Materials Science Forum Vols. 828-829

Abstract: To develop CCT diagram using DSC requires the use of multiple devices in order to measure across the range of cooling rates required to develop the diagram. In the current work one dilatometer is used to characterize the precipitation reactions of the AA7020 alloy. Precipitation and dissolution reactions resulted in changes in the rate of change in the coefficient of thermal expansion. This was used to determine the start and finish temperatures of the MgZn₂ precipitation reaction and produce the CCT diagram. Good agreement was found between the results of this technique and DSC results from the literature.

Abstract: The effects of ageing temperature, time and cooling medium on the microstructure and hardness of a solution treated Ti6Al4V alloy were investigated. The furnace cooling after ageing for 0.5 hours gave a homogenous structure with higher hardness values than the solution treated and water quenched Ti6Al4V alloy. Increasing the ageing time to 2 hours reduced the alloy hardness. Ageing at temperatures between 500 and 700°C, followed by furnace cooling, led to homogenously distributed α- and β-phases within a fully martensitic matrix leading to improved hardness. A heterogeneous structure with a high variation in microhardness was revealed when ageing at 800 and 900°C.

Abstract: Commercially pure, dilute aluminium alloys, such as AA1050, dynamically generate sub-grains during cold rolling. If AA1050 is rolled at cryogenic temperatures (liquid nitrogen), this large decrease in temperature minimises the occurrence of dynamic recovery during rolling. The result is that the material has a large amount of stored energy and a high dislocation density, thereby giving it a high strength. This research looks at the recovery and recrystallization processes during annealing after cryo-rolling, and compares the formation of sub-grains and recrystallized grains to those where rolling was performed at room temperature. The type of dislocation structure that forms during the rolling process directly affects the evolution of the microstructure post-deformation. Owing to the extreme temperatures of cryo-rolling, the dislocation structure cannot undergo dynamic recovery. Instead, a distinct cell-like structure forms, with dense dislocation walls that are high energy. During subsequent annealing, the driving force for recrystallization is increased with a decrease in the rolling temperature, with the cryo-rolled material having a greater number of nucleation seed and consequently, a fine grained recrystallized microstructure.

Abstract: Combining different materials allows designing lightweight structures with tailor-made properties at the macroscopic as well as at the microscopic scale. In this context co-extrusion by lateral angular co-extrusion process (LACE) offers a great potential for advanced products by joining different light alloys, such as titanium and aluminum alloys. While titanium alloys show particular high mechanical strength values and a good corrosion resistance, aluminum alloys feature a considerable high specific bending stiffness at low materials costs. Within the presented research study metallic compounds were manufactured by co-extrusion using a LACE process. Since the mechanical properties of metallic composites highly depend on the existence and formation of an intermetallic layer, the bonding zone of the compounds were investigated. Experiments were carried out at the example of the material combinations Al99.5 & Ti99.2 and AlSi1MgMn & Ti99.2, respectively. The results of co-extruded samples concerning the intermetallic layer that was formed during heat treatment were compared with those directly after the co-extrusion. The investigations were focused on the formation of the bonding zone after both co-extrusion and the subsequent heat treatment. The bonding zone was characterized by scanning electron microscopy as well as electron probe micro analysis. Finally, the bond strength of each compound variant was determined and evaluated.

Abstract: The effects of different process parameters (temperature and time) during the ageing treatment on the microstructure and the mechanical properties of a secondary rheocast AlSi9Cu3(Fe) alloy have been examined. Optical microscope investigations have been performed to qualitatively study the microstructure of the as-rheocast and thermal treated alloys. Transmission electron microscopy technique and selected area electron diffraction analyses have been used to characterize the hardening phases precipitated in the Al-matrix during the different ageing stages. The evolution of mechanical properties of the Al matrix has been monitored by micro-hardness testing.

