Interfacial microstructure and mechanical behaviour in laser clad TiCp/Ni alloy coatings

Abstract

Coatings of TiC_p reinforced composite have been produced by laser cladding. Two kinds of coating with different TiC_p origins were investigated, i.e. undissolved TiC_p and in situ TiC_p. For undissolved TiC_p, epitaxial growth of TiC, precipitation of CrB, and a chemical reaction occur at phase interfaces, and nanoindentation loading curves show pop in marks caused by the plastic deformation associated with crack formation or debonding of TiC_p from the matrix. As for in situ TiC_p, no pop in mark appears. Meanwhile, in situ TiC_p produces hardness and elastic modulus values that are higher than those produced by the coating that contains undissolved TiC_p.     

Correlation between displacement rate and shear force in shear test of Sn–Pb and lead free solder joints

Abstract

An experimental investigation was combined with a non-linear finite element analysis using an elastic–viscoplastic constitutive model to study the effect of ball shear speed on the shear forces of ball grid array (BGA) solder joints. Two solder compositions were examined in the present work: Sn–37Pb and Sn–3·5Ag. Within the Sn–3·5Ag solder, Ag₃Sn intermetallic compound particles were found. For both types of solder used, a Ni₃Sn₄ intermetallic compound layer was observed at the interface between the nickel plated layer and solder, while a gold layer appeared to have dissolved into the liquid solder leaving no observable gold at the interface. The shear force was observed to increase linearly with increasing shear speed and reached a maximum value at the highest shear speed in both the experimental and the computational results. All test specimens fractured in a ductile mode. The failure mechanisms are discussed in terms of von Mises stresses and plastic strain energy density distributions.     

Numerical identification of material model for C–Mn steel using micro-indentation test

Abstract

Identification of the elastoplastic material model for C–Mn steel, using finite element model of micro-indentation test developed by the authors and proposed algorithm of inverse analysis, is one of the objectives of the project. The micro-indentation experiment is widely described in the present paper, especially those parts, which are meaningful in getting input data for direct, further application in the numerical model of micro-indentation test and in the inverse procedure. Finite element solution connected with the inverse algorithm, which is based on the simplex method, is used to search for the unknown parameters of material model. Validation of the developed inverse algorithm is the particular objective of the present work. The present paper shows that material model determined using the inverse analysis is in agreement with that obtained from the tensile test. The results coincide also with the data available in the literature.     

Deformation behaviour and new constitutive equation utilising the grain size of commercial TC6 titanium alloy

Abstract

Isothermal compression tests on a commercial TC6 titanium alloy have been conducted at deformation temperatures of about 800 – 1040°C, strain rates of 0.001 – 50 s^-1 and height reductions of 30 – 50%. The microstructural evolution is represented through the measured grain size of the prior α-phase. Meanwhile, a new constitutive equation, which includes the grain size, is established for high temperature deformation behaviour. The procedure required to formulate a constitutive equation from the experimental results is presented. The constitutive equation to model the behaviour of the TC6 titanium alloy during high temperature deformation is validated and its formulation is presented. The results show that the present equation is satisfactory for describing the behaviour of the TC6 titanium alloy during high temperature deformation. The maximum difference between the calculated and the experimental results is less than 15%.     

Thermodynamic and kinetic modelling: creep resistant materials

Abstract

The use of thermodynamic and kinetic modelling of microstructure evolution in materials exposed to high temperatures in power plants is demonstrated with two examples. Precipitate stability in martensitic 9–12%Cr steels is modelled including equilibrium phase stability, growth of Laves phase particles and coarsening of MX, M₂₃C₆ and Laves phase particles. The modelling provided new insight into the long term stability of new steels. Modelling of the detrimental precipitation of Z phase Cr(V,Nb)N is described, which points to new approaches in alloy development for higher temperatures. Predictions of interdiffusion between a MCrAlY coating and an IN738 bulk alloy by multicomponent diffusion calculations provide a highly versatile tool for life assessment of service exposed gas turbine components as well as for the development of improved coatings.     

