Ida Westermann

Effect of quenching rate on microstructure and mechanical properties of commercial AA7108 aluminium alloy

A commercial AA7108 aluminium alloy subjected to different quenching procedures from the solid solution temperature of 480 °C was considered. Both continuously cooling and quenching to intermediate temperatures (400, 300 and 200 °C) followed by different holding times were applied. All the materials were subsequently subjected to the same industrial two-step artificial ageing procedure to obtain a maximum strength (T6). Tensile testing of the different quenched materials in the T6 temper reveals a large deviation in strength dependent on the cooling/holding time, compared with the reference sample. A collection of the different quenched materials was chosen for further investigation of the precipitate structure by transmission electron microscopy (TEM) to

Anisotropy of Bending Properties in Industrial Heat-Treatable Extruded Aluminium Alloys

In the present work, three-point bending tests have been performed on four commercially extruded 6xxx- and 7xxx alloys, one with a fibrous and one with a recrystallized grain-structure for each alloy class, with the bending axis orientated 0, 45 and 90° with respect to the extrusion direction. Microstructure and texture characterization as well as tensile testing of the same materials have been performed and correlated with the bending results. In general there is good agreement between the bending angle and the fracture strain for all alloys, with the highest values in the extrusion direction. However, there are no indications in the microstructure and texture that explain the large differences in bendability observed. Die lines and recrystallized layer on a fibrous alloy have been removed to investigate their effect on the bending behaviour. However, these effects also seem to be limited, and cannot explain the anisotropy effects observed in bending angles.

Influence of Processing Route on the Work-Hardening and Ductile Fracture of an AA6060 Aluminium Alloy

Tensile tests on smooth and notched axisymmetric specimens were carried out to determine the large strain work-hardening curves and the ductile fracture characteristics of an AA6060 aluminium alloy for three different processing routes. The alloy was processed in three subsequent steps: 1) casting and homogenization, 2) extrusion, and 3) cold rolling and heat treatment to obtain a recrystallized grain structure. After each processing step, the material was tested after natural ageing for more than one week. A laser-based extensometer was used to continuously measure the average true strains to failure in the minimum cross-section of the specimens and the true stress-strain curves were calculated. Since these curves are influenced by necking, they do not represent the correct work-hardening of the material. Accordingly, finite element (FE) simulations of the tensile tests on the smooth axisymmetric specimens were conducted to determine the work-hardening curves to failure, using an optimization tool that interfaced with the nonlinear FE code and the experimental stress-strain curves as objectives. The microstructure of the alloy was characterized after the three processing steps by optical and scanning electron microscopy, and fractography was used to investigate the failure mechanisms.

Mechanical Behaviour of an AA6082 Aluminium Alloy at Low Temperatures

Aluminium alloys are known to have good cryogenic properties. However, little work is reported on aluminium alloys in the temperature range from room temperature and down to-100°C, which are likely operating temperatures in arctic regions. This work is an experimental study of the low temperature mechanical properties of an extruded AA6082 aluminium alloy in the T6 condition approved for marine applications. Quasi-static and dynamic tensile testing has been carried out at different temperatures from room temperature and down to-70°C. This decrease in temperature leads to a 10 % increase in yield strength. No significant influence of temperature was found on the area reduction to fracture. The work-hardening behaviour has been analysed for different temperatures and strain rates by fitting a generalized Voce rule to the tensile data. The initial work-hardening rate is found to depend weakly on the temperature and strain rate.

Effect of alloying elements on stage-III work-hardening behaviour of Al–Zn–Mg(–Cu) alloys

Abstract Four commercial Al–Zn–Mg(–Cu) alloys have been investigated with respect to the age-hardening response and how the stage-III work-hardening behaviour is influenced by the alloying elements. Tensile specimens of the as-cast and homogenized alloys have been exposed to a solution heat treatment and a two-step age-hardening treatment with varying time at the final temperature. The work-hardening parameters obtained by fitting an extended Voce equation to the experimental stress–strain curves have been analysed and discussed. Addition of Cu was found to increase the strength, whereas Mg seemed to be a limiting factor in age-hardening potential. Accelerated precipitation kinetics was observed when the Zn content was increased.

Influence of Stacking Sequence and Intermediate Layer Thickness in AA6082-IF Steel Tri-Layered Cold Roll Bonded Composite Sheets

In tri-layered Cold Roll Bonded (CRB) composite sheets of dissimilar metals, uneven thickness reduction of the different layers have been observed. This has been explained by the difference in yield strength/flow stress of the metals. The aim of this research was to study if these observations also depend on different parameters such as stacking sequence and initial sheet thickness, as opposed to only material properties. Hence, tri-layered CRB composite sheets consisting of AA6082 and IF-steel were produced with two different stacking sequences, St/Al/St vs Al/St/Al. Two different layer thicknesses of the intermediate layer were used. Rolling was performed in a single pass achieving approximately 55-65% total thickness reduction. Comparisons between the samples are given, focusing on the reduction of thickness achieved in each layer, the roll bonded interface characteristics and the bond strength of the joints. The overall thickness reduction achieved in each layer was found to be similar for both metals in both stacking sequences for the considered material combination. The stacking sequence is statistically found not to have any effect on the bond strength of the joints. The thickness of the intermediate layer does not significantly affect the overall bond strength of the composite sheet for both stacking sequences. These findings are opposed to earlier results found in the literature.

