Materials Science Forum Vols. 794-796

Abstract: The grain size dependence of creep behavior from coarse grain to ultrafine grain regions was examined using fully-annealed specimens fabricated from a single process route. For coarse-grained sample, in tensile deformation, stress-strain curves show slow work hardening, and the proof stress shows typical Hall-Petch behavior. On the other hand, creep behavior is observed under the stress above the proof stress, and the creep rate has no grain size dependence. For ultrafine-grained sample, in the tensile deformation, stress-strain curves show yielding behavior, and the yield stress shows Hall-Petch behavior also. On the other hand, creep behavior was observed below the proof stress, but the creep rate decreases with a decrease in grain size.

Abstract: It has been widely accepted that the creep characteristics at high temperatures are mainly evaluated by a minimum creep rate. Although, a shape of creep curve may vary depending on deformation conditions, the apparent steady state or minimum creep rates be the same. Thus,for detailed analysis and prediction of creep behavior, other values which reflect the shape of each creep curve should be considered. For the purpose, authors have proposed Sato’s strain- acceleration-parameter (Strain Acceleration and Transition Objective index, SATO-index) which reflects strain rate change during creep deformation. Based on the concept of SATO-index, the whole creep curve can be represented by a set of small number of numerical parameters, and can be extrapolated from a part of creep curve. In this paper, application of the concept of SATO-index to the creep curves of aluminum-magnesium solid solutions that the creep behavior of the alloys are well investigated and analyzed. The creep curve can be extrapolated by the concept from transient part of creep curve, and the extrapolated creep rates at the minimum creep rate agree well with experiment. Efficiency of the concept of SATO-index to creep experiments is pronounced.

Abstract: In general, aluminum alloy does not exhibit distinct fatigue limit (knee point) in the S-N diagram. The growth of a small fatigue crack of precipitation-hardened Al-Mg-Si system alloy (6061-T6) was investigated to clarify the mechanism of non-appearance of distinct fatigue limit (knee point) in the S-N diagram. The small crack was analysed in detail by replica method, scanning electron microscope (SEM), and Electron Back Scatter Diffraction Patterns (EBSD). On the other hand, the existence of distinct fatigue limit (knee point) of new developed aluminum alloy by adding excess Mg to the 6061 alloy was found. In this study, the resistance of small crack growth of the developed alloy was compared with standard 6061 alloy. It was revealed that the resistance of crack growth of new developed alloy was higher than that of standard 6061 alloy in short crack region (l<1.0 mm).

Abstract: The analysis of deformation and damage behavior of aluminum cast components is very complex, since local mechanical properties in the components are inhomogeneous as a consequence of spatial distribution of microstructure e.g. pore size, grain size and arm spacing of secondary dendrites. Moreover, the damage behavior of aluminum alloys depends strongly on stress state. Until now it is not clear how the pore morphology affects the damage behavior under different loading situations. In this work the damage behavior of the aluminum die casting alloy AlSi9Mn was characterized with tension specimens extracted from different positions in a component. Damage effect was modeled with representative volume elements (RVE) with variation of porosity, pore size and distribution of pores under uniaxial and biaxial tension, plane strain, compression and shear. Not only pore growth and coalescence but also damage of the matrix material was taken into account in the simulations. A large influence of pore morphology on fracture strain was found and relationships between fracture strain and pore morphology were proposed for different stress states.

Abstract: The crack initiation and propagation during bending have been considered to be affected by second phase particles, micro-voids and shear-bands. However, the effects of the second phase particles and the micro-voids on the crack initiation and propagation during bending have not been fully investigation. In this study, the effect of the second phase particle distribution on the formation of micro-voids, and the effect of the micro-voids on the crack initiation and propagation during bending were investigated using the largest synchrotron radiation facility “SPring-8” and FE-SEM/EBSD. With the bending ratio increasing, the micro-voids increased around the coarse particles near the outer surface. In particular, coarse micro-voids were formed around coarse particles with a high aspect ratio on the shear-bands. At a large cracked part, coarse micro-void was observed at the outmost layer section as a crack initiation site, and coarse micro-voids and asheared fracture surface were observed at the crack propagation site. At the small cracked part with no propagation, cube orientation grains were located under the small crack. It was considered that these cube orientation grains inhibited the formation of shear-bands, therefore, propagation of the cracks did not occur at the small cracked area.

