Effects of combined Zr and Mn additions on the microstructure and properties of AA2198 sheet

The effect of individual and combined zirconium and manganese additions have been compared for an AA2198 6 mm thick sheet in T351 temper regarding their influence primarily on recrystallisation resistance and secondly on fracture toughness and overageing resistance. A complete characterisation of the dispersoid distributions was carried out for a deeper understanding of the effects of the Al3Zr and Al20Cu2Mn3 particles, involving studying their formation from the as-cast and homogenised stage.The most important finding in this work was the lower recrystallisation resistance in the alloy containing 0.1 wt%Zr + 0.3 wt%Mn compared to that containing only 0.1 wt%Zr. This result was rather unexpected, if one considers the opposite microsegregation patterns of Zr and Mn during casting, which leads to dispersoids occupying the majority of the grains’ volume and minimising dispersoid-free zones that could be potential sites for nucleation of recrystallisation. The other two alloys with dispersoid additions 0.05 wt%Zr + 0.3 wt%Mn and 0.4 wt%Mn, were partially and fully recrystallised respectively in the rolled T351 condition.Equally important in this work, was the observation that the opposite microsegregation trend of Zr and Mn sufficed to restrict grain growth in unrecrystallised areas. The 0.1Zr-0.3Mn alloy exhibited the lowest grain size of all alloys, both in the T351 temper and after annealing at 535oC for up to 144 hours. The reason for this was the combined action of Al20Cu2Mn3 dispersoids and Mn solute in the regions where the Zr concentration was low (i.e. near the grain boundaries), which offered additional pinning pressure to those areas compared to the 0.1Zr alloy.The lower recrystallisation resistance of the 0.1Zr-0.3Mn alloy was explained on the grounds of two main factors. The first was the lower subgrain size and hence stored energy within bands of Al20Cu2Mn3 dispersoids, which increased the driving force for recrystallisation in these regions. The second was the interaction between Zr and Mn that led to a decrease in the Al3Zr number density and pinning pressure. Since Zr was the dominant dispersoid family in terms of inhibiting recrystallisation, inevitably this alloy became more prone to recrystallisation. The Al3Zr pinning pressure was found to be much lower especially within bands of Al20Cu2Mn3 dispersoids. The detrimental effect of the Mn addition on the Al3Zr distribution was proven not to result from the dissolution of Zr within Mn-containing phases, and several other phases, at the grain interior and also in grain boundaries. The observed effect could not be precisely explained at this stage.Concerning mechanical properties, the 0.1Zr alloy exhibited the best combination of properties in the Kahn tear tests for fracture toughness. Further, it had a higher overageing resistance compared to the 0.1Zr-0.3Mn alloy.As an overall conclusion from this work, considering all the studied properties here that are essential for damage tolerant applications, the addition of 0.1 wt%Zr to the AA2198 6 mm thick sheet was found to be superior to that of the combined addition of 0.1 wt%Zr + 0.3 wt%Mn.EThOS - Electronic Theses Online ServiceEPSRC : Alcan CRVGBUnited Kingdo

Systematic Evaluation of the Advantages of Static Shoulder FSW for Joining Aluminium

Static Shoulder Friction Stir Welding (SS-FSW) is a modification to conventional FSW that was originally developed to improve the weldability of titanium alloys by reducing through thickness temperature gradients. Surprisingly, to date, there have been no published systematic studies comparing SS-FSW to FSW for aluminium welding. This may be because the high conductivity of aluminium means the heat input produced by the shoulder is thought to be beneficial. In the work presented when welding a high strength 7050 aluminium alloy, even in a relatively thin 6 mm plate, it is shown that SS-FSW has several advantages; including a reduction in the heat input, a massive improvement in surface quality, and a more uniform through thickness temperature distribution, which leads to narrower welds with a reduced heat affected zone width and more homogeneous through thickness properties. The reasons for these benefits are discussed.</jats:p

Friction stir processing of aluminium-silicon alloys

Friction Stir Processing (FSP) has the potential for locally enhancing the properties of Al-Si alloy castings, for demanding applications within the automotive industry. In this thesis, the effect of FSP has been examined on three different cast Al-Si alloys:i) A Hypoeutectic Al-8.9wt%Si Alloyii) A Hypereutectic Al-12.1wt%Si Alloyiii) A Hypereutectic Al-12.1wt%Si-2.4wt%Ni AlloyThe influence of different processing parameters has been investigated at a fundamental level. Image analysis of particle size distributions and growth method of tessellation were used to quantify the level of particle refinement and the homogeneity of the second phase spatial distribution. Stop-action experiments were also carried out, to allow the microstructural changes around the tool during FSP to be studied. Two computer models have been explored, in order to predict the temperature distribution and the material flow behaviour. Furthermore, the stability of the microstructure of the friction stir processed material was studied after being heat treated at elevated temperatures. The changes in particle size and grain structure were examined, hardness measurements were taken across the PZ, and tensile testing were carried out at room and elevated temperatures.After FSP, the microstructure of the cast Al-Si alloys was greatly refined. However, differences in microstructure have been observed throughout the PZ, which tended to be better refined and distributed on the advancing side, than the retreating side of the PZ. Changing the processing parameters also influenced the size and spatial distribution of the second phase particles. By studying the changes in microstructure around the tool from the stop-action experiments, and comparing the results to the thermal distribution and material flow behaviour predicted by the computer models, it has been shown that the flow stress, pitch, and temperature of processing, all needed to be considered, when determining the effects that FSP have on the microstructure. FSP caused very little changes to the hardness of the material, while tensile properties were greatly improved, due to the elimination of porosity and refinement of large flawed particles. In terms of the stability of the microstructure after FSP, particle coarsening and abnormal grain growth has been observed during high temperatures heat treatment. Furthermore, the Al2Cu phase was found to dissolve into solid solution at elevated temperatures, so GPZs and solute clustering can then develop within the alloy during natural ageing.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Preferred EAC initiation sites in 7xxx aluminum

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