Impression creep behaviour of tin based lead free solders

Abstract

Growing concern about the toxic effects of lead in conventional solders has prompted the development of lead free solders. Creep owing to heating in service is one of the causes of solder joint failures in electronic packages. The present study deals with the impression creep behaviour of eutectic Sn – 58Bi, Sn – 57Bi – 1˙3Zn and Sn – 38Pb alloys in the temperature range 303 – 393 K and stress range 2˙6 – 180 MPa. Power law creep with stress exponent n varying from 2 to 6˙3 is observed. All the alloys reveal a strong stress dependence of activation enthalpy with values 155, 120 and 112 kJ mol^-1 for Sn – 58Bi, Sn – 57Bi – 1˙3Zn and Sn – 38Pb, respectively, which are well above those for self-diffusion. The steady state impression velocity varies linearly with punch diameter for all three alloys. It is concluded that a mechanism such as forest intersection involving attractive junctions controls the creep flow in these alloys.     

Modelling flow stress behaviour of aluminium alloys during hot rolling

Abstract

A mathematical model is proposed to predict the flow stress behaviour of aluminium alloys under hot rolling conditions. To do so, a dislocation model for evaluating flow stress during deformation is coupled with a finite element analysis to access metal behaviour under non-isothermal and variable strain rate conditions. Then, with the aid of the proposed model, a hot strip rolling process was simulated. In order to verify modelling results, flow stress behaviour of an aluminium alloy is studied employing hot compression tests in various temperatures and strain rates and the model was examined on this material. Non-isothermal hot rolling experiments were carried out and good agreement was found between predictions and experiments.     

Residual stress and thermal cycling of planar solid oxide fuel cells

Abstract

The published literature relating to damage to planar solid oxide fuel cells caused by thermally induced stresses and thermal cycling is reviewed. This covers reported studies of thermal cycling performance and stresses induced by temperature gradients and differences in thermal expansion coefficients in typical planar SOFC configurations, namely electrolyte supported; anode supported and inert substrate supported cells. Generally good agreement is found between electrolyte residual stresses measured by X-ray diffraction or cell curvature and stresses calculated from simple thermo-elastic analysis. Finite element modelling of temperature distributions in cells and stacks in steady state operation are well advanced and capable of being extended to compute stress distributions. Failure criteria are then discussed for laminated cell structures based on critical energy release rate fracture mechanics models developed originally for coatings. However, in most cases the data required to apply the models quantitatively (such as elastic moduli of actual laminated material and fracture energies of materials and interfaces) are not available. Where data are available there are inconsistencies that require resolution. Seals are critical components in many planar solid oxide fuel cell configurations, but again there are discrepancies in experimental mechanical properties and the role of internal stresses in their fracture. In addition, there is as yet no firm evidence that thermal cycling damage involves any true materials fatigue process.     

Evaluation of effect of chromium on wear performance of high manganese steel

Abstract

The effects of elements such as Ni, Mo, Cr, Cu, and V on the wear behaviour of high manganese steels have been reported extensively. Most researchers agree on the influence of many of the elements but disagree on the effect of Cr. A study has been conducted to evaluate the effect of 1.7 and 2.3 wt-Cr on high manganese steel when subjected to various wear conditions: impact loading, abrasion, combined impact-abrasion, and combined abrasion-corrosion. The study has revealed that adding Cr to high manganese steel resulted in an increase in hardness and hardenability, and a decrease in toughness. The effect on wear resistance was found to depend on the wear conditions. Chromium alloyed high manganese steels showed superior wear resistance compared with plain Hadfield steels where corrosion, abrasion, and combined impact-abrasion conditions prevailed. Such conditions required a high surface hardness. Plain Hadfield steel showed superior wear resistance in conditions where pure impact wear is encountered. Such conditions required an increase in toughness rather than surface hardness.     

