Papers by Keyword: Thermo-Mechanical Simulation

Abstract: Friction stir spot welding process is a solid state joining process which has attracted great attention due to its ability to join low melting point light weight alloys such as aluminium and magnesium with high efficiency. In order to understand the complex thermo-mechanical joining process involved with friction stir spot welding, a numerical simulation study was done using ABAQUS finite element software. The simulation primarily aims to interpret the effect of a set of process parameters and tool geometry on the workpiece plates. Johnson-Cook damage criteria model was used to obtain the stress and strain distribution on the workpiece consisting of aluminium 6061 and magnesium AZ-31B placed in a lap configuration. Temperature distribution of the workpiece was obtained by simulating a penalty based frictional contact between the tool and the plate. The thermal results showed that the maximum temperatures attained were significantly lower than the melting points of the base materials indicating that the material mixing and joining occurred as a result of superplastic deformation process instead of melting. Change in material flow behaviour was also observed by the model as pin and shoulder geometries changed.

Abstract: Microstructure evolution and tensile properties were studied in a bainitic pipeline steel grade by performing a number of physical simulations on samples machined out of an industrially produced transfer bar. In these simulations, the cooling interval between roughing and finishing stages (t_V) was varied from 5 s to 180 s. The austenite status after this cooling interval, regarding the prior austenite grain size and precipitates, simulates the condition of austenite before entering the finishing mill. The finishing parameters and the subsequent cooling strategy were kept unchanged throughout all the applied simulation processes. The gradual increase in t_V resulted in a gradual increase of the granular bainite phase on the expense of the aciculare ferrite. This resulted in an incremental decrease in ultimate tensile strength and yield strength with increasing t_V. However, this behavior approached a steady state condition after which the t_V has limited/insignificant effect on the ultimate-and yield strength. This saturating value of t_V is process parameter dependent.

Abstract: The packaging of liquid products is conventionally realized by using two production stages, which are the stretch blow molding and the filling. In the stretch blow molding process, hot polyethylene terephthalate (PET) preforms are inflated by pressurized air into a cavity to form plastic bottles. In a follow-up process, these packages are filled by a separate machine with the desired liquid product. In contrast to that, liquid-forming combines the blowing and filling stages by directly using the liquid product to form a plastic bottle. Through this substitution, two main challenges arise. Firstly, there are significant inertia effects through the liquid mass, leading to additional reaction forces and a spatially inhomogeneous pressure distribution inside the preform. Secondly, the heat transfer between preform and fluid is drastically increased. Because of this cooling effect, a specific combination of forming speed as well as initial preform and liquid temperatures is necessary to avoid thermally induced preform rupture. This is based on the fact that the formability of PET rapidly declines below its glass transition temperature (T_g). Consequently, a process control requires the knowledge of how the process parameters influence the preform cooling. In this paper, a numerical simulation of the liquid-forming process (LF) is introduced including the preform cooling during forming. In addition, the strain-dependent self-heating effect of PET is implemented. Process experiments under different parameter combinations are conducted using simplified bottle geometry. Through a comparison of the results from experiments and from simulation, the influence of process parameters on the temperature drop and thus on thermally induced failure is determined. In this way, process understanding and control are increased.

Abstract: The reheat cracking, also known as stress relief cracking, has occurred many times in the welding coarse grained heat affected zones (CGHAZ) of Vanadium-modified (V-mod) 2.25Cr1Mo steel, but seldom researches on this problem have been conducted until now. In this paper, reheat cracking in welding CGHAZ of V-mod 2.25Cr1Mo steel was studied by the thermo-mechanical simulation methods. A screening test was carried out for simulating similar material to the real CGHAZ. The high temperature ductility of simulated CGHAZ was measured in the range of 600~705°C for determining the most sensitive temperature to reheat cracking. And then, the reheat cracking phenomena at the sensitive temperature was reproduced by Isothermal stress relaxation test and the micro-morphology of cracks was observed and analyzed. The results show that the thermal simulation parameters for acquiring the most similar materials to real CGHAZ are heat input of 35kJ/cm, heating rate of 1000°C/s and holding time at 1320°C of 1s. The reduction of area (RoA) of the simulated CGHAZ specimen first decreases and then increases with increasing temperature. It exhibits a minimum value at 675°C, corresponding to the most sensitive temperature to reheat cracking. In the stress relaxation test, the sample failure typically occurs with very low stress relaxation proportion (less than 15%) and extremely poor ductility (less than 5% in RoA). Both microvoid coalescence along the PAGB and intergranular wedge type cracks were observed in the fractured sample, proposing a mixed cracking mode of W-type and R-type in stress relaxation.

