Influence of fine grained structure and superplastic deformation on the strength of aluminium alloys

The influences of fine grained (FG) structure and superplastic deformation on the mechanical properties under quasistatic, impact and cycling loading conditions have been established for the aluminium alloys 1560 (Al-Mg-Mn), 1141 (Al-Cu-Mg-Ni-Fe) and 1960 (Al-Zn-Mg-Cu-Zr). FG materials compared with recrystallized coarse grained (CG) ones improve tensile strength and ductility and high-cycle fatigue endurance, but reduce static and impact toughnesses. The effect of grain refinement on crack resistance is directly manifested in the difficulty of crack initiation and in facilitating its growth. Blanks with FG structure, after superplastic processing, are recommended instead of CG recrystallized ones for the production of principal parts whose service life is limited by first crack appearance. In the presence of cracks, higher strength can be obtained by the use of blanks made of CG commercial semifinished products and those manufactured by traditional methods of hot die forging.     

The effect of hydrogen on the strength and superplastic deformation of beta-titanium alloys

The effect of hydrogen on mechanical properties and superplastic deformation of two commercial titanium alloys, Ti-4Al-7Mo-10V-2Fe-1Zr (Ti-471021) and Ti-10V-2Fe-3Al (Ti-1023), was studied in the range of hydrogen concentration up to 1.3 wt%. The elevated temperature hardness of Ti-471021 alloy increased with hydrogen concentration. The stress levels during the superplastic deformation in both alloys increased with increasing concentrations of hydrogen in the -phase region. X-ray diffraction results and examination of the microstructure with TEM revealed that no hydrides had formed up to hydrogen concentrations of 1.3 wt%. The increase in flow stress was mainly due to the solid solution strengthening by hydrogen during the superplastic deformation.     

Control of cavitation during superplastic forming of high strength aluminium alloys

Abstract

This investigation deals with superplastic deformation and cavitation behaviour of a 7475 high strength aluminium alloy in uniaxial tension. Intergranular cavitation increases sharply with strain as a result of continuous nucleation, thus limiting the ductility to relatively small strains in the superplastic range and affecting the room temperature service properties. To reduce the overall cavitation, several processes have been carried out. They involve treatments of the material with or without superimposed hydrostatic pressure and they can be applied before, during, or after deformation. The results of these processes are presented and compared. It is concluded that cavitation can be significantly reduced and even eliminated. Such a result can increase the use of superplastic forming for industrial applications in aeronautics and aerospace technology.

MST/985     

Influence of electric current on superplastic deformation mechanism of 5083 aluminium alloy

5083 aluminium alloy superplastic forming adopted resistance heating can not only improve efficiency and cut energy but also generate electroplastic/electrosuperplastic effect to make the material deformation possess lower flow stress and higher plasticity. By analysing the influence of current on dislocation slipping, grain boundary migration and dynamic recrystallisation, it is found that the electron wind force can enhance the mobility of dislocations; meanwhile, the current also can reduce the activation heat of dislocation motion by joule heating effect. What is more, the grain size of resistance heating forming sample is significantly smaller than furnace heating, and the cavities in the sample become small and dispersive, so the resistance heating forming specimen possesses better performance.     

Influence of rolling direction on strength and ductility of aluminium and aluminium alloys produced by accumulative roll bonding

Sheets from commercial purity aluminium AA1050 and aluminium alloy AA6016 were processed by accumulative roll bonding to obtain an ultrafine-grained microstructure. The accumulative roll bonded samples showed a significant increase in specific strength paired with high ductility. Despite a strongly elongated grain structure, tensile testing of samples oriented 45° to the rolling direction revealed considerable improvement in elongation to failure compared to the samples oriented parallel to the rolling direction. From hydraulic bulge tests, it was observed that the accumulative roll bonded samples reached higher burst pressures and slightly lower equivalent strains in comparison to the as-received conventionally grain-sized samples. This behaviour reflects the extraordinary mechanical properties of the ultrafine-grained materials and indicates promising metal sheet formability.     

