54 CHAPTER 3 SELECTION AND PROCESSING OF THE SPECIMEN MATERIAL 3.1 HIGH STRENGTH ALUMINIUM ALLOY In the proposed work, 7075 Al alloy (high strength) has been identified, as a material for the studies on superplastic behavior. An inherent limitation of the conventional aluminium (Al) alloys like many metals and alloys, is that they are unstable when plastically deformed by stretching. This is responsible for the catastrophic necking observed in tensile testing, and also for the limited extent of uniform deformation possible during processes, such as stretch forming which involve mainly tensile stresses. Superplastic materials are relatively stable when deformed in tension. For Al alloys, the die sets are relatively inexpensive, because of the moderate temperatures associated with the process, and they can be produced quickly. For Al alloys the forming temperatures are likely to lie in the range of 460-530 C, which is ~0.9Tm. So, the problem of maintaining small grain sizes requires special attention. Further, Al alloys are prone to cavitation during superplastic flow. The typical composition of the 7075 Al alloy is given in Table 3.1. This alloy has a melting point of 660 C, density of 2.7 g/cm 3, FCC crystal structure, and the yield strength of 500 MPa.
55 Table 3.1 Chemical composition of the 7075 Al alloy Zn Grade Mg Cu Mn Cr Fe Si Al (Wt%) 7075Al 5.9 2.0 1.32 0.2 0.12 0.42 0.19 Bal. 3.2 GRAIN REFINEMENT A fine grain size is a microstructural requirement in superplastic alloys. A number of microstructural features are important for developing superplasticity. These include the size of the grains, mobility and the shape of grain boundaries, and the strength, size and distribution of the second phase in the matrix. The connection between a fine grain size and superplasticity arises because of the nature of the flow stress-strain rate relationship in a material with a fine grain size. An understanding of the basic metallurgical principles underlying grain refinement and grain growth is therefore important, for the development of superplasticity in materials which would not normally be superplastic. Several methods are available for grain refinement, including phase separation, phase transformation, and mechanical working with recrystallisation. The particular method selected for grain refinement for an alloy, depends on the nature of the alloy. When recrystallisation is used for grain refinement, the aim is to nucleate several product (recrystallised) grains within each grain of the parent microstructure. It should be possible in principle to develop a fine grain microstructure, using thermal treatments alone. However, the imposition of mechanical working during any stage of the heat treatment produces the required grain size in a fewer number of processing steps.
56 Achieving a fine grain size is, in itself not sufficient to guarantee that a material will exhibit Superplasticity, since the grain size needs to remain stable throughout the deformation process. Grain growth during superplastic flow has been reported for a number of materials, with the extent of grain growth being greater in the superplastically deformed part of the samples studied, than in the underformed areas. It is clear that strain enhanced grain growth is a widespread property of superplastic deformation in both the pseudo single phase and in microduplex materials. 3.2.1 Grain Refinement by Mechanical Working Grain refinement, as a result of warm working and recrystallisation treatments, has been used extensively in the development of alloys. The alloys are termed pseudo single phase, since they consist almost entirely of a solid solution strengthened matrix, with <10% by volume of a precipitate phase which is present to stabilize the microstructure against grain growth in which the grains would normally tend to rearrange themselves by climbing to form dislocation walls, and possibly, sub grain boundaries. In the absence of any fine particles, the dislocation would allow the microstructure to undergo continuous recovery and ultimately recrystallisation. However, the presence of fine particles, which are usually less than 0.2 µm in diameter, prevents recovery by exerting a drag on the migrating dislocations, dislocation walls and sub grain boundaries. Mechanical working in the presence of the fine particles, therefore, generates and maintains a large amount of stored energy, and introduces into the microstructure a large number of potential nucleation sites for subsequent recrystallisation. Alloys which contain predominantly fine particles, develop a fine grain equiaxed microstructure during the initial stages of superplastic deformation, by in situ recrystallisation. As high temperature deformation, and hence, grain boundary sliding proceeds, the misorientation between the grains increases, and leads to the formation of
