Ageing response and mechanical properties of a SiCp/Al-Li (8090) composite

The ageing kinetics of a silicon carbide particle-reinforced Al-Li (8090) matrix composite and unreinforced alloy, both made by spray forming, were investigated. Ageing treatments, without any straining after solutionizing, and with a 2% plastic strain after solutionizing, were employed. The peak ageing times of the matrix in the composite was shorter than that of the unreinforced alloy. The enhanced hardening rate of the matrix in the composite was attributed to the higher dislocation density induced as a result of the plastic deformation occurring at the particle/matrix interface. This plastic deformation is a result of the large difference in the coefficient of thermal expansion between the particles and matrix. Subjecting the samples to a 2% plastic strain reduced the peak ageing times even further. The tensile strength of the composite samples was marginally higher than that of the unreinforced alloy. Samples subjected to 2% plastic straining prior to ageing also exhibited higher strength values. The strain to failure of all the samples did not recover in the over-aged state.     

8090Al—Li合金蠕变断裂行为的研究

本文研究了8090型Al—Li合金在190～290℃以及30～200MPa的应力条件下的蠕变断裂行为和蠕变过程中的显微组织变化。实验结果表明,蠕变断裂行为与蠕变过程中发生的沉淀相粗化、平衡相在晶界的析出以及晶界无析出带的宽化等有关。和T_o处理的试样相比,T_8处理试样具有较高的蠕变断裂寿命。这是因为经T_8处理的试样中析出大量S＇相分散了共面滑移,同时能稳定亚晶界阻止动态回复过程因而提高了蠕变断裂阻力。     

Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

The shear strength () of overlap shear test pieces made by solid state diffusion bonding or by machining thin (2.5 or 4 mm thick) Al-Li 8090 alloy sheet has been determined for various overlap lengths (/). When / < 3 mm, was independent of / and equal to 188 to 202 MPa for the bonded joint and 199 to 209 MPa for the base metal sheet. The lower mean shear strength of the bonded joint was caused by the lower resistance of intergranular fracture in the planar grain boundary at the bond interface. The bond strengths were, however, greater than those previously reported for joints in 8090 alloy made by solid-state or liquid-phase diffusion bonding and about a factor of 7 greater than those for adhesive bonded joints.     

Microstructural and mechanical characterization of laser-beam welding of a 8090 Al-Li thin sheet

In extension to a previous study on electron-beam welding (EBW) under vacuum on a 8090 thin sheet, the current paper reports the parallel results of laser-beam welding (LBW) of the same material. Autogenous bead-on-plate laser-beam welding was performed by a 3 kW CO₂ LBW machine. The power of the input laser beam, the specimen moving speed, and the focusing condition was varied from 700 to 1300 W, 1500 to 9000 mm min^–1 and 1 to 3 mm below the specimen top surface, respectively. The protection atmosphere and plasma jet were achieved by blowing either Ar or N₂ gas. The effects of using different gases were evaluated in terms of weld-line appearance, fusion-zone dimension, solute evaporation, microhardness, post-weld tensile properties, as well as porosity distribution. In comparing with the EBW results, LBW on the 8090 alloy was characterized with a higher fusion-zone depth/width ratio, cooling rate and porosity amount, and a lower solute loss and post-weld tensile strain. The primary formation mechanism for porosity was thought to be related to the collapsed key-holes during LBW under Ar or N₂ and the hydride-induced gas pores during EBW under vacuum.     

Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

Al-Li 8090 alloy overlap shear test pieces machined from 3 mm thick diffusion-bonded sheets showed two fracture zones at the bond interface. Zone 1 at the ends of the overlap showed predominantly intergranular fracture and zone 2 at the centre of the overlap showed peel-type fracture. The load appeared to be carried entirely by zone 1. Only zone 1 fracture was obtained in the base metal test piece. The fracture zones were caused by the non-planar stress distribution and by the bending moments associated with this type of test piece. The planar bond interface may accentuate the tendency in these alloys towards low ductility and toughness in the short transverse direction.     

