Philip B. Prangnell

The effect of strain path on the development of deformation structures in severely deformed aluminium alloys processed by ecae

Abstract Equal channel angular extrusion (ECAE) has been used to investigate the formation of submicron grain structures in Al-alloys deformed to ultra-high plastic strains by different strain paths. The different strain paths were obtained by rotating billets through 0, 90, and 180° between each extrusion cycle. High resolution EBSD analysis has been employed to measure the boundary misorientations within the deformation structures. This has highlighted great differences in the evolution of the deformed state, as a function of the strain path, even after effective strains as high as 16. It has been demonstrated that the most effective method of forming a submicron grain structure by severe plastic deformation is to maintain a constant strain path. Processing routes involving a 180° rotation reverse the shear strain every second pass and this prevents the build up of significant numbers of new high angle boundaries. When a sample is processed with an alternate clockwise and anticlockwise 90° rotation, between each extrusion cycle the billet is deformed on two shear planes, each of which experiences half the total strain, compared to the single shear plane when there is no rotation. This reduces the rate of formation of high angle boundaries. With a constant clockwise 90° rotation the sample is also deformed on two alternate shear planes, but the total strain becomes redundant every fourth extrusion cycle. However, in this case each shear is reversed out of sequence after first deforming the billet on the alternate shear plane. This appears to be a much more effective means of forming new high angle boundaries than 180° rotation, where the shear strain is immediately reversed each alternate cycle, but is still less efficient than deformation with a constant strain path.

DEVELOPING STABLE FINE-GRAIN MICROSTRUCTURES BY LARGE STRAIN DEFORMATION

Methods of deforming metals to large strains are reviewed and the process of equal channel angular extrusion is analysed in detail. The development of microstructure during large strain deformation is discussed, and it is concluded that the main criterion for the formation of a sub–micron grain structure is the generation of a sufficiently large fraction (> 0.7) of high–angle grain boundary during the deformation process. For aluminium alloys, it is found that a low–temperature anneal is required to convert the deformed microstructure into an equi–axed grain structure. The material, microstructural and processing factors that influence the formation of such fine–grain microstructures are discussed, and the stability of these microstructures at elevated temperatures is considered.

Stability of nugget zone grain structures in high strength Al-alloy friction stir welds during solution treatment

Abstract From studying the solution treatment behaviour of friction stir welds, in a typical high strength Al-alloy (7010), it has been established that the nugget zone grain structure is inherently unstable at high temperatures, despite the presence of Al 3 Zr dispersoids that inhibit grain boundary mobility. Good agreement has been found between experimental observations and a unified theory of the stability of cellular microstructures, proposed by Humphreys, which has shown that the condition for instability is highly dependent on the welding parameters. Low heat inputs result in an exceptionally fine nugget grain structure, and abnormal grain growth occurs throughout the nugget zone, encouraged by the dissolution of soluble precipitates. When welds are produced with higher heat inputs, instability is more marginal, as the grain structure after welding is coarser relative to the dispersoid density. However, grains can still grow abnormally into the nugget zone with a planar front and this leads to very large, mm-scale, grains being formed.

Analysis of the billet deformation behaviour in equal channel angular extrusion

Equal channel angular extrusion is a relatively novel method for deforming materials to very high plastic strains, with no net change in the billets shape. However, before the technique can be exploited it is important to understand the deformation behaviour within the die and its relationship to the tooling configuration and friction conditions. Billets containing scribed grids, simple finite element analysis, and microstructural evidence have been used to investigate this issue. It has been found that the strain achieved is sensitive to the die angle, friction conditions, and the application of a back-pressure, all of which can have a large effect on the microstructure and strain inhomogeneity within the processed billet. The distribution of strain is most uniform, and approximates most closely to a simple shear, if the deformation zone is constrained to be as narrow as possible. The best processing conditions appear to be obtained with a sharp die corner, low friction, and a constraining back-pressure. Embedded marker wire experiments have shown that on repeatedly extruding a billet with a constant strain path rotation of material occurs around the billets ends. This results in the sheared billet wrapping around on itself during the process and thus maintaining the billets shape, despite the increasing shear strain in each extrusion cycle.

The effect of coarse second-phase particles on the rate of grain refinement during severe deformation processing

Abstract The effect of second-phase particles on the rate of grain refinement during severe deformation processing has been investigated, by comparing the microstructure evolution in an AA8079 aluminium alloy, containing 2.5 vol.% of ~2 μm particles, with that in a high purity, single-phase, Al-0.13% Mg alloy, deformed identically by ECAE to an effective strain of ten. The materials were analysed by high-resolution EBSD orientation mapping, which revealed that grain refinement occurred at a dramatically higher rate in the particle-containing alloy. A submicron grain structure could be achieved by an effective strain of only five in the particle-containing alloy, compared to ten in the single-phase material. The mechanisms that contribute to this acceleration of the grain refinement process are discussed.