57 high angle grain boundaries and a true superplastic microstructure, which can undergo grain boundary sliding. The 7xxx series alloys require a more complex thermomechanical processing treatment to develop a fine grain size in what is not normally a superplastic material. Two modes of recrystallisation-the discontinuous and continuous modes-can be used for grain refinement. Both rely on particles to produce a fine grain size. In the discontinuous recrystallisation mode, the key factor is the formation of a high density of nucleation sites for the recrystallisation, has been successful in various precipitation hardening aluminium alloys. Grain sizes of the order of 10 µm are frequently obtainable and the alloys display good superplastic properties. Continuous recrystallisation, which is the alternative recrystallisation mode available for grain refinement, proceeds by sub grain coarsening until high angle boundaries appear, and the structure is recrysatallised. The nucleation of the individual recrystallised grains does not occur in continuous recrystallisation. This technique has been extensively used for grain refinement in an Al-Cu-Zr alloy. As mentioned earlier, the microstructural requirements for superplasticity are a fine grain size, and resistance to grain coarsening, while the structural requirements demand high yield strength. Precipitation hardening in Al-Zn-Mg alloys meets the strength requirements; but, conventionally produced, they are not superplastic. However, processing methods have been found to grain refine these alloys by recrystallisation, and to control grain coarsening by particle dispersions. The Rockwell International Service Centre has developed a thermo mechanical treatment to render a conventional Al alloy superplastic. The same treatment has been successfully applied for grain refining a 7075 aluminium alloy.
58 Normally, a four step grain refinement process used for the 7075 alloy, consists of a solution treatment overageing warm working and recrystallisation. As explained below, each step has a role in producing the fine recrystallised grain size. (i) Solution treatment This step is designed to produce a uniform starting condition, by dissolving the soluble precipitates, placing in solution the Zn, Mg and Cu alloying elements. The E-phase particles, formed by the Cr addition to the alloy, are not dissolved during the solution treatment. These insoluble particles, typically 0.1 µm in diameter, are called dispersoid particles. The estimated volume fraction of the dispersoid particles is about 0.01. The latter particles, probably Mg 3 Cr 2 Al 18, CrAl 7 or Cr 2 Al 9 are insoluble and typically 0.1-0.2 µm in diameter. (ii) Overageing treatment Precipitate particles formed during overageing create nucleation sites for recrystallising grains. The particles are equilibrium m-phase (a mixture of Mg 3 Zn 2 and CuMgAl and must be larger than about 0.75 µm in diameter, to affect the nucleation of recrystallising grains). T phase (a mixture of Mg 3 Zn 3 Al 2 and CuMg 4 Al 6 ) precipitates with a diameter between 1 and 2 µm. The latter particles have two functions. Firstly, to effect inhomogeneous deformation during warm rolling, and secondly to act as nucleation sites for the recrystalising grains during the subsequent heat treatment.
59 (iii) Warm rolling Defects introduced into the alloy during this step are essential for recrystallisation. In addition, warm deformation and substantial lattice reorientation harden the precipitate particles. The flow around the non-deforming particles produces deformation zones, which extend about 1 particle radius away from the particle interface. The deformation zones are characterized by a small dislocation cell size and by lattice orientations that are substantially different from the general lattice orientation in the surrounding matrix. (iv) Recrystallisation A high temperature annealing treatment is applied to produce a fully recrystallised microstructure nucleation of recrystallising grains, that occurs in the deformation zones surrounding the precipitate particles. The recrystallisation process can be arrested by quenching after short recrystallisation times. The fine scale dispersion of large precipitates and a rapid heating rate result in a large number of recrystallisation nuclei, and thus a small grain size is developed. Grain growth during annealing and subsequent superplastic deformation is restricted, by the drag effect imposed on the grain boundaries by the fine insoluble Cr-rich dispersoid phase. After warm working, a rapid recrystallisation heat treatment is carried out at 550 C to produce the fine grain size necessary for superplasticity. It consists of various time cycles, all having a high time cycle. 3.2.2 Modified Grain Refinement Method Among the many grain refinement techniques, the thermomechanical treatment has been considered, and the steps are shown in Figure
