Tensile creep behaviour of NiAl-Cr(Zr) multiphase intermetallic alloy

Abstract

The microstructure of a multiphase NiAl-33.5Cr-0.5Zr intermetallic alloy was examined by SEM with energy dispersive spectroscopy and TEM. The tensile creep behaviour of the hot isostatically pressed NiAl-33.5Cr-0.5Zr alloy was studied. The results of the creep test indicated that all of the creep curves under the present test have similar characteristics: a short primary creep stage, a dominant tertiary creep stage, and nearly identical creep strains (~45%). The apparent stress exponent and the apparent activation energy were analysed and discussed. The mechanism of the creep deformation was also analysed by the observation of TEM.     

Fatigue crack growth in some PM low alloy steels consolidated by rotary forging and sintering

Abstract

Fatigue crack propagation rates under plane strain conditions have been investigated for three PM low alloy steels consolidated to high densities by rotary compaction followed by sintering and heat treatment. It is shown that the densities and properties are intermediate between those of pressed and sintered materials and of powder forged materials. Threshold stress intensities compare satisfactorily with those for wrought counterparts, but resistances to crack growth are inferior to those of wrought steels. Possible reasons for the properties of the rotary compacted materials are considered in the light of their microstructures and the behaviour of other PM materials.     

The Coupled FEM Analysis of the Transient Temperature Field During Inertia Friction Welding of GH4169 Alloy

The inertia friction welding process is a non-linear process because of the interaction between the temperature field and the material properties as well as the friction force. A thermo-mechanical coupled finite element model is established to simulate the temperature field of this process. The transient temperature distribution during the inertia friction welding process of two similar workpieces of GH4169 alloy is calculated. The region of the circular cross-section of the workpiece is divided into a number of four-nodded isoparametric elements. In this model, the temperature dependent thermal properties, time dependent heat inputs, contact condition of welding interface, and deformation of the flash were considered. At the same time, the convection and radiation heat losses at the surface of the workpieces were also considered. A temperature data acquisition system was developed. The temperature at some position near the welding interface was measured using this system. The calculated temperature agrees well with the experimental data. The deformation of the flash and the factor affecting the temperature distribution at the welding interface are also discussed.     

Study of microstructural evolution of friction taper plug welded joints of C–Mn steels

Abstract

This paper describes the microstructural evolution of friction taper plug welded joints of C–Mn steels. Experimental and numerical analyses included calculations based on Calphad and continuous cooling transformation curves, and characterisation techniques. The studied friction taper plug welded joint contains three macroregions: plug material, thermomechanically affected zone (TMAZ) and base material. The thermomechanical conditions imposed in the studied friction taper plug welded joint precluded the formation of a heat affected zone. However, seven subregions were identified within the TMAZ region and details are discussed. The interface zone is found in the TMAZ region, where the most relevant phase transformations take place. It is suggested that the phase transformations in TMAZ region depend on local conditions, such as chemical composition, deformation rate, thermal history and the previous thermomechanical history of the parent materials.     

Microstructural analysis of stir zone of Al alloy produced by friction stir spot welding

Abstract

Friction stir spot welding (FSSW), which was recently developed as a spot joining technique, has been applied to a lap joint of Al alloy 6061 sheets, 1 mm in thickness, to clarify its microstructural features of the joint. A nugget shaped stir zone having finer grain size was observed around the exit hole of the probe. Crystallographic texture analyses using EBSD method suggested that the material flow occurred along the rotating direction of the FSSW tool in the wide region including the stir zone. In the periphery of the nugget shaped stir zone, which was characterised by finer grain size than the stir zone interior, no inclusions or precipitates were found on the SEM scale. A softened region was formed around the joint centre, which could be explained as resulting from dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSSW.     

Kinematic modeling of wheeled mobile robots with slip

This work presents a kinematic modeling method for wheeled mobile robots with slip based on physical principles. First, we present the kinematic modeling of a mobile robot with no-slip considering four types of wheels: fixed, centered orientable, off-centered orientable (castor) and Swedish (also called Mecanum, Ilon or universal). Then, the dynamics of a wheeled mobile robot based on Lagrange formulation are derived and discussed. Next, a quasi-static motion is considered to obtain the kinematic conditions that provide the slip modeling equations. Several types of traction models for the slip between the wheel and the floor are indicated. In particular, for a frictional force linearly dependent on the sliding velocity, the no-slip kinematic equation of the wheeled mobile robot is related, through the weighted least-squares algorithm, with the slip modeling equations. To illustrate the applications of the proposed approach a tricycle vehicle is considered in a real situation. The experimental results obtained for the slip kinematic model are compared with the ones obtained for the well-known Kalman filter.     