Abstract: An aluminum alloy with a chemical composition of Al–6%Mg–0.35%Mn–0.2%Sc–0.08%Zr–0.07%Cr (in wt.) was rolled up to different reductions of 75, 88 and 95% at 360^oC and at ambient temperature. The static mechanical properties and the high-cyclic fatigue (HCF) life were examined. It was shown that the hot rolling results in increased yield stress (YS) and ultimate tensile strength (UTS). However, ductility and fatigue limit of the hot rolled alloy and initial as-cast ingot are nearly the same. The combination of hot and cold rolling leads to significant improvement of tensile strength and fatigue resistance, while ductility tends to reduce with increasing the rolling reduction. The cold rolled alloy exhibits the endurance limit under fatigue conditions, while the alloy in the both as-cast and hot rolled conditions exhibits only fatigue strength. The effect of the deformation structure on the mechanical properties is discussed.

Abstract: In the present investigation the new HYB spindle extruder has been used for butt joining of 4 mm thick aluminium plates of the AA6082-T6 type at RT in one pass, employing a 1.6 mm diameter filler wire of matching composition. The test joint produced was subsequently sectioned and subjected to thorough examination in the laboratory, which included visual inspection of the surface quality and bead penetration depth, optical microscopy for visualisation and documentation of the material flow pattern and the microstructure within the joining zone and Vickers hardness testing. It is concluded that strict control of the bead penetration depth is necessary in order to obtain full bond strength. This is because the bead penetration determines the contact pressure between the filler metal and the base metal in the groove during filling.

Abstract: In this paper selected results about process control and stability during ultrasonic torsion welding of aluminium to titanium sheets are discussed. The process parameters welding force, welding energy and oscillation amplitude were optimized for Al4N/cp-Ti-joints and AA7075/TiAl6V4-joints using modern statistical test methods. The hybrid welds are evaluated and compared based on their mechanical properties. Furthermore thermal characteristics are determined with high resolution during the welding process by thermometry. Central aspects of the current research project are detailed microscopy of the hybrid interface as well as fracture surface analysis to understand the interfacial formation. Relationships between process parameters, joint strength and the related microstructure should be understood.

Abstract: High strength 2024 aluminum alloy studs were joined to galvanized, galvannealed and non-coated steel sheets by using an advanced stud welding method. Effect of the coating layer on the interfacial microstructure and the tensile fracture load of the joints were evaluated. A specially-designed stud having a circular projection at its bottom was pressed against a sheet surface. A discharge current was introduced from the upper part of the stud. Local heating could be achieved by a high current density at a contact point between the projection and sheet. The observation of joint area revealed the projection was severely deformed and spread along the sheet surface. The coating layer of the galvanized steel sheet was removed at the joint interface under the charging voltage of 200 V, while that of the galvannealed one locally remained on the steel surface even at 400 V. This would be strongly related to the melting or liquidus and solidus temperatures of each coating layer. Joining was not achieved at a low charging voltage in the non-coated and galvannealed steel sheets, while high tensile fracture load was obtained even at 200 V in the galvanized ones.

Abstract: Solid-state welding is useful to join dissimilar metal couples, in particular, with a large difference in physical and mechanical properties. However, conventional solid-state welding methods such as diffusion welding and roll bonding are not necessarily applicable to all metal combinations. In addition, they are time-consuming. In the present study, various dissimilar metal joints (e.g. Al/Fe, Al/Cu, Al/Ni, A2024/A5052, A6022/steel, A6022/Plated steel, A2024/AZ80) were fabricated by using several types of high-speed solid-state welding methods; friction stir spot welding, advanced stud welding and impact welding. The strength and characteristic interfacial morphology of the joints were investigated, and each joining mechanism is discussed. In particular, for the impact welding, both experimental and numerical analyses were performed. Two metal sheets were obliquely collided at a very high speed and joined by magnetic pressure or explosive force. Smoothed Particle Hydrodynamics (SPH) method was used to simulate the impact welding process. The emission of metal jet and the evolution of characteristic wavy interface at the joint interface could be clearly visualized. The effects of collision angle, collision velocity and difference in density of the metals on the wave morphology were revealed.