60 3.1. Warm rolling was carried out in a number of steps to obtain the required thickness. In each pass the sheet blank was turned through 90 in order to obtain equi-axial grains. Intermediate heating was introduced, to relieve the internal stresses of the rolled blank sheet. The final grain size was measured, and it was found to be less than 10 µm. The microstructure analysis was carried out, using Biovis materials plus image processing software. INITIAL SHEET THICKNESS 5 mm SOLUTION TREATMENT 500 C FOR 1HOUR THEN FURNACE COOLING TO 380 C OVERAGING AT 380 C FOR 2 HOURS THEN FURNACE COOLING TO190 C WARM ROLLING AT 180 C WITH INTERMEDIATE REHEATING SO AS TO GET 1% REDUCTION PER PASS (65-85% REDUCTION OF THICKNESS) RECRYSTALLISATION AT 500 C FOR 0.5 HOUR AND THEN WATER QUENCHED AGING AT 180 C FOR 0.5 HOUR THEN WATER QUENCHED GRAIN SIZE MEASUREMENT Figure 3.1 Flow chart of the modified thermo-mechanical treatment
61 In this modified thermomechanical treatment of the 7075 alloy, the overall cycle time was reduced by one hour, compared to the Taharsahraoui method. The modified thermomechanical treatment compared with the Taharsahraoui method is given in Table 3.2. Table 3.2 Thermomechanical treatment process Step Temperature ( C) Taharsahraoui method ( Time) Modified method (Time) Operations Solution treatment 500 1.0h 1.0h Furnace cooling to 380 C Overaging 380 2.5h 2.0h Furnace cooling to 190 C Warm rolling 180 65-85% 65-85% Reduction of thickness Recrystallisation 500 0.5 h 0.5 h Water quench Aging 180 1.0 h 0.5 h Water quench 3.3 CONTROL OF GRAIN COARSENING IN 7075 ALUMINIUM ALLOY In addition to grain refinement, the restriction of grain growth is essential in superplastic alloys. Since the high strength precipitation hardening aluminium alloys do not have duplex microstructures, the particles must be used to control grain coarsening. The insoluble dispersoid particles are present in the alloy at the temperature at which superplastic forming is conducted. The role of the dispersoid particles in restricting grain growth where several dispersoid particles produce substantial distortions in a grain
62 boundary. The dispersoid particles are roughly 0.1µm in diameter, and the volume fraction of the dispersoids is close to 0.01. 3.4 RESULTS AND DISCUSSION The sheet was polished to a mirror-like finish, and etched using Keller s reagent for 15 seconds. It is evident that the as received sheet blank has an average grain size of 25 µm, which is not suitable for superplastic forming. Hence, the grain refinement technique was required to reduce the grain size. The final grain size was measured, and it was found to be less than 10µm.The specimen warm rolled up to 80% reduction, achieved the highest elongation to failure. That deformation beyond a strain of almost 80% does not create intense deformation zones around small particles. Increasing the amount of deformation beyond 80% does not produce a finer grain size, when the thermo-mechanical treatment process is used. A four step thermo-mechanical processing sequence has been devised for the grain refinement of 7075 Al. An overageing step is used to precipitate a high density of particles, larger than approximately 0.75µm in diameter. The deformation zones that form around the particles during subsequent rolling, serve as nucleation sites for recrystallising grains, when the alloy is rapidly heated above the recrystallisation temperature. To reduce the ageing time, the precipitate particles formed during overageing are generally be of the same order of strength as the matrix phase. However, the second phase particles are slightly harder than the matrix phase. If it is distributed uniformly as fine particles within the matrix, less cavitation during superplastic flow can occur. Figures 3.2 and 3.3 show the microstructure of the Al alloy before TMT (Thermomechanical treatment) and after TMT respectively.
63 Figure 3.2 Microstructure of the Al 7075 (before TMT) Figure 3.3 Microstructure of the Al 7075 (after TMT)
64 3.5 SUMMARY Grain refinement and grain size control are essential for superplasticity. The methods selected for use in a particular case, depend on the nature of the alloy system. Recrystallisation is useful for grain refinement in alloys that cannot be grain refined by other methods. Particles are useful to control the recrystallisation process, and have been used for grain refinement by recrystallisation. The modified thermomechanical treatment was used to reduce the overall cycle time without change in the final grain size.