The Effect of Heating Rate on the Density and Spatial Distribution of Dispersoids during Homogenisation of 6xxx Aluminium Alloys

Two 6xxx alloys with different Mn-content have been homogenised in a furnace at 575 oC for 2 hours and 15 minutes. Three different heating rates to the homogenisation holding temperature were chosen, as this was expected to affect the precipitation behaviour of the dispersoids. The study focused on developing a reliable procedure for the characterization of the density and spatial distribution of dispersoids in aluminium alloys; both in terms of sample preparation, microscopic techniques and quantitative analyses of results. Scanning electron microscopy (SEM) has been used to evaluate the dispersoid characteristics for the different alloys and heating rates. The results indicate an increase in dispersoid number density and a more uniform distribution of dispersoids for the lowest heating rate, as compared to the more rapid heating rates, for the alloy with 0.05 wt% Mn. For the alloy with 0.15 wt% Mn the number density increased with the heating rate. This is suggested to be due to particle coarsening as an effect of the low heating rate where the samples spend longer time in the furnace.

Permanent effect of a cryogenic spill on fracture properties of structural steels

Fracture analysis of a standard construction steel platform deck, which had been exposed to a liquid nitrogen spill, showed that the brittle fracture started at a flaw in the weld as a consequence of low-temperature embrittlement and thermal stresses experienced by the material. In the present study, the permanent effect of a cryogenic spill on the fracture properties of carbon steels has been investigated. Charpy V-notch impact testing was carried out at 0 °C using specimens, from the platform deck material. The average impact energy appeared to be below requirements only for transverse specimens. No pre-existing damage was found when examining the fracture surfaces and cross sections in the scanning electron microscope. Specimens of the platform deck material and a DOMEX S355 MCD carbon steel were tensile tested immersed in liquid nitrogen. Both steels showed a considerable increase in yield- and fracture strength and a large increase in the Luders strain compared to the room temperature behavior. A cryogenic spill was simulated by applying a constant tensile force to the specimens for 10 min, at -196 C. Subsequent tensile tests at room temperature showed no significant influence on the stress-strain curve of the specimens. A small amount of microcracks were found after holding a DOMEX S355 MCD specimen at a constant force below the yield point. In a platform deck material tensile tested to fracture in liquid nitrogen, cracks associated with elongated MnS inclusions were found through the whole test region. These cracks probably formed as a result of the inclusions having a higher thermal contraction rate than the steel, causing decohesion at the inclusion-matrix interface on cooling. Simultaneous deformation may have caused formation of cracks. Both the microcracks and sulphide related damage may give permanently reduced impact energy after a cryogenic exposure.

The Effect of Elastic Straining on a 6060 Aluminium Alloy during Natural or Artificial Ageing

The effect on hardness and precipitate microstructure of elastically straining a 6060 Al-Mg-Si alloy during natural ageing or artificial ageing has been investigated. The elastic strain is here defined as 50 % of the material yield strength. All heat treatments where elastic straining was applied led to an increased hardness compared to the unstrained reference material. Quantitative investigations of the precipitate microstructure using transmission electron microscopy (TEM) did not indicate any significant difference in precipitate parameters as compared to the unstrained reference material. Therefore the increased strength in the elastically strained material is being linked to strain induced dislocations based on faster ageing kinetics compared to unstrained reference samples.

Work- and Age-Hardening Behaviour of a Commercial AA7108 Aluminium Alloy

Understanding and prediction of the mechanical properties of aluminium alloys are of great importance with respect to e.g. strength requirements and forming operations. In the 7xxx alloying system several mechanisms influence the hardening behaviour of the alloys, e.g. particle size and distribution, dislocation density, and alloying elements in solid solution. This work is an experimental study of work- and age-hardening considering a commercial AA7108 alloy in the as-cast and homogenized condition. Tensile specimens have been exposed to a solution heat treatment and a two-step age-hardening treatment with varying time at the final temperature. The tensile data for the different tempers have been evaluated in elucidation of already existing models based on the one-parameter framework by Kocks, Mecking, and Estrin. The particle size has been further investigated in the transmission electron microscope for one under- and one over-aged condition and the influence of particles on work-hardening behavior has been discussed.