Fracture behaviour of Cu-Zn-Co and Cu-Zn-Ni alloys during superplastic deformation

Cavitation and fracture behaviour in two commercial /gb brasses, one modified with 2wt% Co (Cu-Zn-Co) and the other with 2wt% Cr (Cu-Zn-Cr), have been investigated in Region II of superplastic flow. These alloying elements form cobalt-rich (0.3 m average diameter) and chromium-rich (5 m average diameter) precipitate particles which are distributed uniformly in the matrix and which play an important role in cavitation and inhibiting grain growth during deformation. Void size distributions, volume fraction of voids and the number of voids per unit area have been measured as a function of strain in Region II and the results show a very marked difference in the degree of cavitation in Cu-Zn-Co and Cu-Zn-Cr alloys. Experiments show that the deformation is quasi-uniform with little or no necking in the specimens of Cu-Zn-Co alloy in Region II, and the final fracture occurred by the growth and interlinkage of internal voids. On the other hand, in the specimens of Cu-Zn-Cr alloy a sharp or localized neck developed early in the deformation in Region II and the specimen pulled down to a fine point leading to failure by necking. The importance of diffusion or slip accommodation of grain boundary sliding in void formation during superplastic flow is discussed and a criterion for failure is suggested.     

Microstructure and strength of Si-Ti-C-O,fibre-reinforced aluminium and aluminium alloys

Microstructures of Si-Ti-C-O fibre-reinforced aluminium and aluminium alloys were investigated by scanning electron microscopy, and both conventional and analytical transmission electron microscopy. In the latter samples, some inclusions were observed between the matrix and the fibres. From the electron diffraction, high resolution microscopy and compositional analysis by energy-dispersive X-ray, the inclusions were identified as the α-Al-Si-Fe phase. Since the longitudinal three-point bending strength decreases with the increase of iron content, it was concluded that the α-phase inclusions on the surface of the fibre contribute to the lower strength of the composites based on this alloy.     

Texture changes during superplastic deformation of mechanically alloyed aluminium IN90211

Superplastic deformation of mechanically alloyed aluminium IN90211 was studied by texture analysis. The textures in three deformed specimens were investigated as a function of strain using the three-dimensional crystal orientation distribution functions (CODFs). The results for the two superplastically deformed specimens (425 °C, strain rate of 1 s^–1, stress near 50 MPa, and 475 °C, initial strain rate of 77 s^–1, about 110 MPa) indicate that at strains below about 2.0, the specimen deforms by grain-boundary sliding and single (or double) slip, and at larger strains the deformation is dominated by grain-boundary sliding, multiple slip and some recrystallization. At 475°C, 330s^–1, and stress near 160 MPa, the specimen was above the superplastic regime, and the resulting texture changes with deformation were markedly different from superplastic results, and quite unusual.     

Particle characterization and cavity nucleation during superplastic deformation in Al-Li-based alloys

The role of intermetallic particles on the cavity formation during superplastic deformation has been studied in two aluminium-lithium-based alloys of identical chemical composition. They were, however, manufactured by two different routes — one by ingot metallurgy (IM) and the other by rapid solidification powder metallurgy (PM). A large number of particles of different shapes and sizes were found in both the alloys. In the IM alloy, particles were aligned in stringers in the direction of rolling. Iron and silicon, which were present as impurity elements, formed intermetallic phases in the IM alloy whereas only silicon-rich particles were found in the PM alloy. The particles of the PM alloy were of smaller size and were rather uniformly distributed in comparison to the IM alloy. During superplastic deformation, cavities first nucleated at the interface of large particles. The cavities of the IM alloy formed around the aligned stringers of large particles, whereas in the PM alloy they were uniformly distributed. It is shown that because both alloys contained particles of varying sizes, cavity nucleation occurred continuously during superplastic deformation.     