Abstract: The aim of the study is to present the influence of quench rate on the artificial ageing response of Al-8%Si-0.4%Mg cast alloy in terms of Brinell hardness and yield strength. The investigated material was produced by a gradient solidification technique and exhibited a microstructure that corresponds to the one of gravity die castings, with a dendrite arm spacing of approximately 25 µm. The study comprises two solution treatment temperatures, five quench rates and artificial ageing times exceeding 100 hours at 170 and 220 °C. The microstructure and concentration profiles of Mg and Si were evaluated using energy and wavelength dispersive spectroscopy. Microstructural examination reveals an increment of solutes in the Al-matrix when higher solution treatment temperatures accompanied with high quench rates are applied and shows how both Si and Mg atoms have diffused towards the eutectic during quenching. Consequently, i.e. by increasing the levels of solutes and vacancies, the highest strength levels were realized. The study confirmed that quench rates above 2 °C /s do not offer substantial strength improvement while quenching at lower rates resulted in a lower peak hardness and longer times to peak

Abstract: This paper investigates the optimization of the solution heat treatment parameters of the rheo-high pressure die cast (R-HPDC) 2139 aluminium alloy. Differential Scanning Calorimetry (DSC) and optical microscopy were used to investigate the incidence of incipient melting and therefore determine suitable solution heat treatment temperatures. A three-step solution heat treatment where the alloy was heat treated from 400°C to 513°C using controlled heating conditions and held at 513°C for 2 hours and finally heated up from 513°C to 525°C and held there for 16 hours was done. R-HPDC is known to produce surface liquid segregation and when processing the alloys these areas are most prone to incipient melting. The applicability of a single (525°C for 16h) and three-step solution heat treatments on the R-HPDC 2139 aluminium alloy was also investigated. A single-step solution heat treatment results in incipient melting, whereas this is mostly eliminated using the three-step solution heat treatment. However, a high volume fraction of undissolved phases remain in the liquid segregated areas, even after the three-step solution heat treatment.

Abstract: The β-phase transforms to α′, α and α" within a range of temperature from the β-transus (T_β) to about 600°C, considering no external stress is applied. Two types of microstructure were obtained: acicular martensite when rapidly cooled and lamellar α/β when slowly cooled from the β phase field. The sequential transformation of β into α′, α-phase, α₂, and α" was revealed as peaks on the coefficient thermal expansion (CTE) curves, however, reversed transformations: α"→β, and α→β, were revealed by the DSC thermograms. The presence of β, α′, α, α₂ and α" was identified by means of XRD analysis and HRTEM.

Abstract: Microstructure evolution and static recrystallization kinetics of a magnesium AZ31 alloy during annealing have been investigated. The Mg alloy sheets were rolled at 100°C at a low rolling speed of 15m/min and a high rolling speed of 1000m/min to reductions of 8%, 23% and 30%. Annealing was then conducted on the as-rolled specimens at 200°C for different times. The microstructure was characterized by optical microscopy and recrystallization kinetics were evaluated by means of Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The softening behavior was found to be divided into two annealing stages. The first stage was related to the recrystallization on high stored energy regions, such as shear bands and twins, while the second stage was associated to the recrystallization of low stored energy areas.

Abstract: Near-net shape casting of wrought aluminium alloys has proven to be difficult due to hot tearing. The Council for Scientific and Industrial Research (CSIR) has successfully processed wrought aluminium alloy 2139 into plate castings using the Rheo-high pressure die casting process (R-HPDC). Alloy 2139 is a Ag-containing aluminium alloy from the Al-Cu-Mg 2xxx series family. The addition of Ag enhances the age hardening response through the formation of co-clusters that act as precursors to the formation of plate-like Ω precipitates. These co-clusters typically form during natural ageing and 12-24 h of natural pre-ageing is normally specified before artificial ageing in Ag-containing Al-Cu-Mg alloys. The T6 hardness and tensile properties of R-HPDC 2139 alloy were investigated with and without natural pre-ageing. It is shown that there is no significant difference in both peak hardness and tensile properties in R-HPDC alloy 2139 with and without natural pre-ageing. The possible precipitation phenomena in both cases are discussed.