Effect of welding parameters on nugget zone microstructure and properties in high strength aluminium alloy friction stir welds

Abstract Theeffects ofthemainwelding variables,apart form tool design,on themicrostructureandproperties ofthenugget zone in friction stir welds have been investigated for a typical high strength Al alloy (AA7010). It has been found that there is an optimum rotational speed, for a given travel speed, that gives the highest strength and ductility for the nugget zone. As the travel speed is increased, it is necessary to increase the spindle speed to maintain this condition, although the optimum condition does notdirectlyrelatetoa constantratioof rotationalto travel speed. This condition coincides with a heat input, for a given travel speed, that gives the maximum level of solute in solution in the nugget and the lowest density of coarse second phase particles. For low travel speeds, this is limited at excessive heat inputs by reprecipitation occurringduringtheweldcoolingcycles,whereas for high travel speeds incipient melting within the nugget appears to be an important factor.

Modelling texture development during equal channel angular extrusion of aluminium

Abstract The deformation textures that develop in aluminium during ECAE (without rotation of the billet) have been investigated experimentally and modelled using the FC-Taylor approach, for two different die angles (90 and 120°), by using actual deformation histories measured from scribed marker grids. This has shown that the deformation during ECAE can best be described in terms of streamline coordinates and involves a simple shear parallel to the streamline, which becomes aligned with the final extrusion direction, and a plane strain tension and compression component that develops as the material enters and leaves the dies deformation zone. The textures observed were similar to those found following torsion straining and had the main components {001}〈110〉 and {112}〈110〉 along a B partial fibre. However, in the case of ECAE, the positions of maximum intensity were rotated by ~15–20° about the transverse direction (TD). Similar textures were seen for even and odd numbers of extrusion passes, suggesting that the TD rotation is not caused by alignment of the fibre direction with the die’s ‘shear plane’, as has been previously reported. In contrast, texture simulations showed that this rotation occurs as a consequence of the additional plane strain compression component in ECAE deformation.

Production of ultra-fine grain microstructures in Al-Mg alloys by coventional rolling

Abstract The conditions under which micron-scale grain structures can be developed in two Al–3%Mg alloys by a process of continuous recrystallization, during rolling and plane strain compression to large strains, have been investigated using high resolution electron backscatter diffraction (EBSD). In an Al–Mg–Cr–Fe alloy, it was found that a lower limit to the processing temperature for forming ultra-fine grain structures is imposed by the low mobility of grain boundaries, whilst an upper limit is imposed by grain growth. In an Al–Mg–Sc–Zr alloy containing stable second-phase dispersoid particles, a fine-grained microstructure is formed only at larger strains and higher temperatures due to the interaction of grain boundaries with the dispersoids. It is concluded that although micron-scale grain structures can be produced in Al–3%Mg alloys by deformation processing in plane strain compression, the processing window, which is controlled by both the solute and particle content, is severely restricted.

Microstructure refinement and mechanical properties of severely deformed Al–Mg–Li alloys

Abstract Experiments have been conducted to investigate the possibility of producing aluminium aerospace-alloys, with properties similar to those of the powder metallurgical alloy AA5091, by the potentially cheaper route of spray-casting and severe deformation. Two alloys with compositions based on the commercial alloy AA5091 were spray-cast to obtain a large density of Al 3 (Zr,Ti) dispersoids as a substitute for the oxide and carbide particles present in AA5091. The materials were subsequently severely deformed using equal channel angular extrusion (ECAE) to effective strains as high as ten. After deformation the grain sizes in both materials were as fine as 0.3–0.6 μm, while the fraction of high angle grain boundary area was >70%. The severely deformed alloys were stable up to 400 °C, but at higher temperatures abnormal grain growth was observed initiated by the heterogeneous distribution of dispersoid particles inherited from the spray-castings. Encouragingly, for similar heat-treatment conditions, the yield strengths of the experimental materials were found to be comparable to the values specified for the commercial alloy AA5091 while the ductilities were up to three times higher.

The effect of particle distribution on damage formation in particulate reinforced metal matrix composites deformed in compression

Image analysis results are reported on the generation of damage in particulate reinforced metal matrix composites during compressive deformation. The technique allows the automated collection of data on the incidence of particle fracture and void formation in the matrix as a function of important microstructural parameters such as local particle volume fraction and particle size. There is a strong relationship between damage and the local volume fraction of the reinforcement proving that damage formation is accentuated in regions of particle clustering. With the SiC reinforced materials examined, there was observed to be a change in dominance of damage mechanism from particle fracture at low local volume fractions to void formation in the matrix within strongly clustered regions. The results are compared with finite element (FE) modelling of the compressive deformation of clustered particles using a simple cluster of equi-spaced particles. The FE results suggest that plastic flow is generally inhibited in clustered regions. In certain highly clustered configurations shielding is such that flow does not occur in the heart of the cluster even at high levels of average plastic strain. The modelling suggests that the change in dominance of damage mechanism is related to the dramatic increase in tensile hydrostatic stresses in the matrix with higher levels of particle clustering.