Showing papers on "Aluminium alloy published in 2016"

Precipitation Reactions in Age-Hardenable Alloys During Laser Additive Manufacturing

Abstract: We describe and study the thermal profiles experienced by various age-hardenable alloys during laser additive manufacturing (LAM), employing two different manufacturing techniques: selective laser melting and laser metal deposition. Using scanning electron microscopy and atom probe tomography, we reveal at which stages during the manufacturing process desired and undesired precipitation reactions can occur in age-hardenable alloys. Using examples from a maraging steel, a nickel-base superalloy and a scandium-containing aluminium alloy, we demonstrate that precipitation can already occur during the production of the powders used as starting material, during the deposition of material (i.e. during solidification and subsequent cooling), during the intrinsic heat treatment effected by LAM (i.e. in the heat affected zones) and, naturally, during an ageing post-heat treatment. These examples demonstrate the importance of understanding and controlling the thermal profile during the entire additive manufacturing cycle of age-hardenable materials including powder synthesis.

Fatigue crack growth behavior and mechanical properties of additively processed EN AW-7075 aluminium alloy

Abstract: Selective Laser Melting (SLM ® ), an additive manufacturing (AM) technology, allows manufacturing of geometrically complex metallic parts directly. In the SLM technology, a high energy laser beam is applied to melt a thin layer of the metallic powder according to the information provided by CAD files. This layer-wise manufacturing offers the opportunity to create complex parts for application areas e.g., aerospace and automotive industries where the lightweight design has been and still is a priority for material development in recent years. Therefore, the materials such as aluminium alloys come into focus due to their low density and high mechanical characteristics. In view of these aspects, previously unused high strength aluminium alloy EN AW-7075 powder was produced by gas atomization and processed by SLM ® as presented in this paper. Initially, specimens were produced to examine monotonic and fracture mechanical properties in different building directions. The tensile tests and the fracture examinations show an anisotropic material behaviour. The fatigue crack growth curves have the double S shape, which is typical of aluminium. Mechanical characteristics obtained from the experiments are lower in comparison to the conventionally manufactured aluminium alloy properties.

Experimental investigation of tension and compression creep-ageing behaviour of AA2050 with different initial tempers

Abstract: Creep-ageing behaviour of aluminium alloy 2050 with different initial tempers (T34, T84 and as-quenched) has been experimentally investigated under both tension and compression creep-ageing conditions, with different stress levels at 155 °C for 18 h. Corresponding strengthening phenomena have been studied by interrupted creep-ageing tests and subsequent tensile tests. Moreover, the microstructures of some selected specimens after creep-ageing tests have been examined by transmission electron microscopy (TEM) and the precipitation process has been analysed. It has been found that creep strains under tensile stresses are much larger than those under compressive stresses during the tests. A new “double primary creep feature” has been observed in both the as-quenched alloys and the pre-stretched/natural-aged (T34) alloys, in which an intermediate inverse creep stage with an increasing creep strain rate locates between the initial primary+transient steady-state creep stages and the second primary+second steady-state creep stages. While for the alloy with peak-aged initial temper (T84), typical primary and steady-state secondary creep stages are observed during tension creep-ageing tests and little creep strain occurs under compressive stresses of 150 and 175 MPa. The mechanisms for these phenomena are discussed in terms of microstructural interactions among the changing dislocations, solute-matrix bonding and precipitates, and their effects on the creep resistance of the alloy during creep-ageing tests are analysed.

Tensile behaviour of aluminium 7017 alloy at various temperatures and strain rates

Abstract: The objective of the present study is to carry out high strain rate tensile tests on 7017 aluminium alloy under different strain rates ranging from 0.01, 500, 1000 and 1500 s−1 and at temperatures of 25, 100, 200 and 300 °C. Quasi-Static tensile stress–strain curves were generated using INSTRON 8500 machine. Johnson-Cook (J-C) constitutive model was developed for 7017 aluminium alloy based on high strain rate tensile data generated from split Hopkinson tension bar (SHTB) at various temperatures. This study evidently showed an improvement in dynamic strength as the strain rate increases. The predictions of J-C model are observed to be in consistence with the experimental data for all strain rates and temperatures. The fracture surfaces of specimens tested were studied under SEM. The change in fracture mode has been observed at different strain rates. The shear mode of fracture is dominant at lower strain rates (0.01 and 500 s−1); whereas cup- and cone-like surface representing dimple structure is found at the higher strain rates (1000 and 1500 s−1). The numbers of dimples at high strain rates are more than the quasi-static and intermediate strain rates. It is also observed that the flow stress decreases with increase in temperature. The 7017 aluminium alloy demonstrates thermal softening at higher temperatures. So when the temperature is more than 200 °C at these strain rates, thermal softening is predominant mode of deformation mechanism. It is found that when the temperature increases to 200 °C, the number of dimples rises and the dimple size of 7017 aluminium alloy is larger than at lower temperatures.

Effect of impactor shapes on the low velocity impact damage of sandwich composite plate: Experimental study and modelling

Abstract: An experimental and numerical analysis of the influence of impactor shapes on the low velocity impact performance of aluminium sandwich composite plates has been carried out. The aluminium composite panels were manufactured by using two aluminium sheets and a low density polyethylene core under heat and pressure, which shows the outstanding properties of low weight, good rigidity and impact resistance. Experimental tests were performed using drop weight test machine, samples were impacted using steel conical, ogival, hemispherical and flat impactors, all 12 mm in diameter, for different initial impact energies of 29.43 J and 44.15 J and specimen thickness of 4 mm containing three different parts (0.5 + 3.0 + 0.5). A three dimensional non-linear finite element model is developed for simulating the impact behaviour of sandwich composite plate and the ABAQUS/Explicit commercial program was used. The face sheet material aluminium alloy 3003-O of the plate was modelled as isotropic with elastic–plastic characteristics. The description of the material characteristic of the attenuator was made by means of the Johnson–Cook elastic–plastic law. The material constitutive law of the Al 3003 plates has been implemented in a user-defined subroutine UMAT. The foam core was modelled as a crushable foam material. The finite element results showed a good correlation to the experimental data in terms of contact-force histories, energy histories, absorbed energy, and failure of the sandwich composite was observed between the experimental data.