electron microscope. It was found that deformation twins formed frequently at the intersections of several grain bands where the critical stress level is reached locally and a change in the dominant slip system may occur due to the significant plastic deformation. Deformation twinning was also observed to occur via partial dislocations emission from grain boundaries and grain boundary junctions in the ultrafine grained alloy. Twins and stacking faults formed by Shockley partial dislocations dissociated from 0° screw dislocations were characterized and analyzed. Three twinning mechanisms suggested by molecular dynamic simulations were identified and their relevance was discussed.

Microstructure, hardness evolution and thermal stability of binary Al-7Mg alloy processed by ECAP with intermediate annealing

Abstract A binary Al-7Mg alloy was processed by equal channel angular pressing (ECAP) at room temperature via route Bc, combined with intermediate annealing. After 6 passes, a high hardness of 218 HV is achieved. Transmission electron microscopy (TEM) observations demonstrate that ECAP leads to a significant grain refinement and ultrafine grains down to 100-200 nm are developed after 5 or 6 passes. X-ray diffraction (XRD) analysis indicates that the major part of Mg atoms are in solid solution in the deformed material, and the possible strengthening effect of Mg solute atom clusters or precipitates is neglected. The high hardness of the 6 pass-treated materials comes mainly from grain boundary strengthening, which contributes about 41% to the total strength, while dislocations and Mg solid solution contribute about 24% and 35% to the remaining strength, respectively. Also, the thermal stability of this severely deformed material was investigated by hardness measurements. The material is relatively stable when annealed at a temperature lower than 250 °C, while annealing at 300 °C leads to a rapid softening of the material.

Strain localisation in solution heat treated Al–Zn–Mg alloys

Abstract Two Al–Zn–Mg alloys with recrystallised and fibrous grain morphologies are studied with regards to the microstructure after solution heat treatment, cold water quenching and immediate room temperature deformation. It was found that the dislocation movement was localised in narrow slip bands cutting through the dislocation tangles. This observation is related to dynamic strain ageing and to macroscopic shear bands frequently observed in these alloys.

Aging behavior and mechanical properties of 6013 aluminum alloy processed by severe plastic deformation

Structural features, aging behavior, precipitation kinetics and mechanical properties of a 6013 Al-Mg-Si aluminum alloy subjected to equal channel angular pressing (ECAP) at different temperatures were comparatively investigated with that in conventional static aging by quantitative X-ray diffraction (XRD) measurements, differential scanning calorimetry (DSC) and tensile tests. Average grain sizes measured by XRD are in the range of 66−112 nm while the average dislocation density is in the range of 1.20×10 14 −1.70×10 14 m −2 in the deformed alloy. The DSC analysis reveals that the precipitation kinetics in the deformed alloy is much faster as compared with the peak-aged sample due to the smaller grains and higher dislocation density developed after ECAP. Both the yield strength (YS) and ultimate tensile strength (UTS) are dramatically increased in all the ECAP samples as compared with the undeformed counterparts. The maximum strength appears in the samples ECAP treated at room temperature and the maximum YS is about 1.6 times that of the statically peak-aged sample. The very high strength in the ECAP alloy is suggested to be related to the grain size strengthening and dislocation strengthening, as well as the precipitation strengthening contributing from the

Optimization of EBSD parameters for ultra-fast characterization

Ultra‐fast pattern acquisition of electron backscatter diffraction and offline indexing could become a dominant technique over online electron backscatter diffraction to investigate the microstructures of a wide range of materials, especially for in situ experiments or very large scans. However, less attention has been paid to optimize the parameters related to ultra‐fast electron backscatter diffraction. The present results show that contamination on a clean and unmounted specimen is not a problem even at step sizes as small as 1 nm at a vacuum degree of 6.1 × 10−5 Pa. There exists an optimum step size at about 50 data acquisition board units. A new and easy method to calculate the effective spatial resolution is proposed. Effective spatial resolution tends to increase slightly as the probe current increases from 10 to 100 nA. The fraction of indexed points decreases slightly as the frame rate increases from 128 patterns per second (pps) to 835 pps by compensating the probe current at the same ratio. The value 96 × 96 is found to be the optimum pattern resolution to obtain optimum speed and image quality. For a fixed position of electron backscatter diffraction detector, the fraction of indexed points as a function of working distance has a maximum value and drops sharply by shortening the working distance and it decreases slowly with increasing the working distance.