On the effects of second phase distribution on the fracture behaviour of two superplastic aluminium alloys

The significant scatter observed in the elongation to fracture of specimens taken from a single, industrially processed sheet for two Al alloys has been traced to changes in the value of the strain rate sensitivity index (m) and also to the specimen to specimen variation in the concentration and distribution of second phase particles/inclusions. It is shown that these three variables of m, second phase particle content and distribution affect the width of tear ridges, the size of clusters of grains that pull out as a whole during fracture, their connectivity and the extent to which cavitation can be suffered before final fracture. The consequences of these effects for specimen ductility are discussed.     

Texture development in high strength aluminium alloys

Abstract

Texture development during the thermomechanical processing of high strength aluminium alloys is reviewed. The alloys dealt with include both conventional heat treatable alloys, and unconventional materials such as rapidly quenched alloys and metal-matrix composites. The processing routes considered include hot and cold rolling, extrusion, forging, recrystallisation, and superplastic deformation. The information is presented as (111) pole figures and orientation distribution functions, in order to illustrate the much greater degree of detailed information that can be extracted from the latter method of analysis. The implications of texture development are considered by examining the effects that texture can have on tensile property anisotropy and fatigue and fracture behaviour.

MST/1292     

Microstructure and thermal stabilityof superplastic aluminium—lithium alloy after severe plastic deformation

Structure and phase composition of 1420 (Al-Mg-Li-Zr) alloy obtained by equal channel angular pressing during mechanical tensile testing and annealing within the temperature range 423–723 K were studied by means of TEM and XRDA. It was revealed that structural changes of the alloy after the mechanical tests corresponded to two stages of the deformation. The alloy structure was unstable due to its non-equilibrium nature and came to the equilibrium state during the annealing. Changes in the grain size and phase composition during the alloy annealing resulted in a significant decrease of its hardness.     

Workability,hardness, and strength of aluminium alloys

Abstract

Binary alloys of Al with elements of the first long period, from Ca to Zn inclusive, and also with Li, Mg, Si, Ag, Y, La, and Gd have been examined regarding their working response and tensile properties after forging and cold rolling of arc melted specimens. The results are correlated with the solid solubilities of the alloying elements and their positions in the periodic table.

MST/3059     

Cyclic deformation of aluminium alloys after the preliminary combined loading

This work explores sensitivity aluminium alloys under the influence of preliminary combined loading. There were examined 2024-T351 and D16ChATV aluminium alloys. The research was carried out under the conditions of tensile static and constant-amplitude fatigue tests on samples made of virgin material and after pre-combined loading. As a result of the research, it was concluded that the pre-combined loading leads to the modification of the mechanical properties of the tested materials. During the study reached similar results in increased strength for two aluminium alloys after the pre-combined loading, despite some differences in the structure of the output.     

Effect of inverted pressurisation profile on deformation characteristics of 5083 aluminium alloy during superplastic forming

Abstract

Decreasing the cycle time for superplastic forming of a commercially available superplastic 5083 aluminium alloy has been studied in the present work by use of an inverted pressurisation profile. A right cylindrical cup with a depth/ diameter ratio of 0·5 could be superplastically gas pressure formed in less than 100 s. The deformation behaviour was similar to that of constant strain forming during the free bulging stage. In this stage, a stress state gradient from the pole to the edge of the formed dome was observed. Plasticity controlled growth of cavities was thought to be the mechanism for the increase of cavity volume fraction during forming. After the centre point of the deformed sheet touched the die surface, the metal flow pattern was found to be different from that of the traditional approach. The minimum thickness was not located at around the bottom corner of the cylindrical cup rather it was located ~ 7.5 mm away from the bottom centre of the cup with radius 20 mm. Significant cavity nucleation and coalescence caused higher cavity growth rates at large strains, owing to the continuous increase in strain rate resulting from the imposed pressurisation profile.