Strength and fracture behaviour of diffusion bonded joints in Al-Li (8090) alloy

Peel strengths at room temperature and under superplastic forming conditions at 530 °C were measured for diffusion-bonded joints in Al-Li 8090 alloy sheet. The bonds were made in the solid state, or via a transient liquid phase using interlayers. The effect of strain rate, sheet thickness and heat treatment were investigated. The significance of these results for the testing of DB joints and for their use in DB/SPF structures is discussed.     

Critical strength criteria for DB/SPF processing of Al-Li 8090 alloy

Peel strengths are reported for solid state or transient liquid-phase diffusion-bonded (DB) joints between aluminium-lithium 8090 alloy sheets. The joints were tested under superplastic forming (SPF) conditions at 530 °C and with a progressively increasing peel angle,, in the range 0°–60°. The sheet deformed superplastically with or without peel fracture of the joints. A deformation model is proposed which predicts a critical combination of peel strength and superplastic flow stress for DB/SPF processing of the 8090 alloy and indicates peel fracture will occur when sheet thicknesses exceed 2 and 0.8 mm in solid state and transient liquid-phase diffusion-bonded joints, respectively.     

Superplasticity in an alulllinium alloy 2124/SiCp composite

Abstract

The superplastic potential of an aluminium alloy 2124/SiC_p composite, fabricated by powder metallurgy techniques, has been investigated. Instead of any special thermomechanical processing or hot extrusion, simple warm rolling has been employed to obtain a fine grained structure before superplastic testing. Constant strain rate tests were performed to characterise the superplastic behaviour of the composite. All tests were performed in air at temperatures of 743–783 K and in the strain rate range 10^-3-10^-1 S^-l. A maximum elongation of 425% was achieved at a temperature of 763 K and a strain rate of 8.3 × 10^-2 S^-1. The highest value obtained for the strain rate sensitivity index (m) was 0.41. Differential scanning calorimetry was used to ascertain the possibility of any partial melting in the vicinity of optimum superplastic temperature. These results suggested that no liquid phase existed where maximum elongation was achieved and deformation took place entirely in the solid state. Optical and electron microscopy were used to examine the materials microstructure before and after superplastic testing.     

Dynamic mechanical response of a 1060 Al/Al2O3 composite

The flow stress of a 1060 Al/Al₂O₃ composite increases rapidly with strain rate due to the higher dislocation accumulation rate and the increasing strength of dislocation barriers. The Al/Al₂O₃ interfaces were found to be well bonded even after high-rate deformation of the composite. MgAl₂O₄ particles observed at Al/Al₂O₃ interfaces in the composite of the present study are thought to improve the interface strength. Unlike in pure aluminium, a well-developed cell structure was not observed in the deformed 1060 Al/Al₂O₃ composite. The absence of a well-developed cell structure is thought to result from a more homogeneous slip distribution in the composite.     

Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

Microstructure of diffusion-bonded joints in Al-Li 8090 alloy

Joints were produced between Al-Li 8090 alloy sheet by solid state (SSDB) and transient liquid-phase (TLPDB) diffusion-bonding techniques. The bond interface was a planar, thermally stable, large-angle grain boundary in the SSDB joint. Non-planar grain boundaries in a band of coarse grains were present in the TLPDB joint. The origin of these microstructures and the measured shear strengths of the joints relative to that of the parent sheet are discussed.     

Microstructural and mechanical characterisation of premixed spray formed A357/SiCp composite

Abstract

A spray formed aluminium alloy/SiC_p composite, obtained by an alternative production route, was produced by mixing suitable amounts of A357.0 alloy and SiC_p particles in the melting crucible of a laboratory scale gas atomiser, under a protective atmosphere. A mean volume fraction of reinforcing particles close to 15 vol.-% was achieved by properly choosing the melt superheat temperature and stirring speed. Microstructural characterisation of both the precomposite powders and of the spray deposited block was carried out by microscopy examination and X-ray diffraction. The variations of the volume fraction of reinforcement within the formed composite block were determined as a function of position and correlated to the corresponding local mechanical properties. The mechanical properties of the spray formed composite material are compared with those of the unreinforced alloy, spray formed in the same atomiser.     