Friction stir welding of aluminium 7136-T76511 extrusions

Abstract

This research programme evaluates the as welded properties of Al 7136-T76511 extrusions joined through friction stir welding (FSW). Microstructural characterisation and mechanical testing were performed on the baseline material and on panels friction stir welded at 250 and 350 rev min^–1 (all other weld parameters held constant). Transmission electron microscopy revealed the microstructural features in each of the unique weld regions and demonstrated that the precipitate density and morphology in these regions correlates with the temperature profile produced by the FSW process. A thermal model of FSW is developed that utilises an energy based scaling factor to account for tool slip. The slip factor is derived from an empirical relationship between the ratio of the maximum welding temperature to the solidus temperature and energy per unit length of weld. The thermal model successfully predicts the maximum welding temperatures and profiles over a range of energy levels. The mechanical behaviour after welding is correlated to the temperature distribution predicted by the model and to the observed microstructural characteristics. As welded mechanical properties of the alloy trended positively with the energy per unit length of weld, i.e. the highest joint efficiency was achieved at the highest welding temperature.     

Preliminary studies on friction welding of sintered P/M steel preforms to wrought mild steel

Abstract

Continuous drive friction welding studies on sintered powder metallurgical (P/M) steel preforms–wrought mild steel combination are reported in the present study. The work is a preliminary study to optimise the friction welding parameters and data generated by the present work is expected to contribute to friction welding of dissimilar and similar sintered P/M preforms to wrought metals or sintered P/M preforms – a planned future research work. Sound welds were obtained with all welding parameter combinations studied. The mechanical properties of welds were comparable to those of sintered P/M steel. Sintered P/M preforms deformed to a greater extent than wrought mild steel due to their low flow stress and thermal conductivity. The sintered density and other properties of the P/M preforms were found to dictate the deformation at the interface and consequently the weld strength. The results indicate that the current approach can be extended to other combinations of sintered P/M preforms.     

Peak force as function of the embedded length in pull-out and microbond tests: effect of specimen geometry

We have derived the equations which explicitly express the peak force, F _max, and the apparent interfacial shear strength, τ _app, measured in the pull-out and microbond tests, as functions of the embedded length. Three types of test geometries were considered: (1) a fiber embedded in a cylindrical block of the matrix material; (2) microbond test with spherical matrix droplets; and (3) pull-out test in which the matrix droplet had the shape of a hemisphere. Our equations include the local interfacial shear strength (IFSS), τ _d, and the frictional interfacial stress, τ _f, as parameters; the effect of specimen geometry appeared in the form of dependency of the effective fiber volume fraction on the embedded length. The values of τ _d and τ _f were determined by fitting our theoretical curves to experimental F _max (l _e) plots by using the least squares method. Our analysis showed how the local IFSS and the frictional interfacial stress affected the measured F _max and τ _app values. In particular, it was revealed that intervals of embedded lengths could exist in which frictional interfacial stress had no effect on F _max and τ _app, even if the τ _f value was high. We also derived an equation relating the scatter in the interfacial strength parameters (τ _d and τ _f) to the scatter in τ _app, which is experimentally measurable, and proposed a procedure to determine the standard deviations of τ _d and τ _f from experimental pull-out and/or microbond test data.     

Effect of annealing on the cohesion,adhesion, and tribological behavior of amorphous silicon-containing diamond-like carbon (Si-DLC) coatings on steel

Amorphous silicon-containing diamond-like carbon (Si-DLC) coatings were deposited by Ar+ ion beam-assisted physical vapor deposition of tetraphenyl-tetramethyl-trisiloxane (704 Dow Corning diffusion pump oil) on AISI 4340 low alloy and 440° C high alloy steel specimens, as well as on thin wafers of the same compositions, in order to evaluate residual stresses within the coatings. During annealing in an argon atmosphere at 200°C for up to 30 min, the residual compressive stress, attributed to hydrogen entrapment during deposition, gradually changed to tensile due to loss of hydrogen, and the rate of stress increase decreased with increasing annealing time. The cohesion and adhesion failure loads of the coatings decreased with annealing time, as did the friction coefficient between the coating and a diamond stylus. The specific wear rate, measured by pin-on-disk tribometry, increased with annealing time. These properties are affected not only by the change in residual stress state during annealing, but most likely also by devitrification and the accompanying grain growth. If these effects are neglected, then the properties may be correlated directly with residual stresses in the coating.