Abstract: In the present paper, thermo-mechanical simulation of ultra-high strength ferrite-bainite dual phase (DP) steel was performed using a thermomechanical simulator. Continuous cooling transformation (CCT) diagram was constructed for DP steel. The effects of composition and cooling rate on the kinetics and products of phase transformation and the form of the CCT diagram were investigated. The results have shown that the α→γ transformation in DP steel was found to be more sluggish due to the addition of alloying elements. The segregation of manganese and niobium at austenite grain boundaries is expected to cause a solute drag effect, thereby reducing the rate of γ→α transformation in DP steel. The pearlite transformation region disappeared for cooling rates from 0.1 to 20°C/s. The microstructure comprises of bainite and martenite was obtained at fast cooling rate. The present steel is expected to have a higher hardenability.

Abstract: An investigation concerning laser beam – GMA – hybrid welding of high strength steels has been completed for a crane plant. Materials, welding procedure qualification tests and the course of action during welding a demonstrator were studied [1].

Abstract: A novel microalloy Q&P steel with Nb and Mo is designed. The dilatometric curves of low-carton microalloyed Q&P steel with and without Nb and Mo are detected at different cooling rates on DIL-805 thermal dilatometer．The CCT curves are determined using thermal dilation measurement, microstructure observation and hardness measurement. In the Q&P process, the niobium carbide precipitate plays a precipitation strengthening effect, so the tensile strength of Q&P steel is enhanced. The results show that: The element Nb decreases the phase-transformation temperatures of ferrite and pearlite, which leads the proeutectoid ferrite and pearlite phase transformation finish lines to move to lower-right. For the low-carton Q&P steel microalloyed with Nb and Mo, when cooling rate is higher than 5°C•s^－1, the microstructures is only martensite. Its excellent hardenability is propitious to improve the strength of the Q&P steel. While, for the traditional C-Si-Mn Q&P steel, the martensite can be only obtained until the cooling rate higher than 20°C•s^－1.

Abstract: According to the process feature of the coke dry quenching (CDQ), the factors of the destructive mechanism that influence the sequence function properties of the brick were analyzed. The countermeasures have been adopted for the defects of the bricks for CDQ, such as inferior thermal shock stability and short lifetime, etc. In order to search the effect of thermal expansion stress and mechanical load stress on the CDQ, the temperature field and stress field of the flue in the CDQ were analyzed according to the thermal elastic and plastic theory. The Drucker-Prager plasticity model combined with a tension cut-off criterion was described material behavior. All material properties were taken as temperature dependent. The result indicates that the support bracket under high gradient temperature will bring on the highest thermal stress, which is the main reason of fracture of the support bracket. The cycling temperature has the ability to cause repeated crack propagation throughout the whole service period. To solve this problem, the design of the structure, the method of the heat exchange and the properties of the materials will be improved. The obtained results give a good insight into the reasons of material failure and help to find counter-measures for prolonging the lifetime of CDQ.

Abstract: Titanium alloys are usually used as structural parts in forged and subsequent heat treated state. Up to now, there have been rare researches appeared on their shaping or forming by multi-stand tandem hot rolling. In this article, we choose Ti-6Al-4V as experimental material and a simulated research has been carried out in a thermo-mechanical simulator MMS-300 to model its hot-rolling process. The plastic flow behavior for Ti-6Al-4V alloy during hot-working has been determined at various deformation temperatures and deformation rates by considering its hot-rolling in single or two phase region. The deformation mechanisms at elevated temperatures are also analyzed in correlation with its deformation activation energy. This research can provide guidance in designing processing parameters for hot-rolling of Ti-6Al-4V alloy.

Abstract: Non-homogenous cooling rates and solidification conditions during DC-casting of high strength aluminum alloys result in the formation and accumulation of residual thermal stresses with different signs and magnitudes in different locations of the billet. Rapid propagation of micro-cracks in the presence of thermal stresses can lead to catastrophic failure in the solid state, which is called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Simulation of residual thermal stresses of an AA7050 alloy during DC-casting by means of ALSIM5 showed that in the steady-state conditions large compressive stresses formed near the surface of the billet in the circumferential direction. Stresses changed sign on moving towards the centre of the billet and became tensile with high magnitudes in radial and transverse directions, which made the alloy prone to hot and cold cracking.