Numerical analysis of factors influencing Charpy impact properties of TMCR structural steels using fuzzy modelling

Abstract

The influence of chemical composition and thermomechanical processing parameters on the Charpy impact energy of grade 420/460 TMCR (thermomechanically controlled rolled) steels was investigated using fuzzy modelling technology. Fuzzy modelling was applied to develop generic models for the prediction of Charpy impact properties. The fuzzy models obtained, which involved chemical compositions, processing conditions, and Charpy impact energy, were used to reveal correlations between individual influence factors and Charpy impact toughness. Numeric analysis shows that low carbon equivalent value (CEV), low sulphur residual, and low processing temperature are all beneficial to impact toughness, predictions which fit with known metallurgical knowledge thus demonstrating the integrity of the model.     

Studies on process optimisation of multistage atomisation

Abstract

In order to understand more about the multistage atomisation process, process experiments and orthogonal optimisation of multistage atomisation were carried out for pure tin to make clear the effect of every main process parameter and the optimum conditions. The orthogonal experiments showed that the disk rotating speed and superheat temperature have the largest effects on the mean particle size and that spraying height affects the powder shape coefficient remarkably. The delivery tube position has a pronounced effect on the result and rate of atomisation through changing the pressure characteristics of the nozzle tip and is also an important process factor, while the effect of gas pressure on atomisation result is relatively small. The optimum process conditions for multistage atomisation of pure tin are as follows: superheat 350 K; spraying height 85 mm; gas pressure 0·8 MPa; disk rotating speed 4000 rev min^-1; and protrusion height of delivery tube 2–3 mm.     

Gaussian process modelling of austenite formation in steel

Abstract

The present paper introduces the Gaussian process model for the empirical modelling of the formation of austenite during the continuous heating of steels. A previous paper has examined the application of neural networks to this problem, but the Gaussian process model is a more general probabilistic model which avoids some of the arbitrariness of neural networks, and is somewhat more amenable to interpretation. It is demonstrated that the model leads to an improvement in the significance of the trends of the Ac₁ and Ac₃ temperatures as a function of the chemical composition and heating rate. In some cases, these predicted trends are more plausible than those obtained with the neural network analysis. Additionally, it is shown that many of the trace alloying elements present in steels are irrelevant in determining the austenite formation temperatures.     

Tensile properties of thermally exposed aluminium borate whisker reinforced 6061 aluminium alloy composite

Abstract

The effect of thermal exposure on the tensile properties of aluminium borate whisker reinforced 6061 aluminium alloy composite was studied. The interfacial reaction was investigated by TEM and the mechanical properties were studied using tensile tests. The results indicated that the interfacial reaction had an influence on the mechanical properties of the composite, so that the maxima of Young’s modulus and ultimate tensile strength of the composite after exposure at 500°C for 10 h were obtained for the optimum degree of interfacial reaction. The yield strength, however, was not only affected by the interfacial state but also by many other factors.     

Extrusion characteristics of aluminium alloy/SiCpmetal matrix composites

Abstract

Systematic extrusion studies have been carried out on aluminium alloy 2124/SiC_p metal matrix composites. Effects of various extrusion process parameters, such as die design, ram speed, extrusion ratio, reheat temperature, and lubrication, on the pressure requirement and surface quality of the as extruded circular rods have been investigated. Different volume fractions of SiC particles (10, 15, and 20 vol.-%) were used for the synthesis of metal matrix composite billets. These composites were synthesised using two different techniques, namely, stir casting and powder metallurgy. These billets were then hot extruded on a laboratory scale 500 ton vertical hydraulic press. The significance of specially designed dish shaped dies, avoiding the dead metal zone, has also been highlighted. The results indicated that the best extrusion was possible when powder metallurgical processed billets were extruded. Volume fraction analysis of ceramic reinforcement in the extruded rod (typically 2 m long) and in the extruded discard showed no appreciable backward migration of these particles during extrusion.     

Numerical simulation of quenching of large sized blocks of 718 steel used for plastic dies

Abstract

The 718 steel used for plastic dies is required to be prehardened to 29 – 35HRC. For large sized blocks, it is relatively difficult to additionally guarantee uniform hardness with the greatest hardness difference of 3HRC and without any quenching cracks after heat treatment. In this paper, the transient temperature field and microstructure transformation during the quenching process of large sized blocks of 718 steel have been simulated numerically. Based on the simulation and experimental results, an appropriate quenching process, that is, direct air cooled quenching after austenitising at higher temperatures such as 880 – 930°C, was recommended in order to reach hardness uniformity and avoid quenching cracks.     

