Ultrafine grained plates of Al-Mg-Si alloy obtained by Incremental Equal Channel Angular Pressing : microstructure and mechanical properties

In this study, an Al-Mg-Si alloy was processed using via Incremental Equal Channel Angular Pressing (I-ECAP) in order to obtain homogenous, ultrafine grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90˚ rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 - 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53-57 % depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminium with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength - more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications

Different stress states deformation of AA6082 subjected to different artificially aged conditions

The artificial aging response of Al-Mg-Si 6082 aluminum alloy is investigated over a wide temperature range. Samples aged to under aged, peak aged and over aged conditions are further subjected to plastic deformation by simple compression, plane strain compression and simple shear. The flow behavior and the corresponding hardening rates are documented. Equivalent stress - strain curves are generated for the three stress states for an aging temperature of 160°C. Strain reversal experiments in simple shear were carried out in order to characterize the Bauschinger effect. Strain path change experiments were also conducted, in which the gage section that was first deformed by simple shear was further deformed by simple compression.King Saud University, Riyadh, Saudi Arabi

Effect of solution heat treatment on the hot workability of Al-Mg-Si alloy

The current work presents a detailed study on the high temperature processing of solution treated Al-Mg-Si alloy in the temperature range of 623K to 773K and at different strain rates in the range of 5 x 10X-5 to 6X10-2s-1. A constitutive relation that can be used in modeling the forming process of this alloy under similar hot working conditions is established. Also, the prevailing deformation mechanism was investigated through relations of the steady state stress dependence on strain rate which revealed a stress exponent of 8.5 (strain rate sensitivity; m ∼ 0.12). This stress exponent is higher than what is usually observed in Al and Al-Mg alloys under similar experimental conditions. This high stress exponent may arise from the presence of threshold stress that results from dislocation interaction with second phase particles (Mg2Si), precipitating during the deformation at high temperatures. The values of threshold stress showed an exponential increase with decreasing temperature and a dependence with an energy term Qo = 38kJmol-1. The apparent activation energy for solution treated condition was calculated to be about 320kJmol-1, which is higher than the activation energy for self-diffusion in Al (Qd = 143kJmol-1) and for the diffusion of Mg in Al (115-130kJmol-1). By incorporating the threshold stress in the analysis, the true activation energy was calculated to have a value of 111 kJmol-1, and the normalized strain rates can be represented by a power function of the effective stress with stress exponent of ∼3. Ductility was documented to reveal the best working condition for this alloy in solution treated condition. The ductility exhibited a maximum value of about 120% at 773 K at a strain rate of 0.064 s-1. The results of the current work is, also, compared to the results of another heat treatment condition (T4-naturally aged) to reveal which ever condition holds better hot forming characteristics.King Saud Universit

Microstructural and mechanical characterization of friction stir welded-1050 aluminium alloy

Friction stir welding (FSW) is a fairly recent technique that utilizes a non-consumable rotating welding tool to generate frictional heat and plastic deformation at the welding location in the continuously-fed work piece. In the present investigation this welding process is applied to join 1050 cold-rolled aluminium plates. The effects of varying the welding parameters namely welding speed [56, 90 and 140 mm/min] and tool rotational speed [850 and 1070 rpm] on the mechanical and microstructural properties were studied. Vickers micro hardness results across the weldment showed that the weld nugget hardness is dependant upon the welding speed and the tool rotational speed. Increasing the welding speed at 850 rpm reduced the hardness at the weld nugget, whereas, at 1070 rpm the weld nugget hardness merely did not change. However, the hardness achieved at 850 rpm was constantly higher than that achieved with 1070 rpm irrespective to welding speeds. In the same fashion, the yield and ultimate strengths of the joints were influenced by varying the welding parameters. Increasing the welding speed at 850 rpm reduced both strengths whereas; at 1070 rpm they were almost unchanged. Microstructural study showed that the weld region is composed of unaffected base metal and the stir zoned [weld nugget] which is characterised by a fine equiaxed grain structure. Increasing the welding speed at constant tool rotational speed has caused a slight refinement in the weld nugget's grain size, whereas, decreasing the rotational speed has also led to weld nugget grain refinement.King Saud Universit

Effect of Nd:YAG laser parameters on the penetration depth of a representative Ni–Cr dental casting alloy

The effects of voltage and laser beam (spot) diameter on the penetration depth during laser beam welding in a representative nickel–chromium (Ni–Cr) dental alloy were the subject of this study. The cast alloy specimens were butted against each other and laser welded at their interface using various voltages (160–390 V) and spot diameters (0.2–1.8 mm) and a constant pulse duration of 10 ms. After welding, the laser beam penetration depths in the alloy were measured. The results were plotted and were statistically analyzed with a two-way ANOVA, employing voltage and spot diameter as the discriminating variables and using Holm–Sidak post hoc method (a = 0.05). The maximum penetration depth was 4.7 mm. The penetration depth increased as the spot diameter decreased at a fixed voltage and increased as the voltage increased at a fixed spot diameter. Varying the parameters of voltage and laser spot diameter significantly affected the depth of penetration of the dental cast Ni–Cr alloy. The penetration depth of laser-welded Ni–Cr dental alloys can be accurately adjusted based on the aforementioned results, leading to successfully joined/repaired dental restorations, saving manufacturing time, reducing final cost, and enhancing the longevity of dental prostheses. © 2013, Springer-Verlag London

Shear bond strength and characterization of interfaces between electroformed gold substrates and porcelain

This study aimed to determine the effects of two different sandblasting conditions on the shear bond strength between electroformed Au substrates and porcelain, and characterize the interface between the Au substrate and porcelain. Electroformed Au specimens, 0.3 mm thick with a cap-like shape were prepared. The prepared specimens were then divided into two different groups and each group was sandblasted with a different size of alumina grains (100 or 250 μm) prior to dental porcelain application. Bonded specimens from each group were subjected to shear testing. After debonding, the fracture mode was analyzed by optical microscopy and SEM-EDX. One intact bonded specimen from each group was metallographically prepared to characterize the interfacial bonding by SEM-EDX and area scan analysis. The shear bond strength values (MPa) and standard deviations were 8.2 ± 1.8 and 9.1 ± 2.7 for the samples blasted with 100 and 250 μm alumina particles, respectively. No statistically significant difference was found between the two groups. In addition, no differences in fracture mode were found between the two groups. Qualitative analysis showed that, surprisingly, the Au substrate contained O, N, and P which might be related to the Au-sulfite electrolyte used in electroforming. As expected, the retained porcelain comprised Si, O, Al, Ca, Na and K. Mutual diffusion of Au, P, Si, Na, K and O without concentration gradients was found at the interface. Mutual ionic diffusion at the interface between ceramics and electroformed Au substrates (as opposed to mechanical interlocking) seems to be the most possible factor contributing to Au-ceramic bonding. © 2012 Elsevier B.V. All rights reserved