Microstructure Evolution During High-Temperature Deformation of 8090 Al-Li Alloy

The microstructure evolution of an as-processed 8090 Al-Li alloy during high temperature deformation has been investigated with emphasis on the dynamic grain size refinement and the formation of high-angle grain boundaries. Tensile tests were conducted at temperatures 470-560°C and initial strain rates of 10^-2-10^-6 s^-1. The starting and deformed samples were characterized using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Electron Backscattered Diffraction (EBSD). The material showed a maximum elongation to failure of 660% at 530°C and strain rate of 10^-3 s^-1. A microstructural transformation from coarse grains to uniform fine microstructure through dynamic recrystallization (DRX) was observed. The DRX process was characterized by (1) a bimodal microstructure, (2) a gradual increase in average boundary misorientation angles, and (3) a gradual decrease of microtexture. The development of high angle boundaries was attributed to the absorption of dislocations into subboundaries and the grain boundary sliding (GBS)-induced subgrain rotation. The microstructural evolution was suggested to be responsible for the superplastic behavior observed in this as-processed material.     

Fatigue Behaviour of the Al-Li alloy 8090 compared to 2024

The high cycle fatigue and the fatigue crack propagation behaviour of the new Al-Li alloy 8090 were evaluated on 25 mm plate material and compared to the conventional high strength Al alloy 2024. The investigation covered changes of test direction, R-ratio and environment. The results revealed that for most conditions the Al-Li alloy proved to be equivalent or better than the conventional Al alloy.     

Study of dynamic and mechanical behaviour of SiCp/Al composite under tensile impact

In this paper, using our own design of bar-bar tensile impact apparatus with a high speed rotating disk, we have performed an experimental study of Al alloys with SiC particles (V ₀=10%) in the strain-rate range from 100 s^–1 to 1000 s^–1. The complete stress-strain curves of the composite under tensile impact was obtained. On the basis of our experimental results, we studied the initial dynamic behaviour of the composite and derived a one-dimensional macro-constitutive equation for the composite under tensile impact by using the elastic-viscoplastic constitutive theory.Supported by the National Natural Science Foundation of China     

High-strain-rate superplasticity of SiC p /8090 aluminum composite

Superplastic tensile tests of a 17 vol.% SiC _p /8090 Al-Li composite were carried out at strain rates ranging from 7.25 × 10^-4 s^-1 to 3.46 × 10^-1 s^-1 and at temperatures from 773 K to 873 K. A maximum elongation of 300% was obtained at a strain rate of 1.83 × 10^-1 s^-1 when tested at a temperature of 848 K which was slightly above the solidus temperature of the composite. The effect of a small fraction of liquid phase on high-strain-rate superplasticity was discussed. Finally, the activation energy of high-strain-rate superplastic deformation was calculated and high-strain-rate superplastic mechanism was discussed.     

Fracture toughness and fatigue crack propagation behavior of 8090 Al-Li alloy

We present results of fatigue tests of high-strength 8090 Al-Li alloy and data on its fatigue crack growth resistance. High strength combined with fairly high crack growth resistance and endurance limit results in much better service characteristics compared to other high-strength aluminum alloys. We discuss results of tensile and impact tests of Charpy specimens and the critical values of theJ-integral andK _1c for 10-mm-thick specimens in the T-L and L-T orientations subjected to complete and partial aging. The experimental results are compared with published data for 8090 and other high-strength aluminum alloys. We suggest a numerical method for the evaluation of fatigue strength according toda/dN-K diagrams.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 45–58, January – February, 1995.