Tensile creep behaviour of coarse grained copper foils at high homologous temperatures and low stresses

Abstract

Tensile creep behaviour of OFHC copper in the temperature range 850 to 1074°C (0·83 to 0·99T_m) under low stress (0·1 to 0·6 MPa) has been investigated in tension for 0·4 and 0·6 mm thick foils with grain size ～1 mm, in the plane of the foils. Increases in creep rate per unit stress at 0·99T_m were two orders of magnitude higher than predicted for Nabarro–Herring diffusional creep and were nearer to values expected from the operation of grain size independent Harper–Dorn creep, but the stress exponent n was closer to 2 than to the n=1 expected in this mechanism. Observations on specimen surfaces revealed some widely spaced slip bands, some small grain boundary movements and occasional cavitation on grain boundaries nearly perpendicular to the stress. Creep rates were comparable with predictions of the movement of dislocations, controlled by the rate of their generation at Bardeen–Herring sources at a spacing similar to that of the observed slip lines.     

Hot strength of creep resistant ferritic steels and relationship to creep rupture data

Abstract

Experimental data on the tensile strength of ferritic steels designed for prolonged service at elevated temperatures have been assessed as a function of many variables, including the testing temperature. The resulting model has been combined with other data on the intrinsic strength of pure ferritic iron and substitutional solute strengthening to show that there is a regime in the temperature range 780–845 K beyond which there is a rapid decline in the microstructural contribution to strength. This decline cannot be attributed to changes in microstructure, but possibly to the ability of dislocations to overcome obstacles with the help of thermal activation. There is evidence of an approximate relationship between the temperature dependence of hot tensile strength and creep rupture stress.     

Finite element analysis for inductive reheating process of continuously cast steel billets

Abstract

Inductive heating has distinctive advantages for reheating of billets in continuous casting, but the success of this process requires good understanding of the operation principles and effective control of the process variables. In the present study modelling and finite element simulation are performed for the inductive reheating of continuously cast square billets. Important performance characteristics such as temperature profile, magnetic field distribution and the power absorption have been analysed. Representative temperatures obtained from the simulation are compared with experimentally measured temperatures. The results provide better understanding of the inductive reheating process and key information for process design.     

Prediction of creep life of X10CrMoVNbN-91 (P-91) steel through short term stress relaxation test methodology

Abstract

The methodology proposed here adopts short-term stress relaxation testing (SRT) to predict creep strength/life of X10CrMoVNbN 91 (P-91) steel. SRT has been performed at three temperatures (853, 873, 898 K), and at each temperature two prestrains (0.7 and 1.2%) were applied with a hold time of 7 h. The initial data, stress versus time, were converted to stress versus inelastic strain rate by computation and compared with conventional steady state creep rate. Creep life was predicted from the SRT results using the Monkman-Grant relationship, and compared with creep data. The comparison showed the predicted life to agree extremely well with the creep-rupture life (≈30 000 h). The calculated activation energy for SRT (719 kJ mol^-1) was found to be higher than the creep (415 kJ mol^-1). Detailed SEM and TEM studies revealed that the degradation of microstructure in terms of coarsening of M₂₃ C₆, and martensitic lath grains, had taken place and was commensurable to long term creep exposure.     

Cyclic overaging heat treatment for ductility and weldability improvement of nickel based superalloys

Abstract

A new cyclic overaging heat treatment for ductility and weldability improvement of advanced nickel based superalloys has been investigated. The heat treatment consists of intermittent heating cycles during the cooling stage to check the nucleation of fine γ' in the material during the overaging. Using Rene 80 alloy as a model material, the effects of intermittent heating rate and duration were studied and optimised. Quantitative metallography was carried out to compare the microstructures produced by the proposed heat treatment with those produced by other conventional pre-weld heat treatments. Tensile tests and welding tests both showed that the proposed cyclic over-aging heat treatment is effective in improving the tensile ductility and weldability of Rene 80 alloy.