Materials Science Forum Vols. 654-656

Abstract: Low-cycle fatigue behavior of a wrought Al-0.8wt%Mg-0.7wt%Si alloy with and without 0.27wt%Sc has been investigated at room temperature under constant plastic-strain amplitudes. After peak-aging treatments, both the alloys had fine lath-shaped β' precipitates. In the Sc-containing alloy, spherical Al3Sc precipitates of about 11 nm in diameter were co-existed. The alloy with Sc exhibited cyclic hardening to saturation, while the alloy without Sc showed clear cyclic softening after initial hardening. Transmission electron microscopy observation revealed that slip band structures were developed in the Sc-free alloy. Within the slip bands, shearing of the β' precipitates by moving dislocations was often observed. The cyclic softening in the alloy without Sc can then be explained by a loss of precipitation strengthening effect through the precipitation destruction within strongly-strained slip bands. In the Sc-bearing alloy, owing to the existence of non-shearable Al3Sc precipitates, dislocations were uniformly distributed, resulting in the absence of the cyclic softening.

Abstract: We investigate the energy of segregation of solute Ca at symmetric tilt grain boundary in aluminum from the first-principles calculations. As energy of segregation of Ca is negative, Ca atoms tend to segregate at the grain boundary. Furthermore, on basis of the Rice-Wang model, we study the effect of the segregation of Ca on the grain boundary embrittlement of aluminum. Our first-principles calculations of energies of segregation at grain boundary and free surface show that Ca behaves as embrittler.

Abstract: Intergranular corrosion can lead to significant problems such as sub-critical crack growth or loss in section strength, potentially leading to failure, as well as a substantially increased maintenance burden. This type of corrosion is found in most types of aluminium alloys, but is a particularly significant problem in aerospace aluminium alloys. The form of intergranular corrosion can vary widely, and may depend on alloy composition, product form, environmental conditions and the presence or otherwise of local or global stresses. One notable example is the occurrence of intergranular corrosion due to atmospheric corrosion, in which salts and deposits deliquesce on the surface forming discrete corrosion cells. Intergranular corrosion of aluminium alloys is usually most rapid in the rolling or extrusion direction of wrought alloy. The reasons for this are not fully understood, and may include texture effects that produce highly susceptible grain boundaries, the inhomogeneous distribution of noble constituent particles, and stresses acting at a microscale. This paper will review and discuss the evidence for and against for the different effects mentioned.

Abstract: The compressive behavior of two closed cell aluminum foams (Alporas and Cymat) were investigated under various strain rates from 10-3 to 102 s-1 using compressive tests conducted on MTS and Instron test machines. High speed camera videos show that localized deformation occurs in Alporas foam specimens uniformly, but not in Cymat foams. The stress-strain curves for Alporas foams are smooth and regular, while they oscillate dramatically for Cymat foams. Alporas foams exhibit strain rate sensitivity, i.e. with an increase in strain rate, the normalized plateau stress and energy absorption capacity increase. The plateau stress for Cymat foams was more sensitive than Alporas to strain rate, but the energy absorption less so, at least at a nominal relative density of 10%.

Abstract: Recently, heat treatment technologies have been developed by the CSIRO Light Metals Flagship in Australia that allow the 0.2% proof stress of conventional aluminum alloy high pressure diecastings (HPDC’s) to be more than doubled without encountering problems with blistering or dimensional instability [1,2]. A range of other properties may also be improved such as fatigue resistance, thermal conductivity and fracture resistance. However, the current commercial HPDC Al-Si-Cu alloys have not been developed to exploit heat treatment or to optimize these specific mechanical properties, and one potential limitation of heat treating HPDC’s is that fracture resistance may be reduced as strength is increased. The current paper presents the outcomes of a program aimed at developing highly castable, secondary Al-Si-Cu HPDC alloys which display significantly enhanced ductility and fracture resistance in both the as-cast and heat treated conditions. Kahn-type tear tests were conducted to compare the fracture resistance of the conventional A380 alloy with a selection of the newly developed compositions. A comparison has also been made with the current permanent mold cast aluminium alloys and it is shown that the new HPDC compositions typically display higher levels of both tensile properties and fracture resistance.

Abstract: High-intensity ultrasound was applied during the preparation of an Al-5Ti-1B master alloy when reacting fluoride salts with molten Al. The reaction rate was significantly increased with TiB2 particles of much reduced mean diameter and narrow width spread produced in ~4 minutes. The improved grain refining performance of the TiB2 in a commercial purity Al was studied and modelled using free growth model. The increased number and reduced size of TiB2 particles provided an enhanced grain refining capability.

Abstract: Castings may be used more as structural components if friction stir (FS) processing could refine the microstructures satisfactorily. For this, material flow and the formation of various microstructural regions during FS were studied. It was found that -Al dendrites and eutectic Al-Si were sheared into the thread spaces and the microstructures were significantly modified and refined. Material merely dragged into the FS zone remains microstructurally segregated. The volume of each flow was affected by rotation speed and forward speed and the reason for this is explained. Finally, textures determined together with the strain values estimated are presented to discuss further the microstructual development in various stir zones.

Abstract: Aluminum alloys are extensively used in the automotive industry in response to increasing requirements for weight reduction in car body architecture. Laser welding of Al alloys has many advantages such as low heat input, narrow heat affected zone, minimal thermal distortion and flexibility. Recently, the development of high brightness lasers - thin disk lasers and fiber lasers enable long focal distance welding so that laser remote welding with scanning mirrors can be used in laser welding of Al alloys. In this study, laser remote welding was implemented to a heat shield part of an automobile by utilizing a 4kW disk laser and a 2-axis scanner. By replacing conventional resistance spot welding, the weld flange length could be reduced from 15mm to 8mm and a cycle time for each welding point could also be reduced from 3.5s to 0.6s.

Abstract: Friction stir spot welding (FSSW) was applied to lap joining of aluminum alloy sheets and steel sheets. A 1.2 mm-thick non-plated carbon steel sheet and plated steel sheets with zinc alloy (ZAM), pure zinc (GI), zinc alloy including Fe (GA) and Al-Si alloy (AS) were prepared. The melting temperature of the plated layer is 330, 420, 880 and 640°C, respectively. A 1.1 mm-thick 6022 aluminum alloy sheet was overlapped on the steel sheet. A rotating tool was inserted from the aluminum alloy sheet side and the probe tip was kept at the position of 0.2 mm above the lapped interface for 3 seconds. For ZAM and GI, original plated layers were removed from the interface and intermediate layers were formed at the joint interface. This is because the melting temperature of the plated layer was lower than the interface temperature under the rotating probe tip during the FSSW. In contrast to that, the partial original plated layer remained after welding, and additional layer formed at the plated layer /aluminum alloy interface for GA. For AS, Al-Fe intermetallic compound layer, which was formed at the original Al-Si alloy plated steel surface remained.

Abstract: Effects of Cr addition on morphology of β-Al5FeSi phase formed in recycled aluminum alloy were studied as a function of cooling rate by technology of time-resolved radiography. The Cr addition to iron-contaminated recycled alloys modified the needle-like β-Al5FeSi into faceted dendritic α-AlFeCrSi phase in condition of conventional casting. Time-resolved radiography of the formation of α-AlFeCrSi revealed the transition of its morphology from polyhedron at about 10K/min of cooling rate to equiaxed dendrite of faceted arms at about 50K/min. of cooling rate.