Thermal Affected Zone Research Articles

Analysis of the Temperature Distribution in Friction Stir Welding Using the Finite Element Method

Welding is a fabrication process widely used in several industrial areas. The welding of metallic alloys presents some basic characteristics as the presence of a localized intensive heat input that promotes mechanical and metallurgical changes. Different from conventional welding processes, where macroscopic fusion is observed, friction welding is a solid state welding process where the joint is produced by the relative rotational and/or translational motion of two pieces under the action of compressive forces producing heat and plastic strain on the friction surfaces. Friction Stir Welding (FSW) process has received much attention for its special characteristics, like the high quality of the joints. Although there are several experimental works on the subject, numerical modeling is not well stated, as the process is very complex involving the coupling of several non-linear phenomena. In this contribution a tridimensional finite element model is presented to study the temperature distribution in plates welded by the FSW process. A weld heat source is proposed to represent the heat generated during the process. The heat source model considers several contributions present in the process as the friction between the tool and the piece and the plastic power associated to the plastic strain developed. Numerical results show that the model is in close agreement with experimental results, indicating that the model is capable of capturing the main characteristics of the process. The proposed model can be used to predict important process characteristics, like the TAZ (Thermal Affected Zone), as a function of the welding parameters.

Metallurgical modifications and residual stress in welded steel with average carbon content

Welded joints of metallic materials are subject to residual stress and metallurgical transformations, in particular in the thermally affected zone (TAZ), with the possible formation, especially in steels with a high carbon content, of brittle structures and a loss of tenacity. The loss of tenacity in the TAZ facilitates the formation of cracks and possible brittle fracturing or failure, following the progressive propagation of the cracked zone. Moreover, residual stresses may cause premature failure in structural elements as they have an influence on the conditions of collapse, for example on account of instability of the compressed members. In this work, the order of magnitude of residual stress is estimated for a specific case, identifying by means of an empirical–experimental approach, the portion of material more susceptible to the formation of cold cracks under the combined effect of residual stress and brittleness of the microstructure.

Application of radiation focusators for creation of nanoporous metal materials with high specific surface area by laser action

Nanoporous metal material with high specific surface area was created under the action of laser radiation with the use of radiation focusators. The regimes of laser action under which nanoporous layer is formed in the centre of the thermal-affected zone on the surface of the material being investigated containing the products of reaction of cuprum and zinc in the alloy with the oxidizing medium. After laser treatment samples of brass L62 were exposed to plastic deformation and high temperature impact for the estimation of mechanical strength and heat resistance of the produced nanoporous material. Main part of the surface does not have destructions after the plastic deformation in two different modes and two high-temperature annealings, exfoliation of the porous layer was not observed, that is the evidence of sufficiently high mechanical strength and high resistance of the derived nanoporous material.

Development of the thermally affected zone (TAZ) around a groundwater heat pump (GWHP) system: A sensitivity analysis

Open-loop groundwater heat pumps (GWHPs) are considered one of the most energy efficient and environmentally friendly air-conditioning systems for temperate zones. A fundamental aspect in GWHP plant design is early evaluation of the thermally affected zone (TAZ) that develops around the injection well. This is particularly important to avoid interference with previously existing groundwater uses (wells) and subsurface underground structures. Numerical modelling is useful for delineating temperature anomalies. We carry out numerical simulations and a sensitivity analysis for the subsurface parameters affecting the TAZ. Using the simulation results we obtain a relative hierarchy of significance for the parameters with respect to the final result and then apply this analysis to an actual site. The results of the analysis indicate that the hydrodynamic parameters correlated with groundwater flow such as the hydraulic conductivity and the gradient are highly important, particularly those relating to the advective heat flow component.

Finite Element Analysis of Ti-6Al-4V Alloy Crater Geometry for EDM Based on a Single Discharge

Temperature theoretical model of Ti-6Al-4Valloy during electrical discharge machining (EDM) process is established in this paper. Based on finite element thermal analysis, the temperature field of EDM Ti-6Al-4V on a single discharge is simulated with the aid of ANSYS software. The results show that there is a high temperature distribution around the spark point while the thermal-affected zone is quite small. Additionally, the profiles of discharge crater geometry have been predicted and the relation between crater shape and parameters under different current and pulse duration are also explored.

Sensitivity Analysis of Parameters for the Simulation of Thermal Transport in Aquifer: A GSHP System in Chengdu, China

Based on the theory of sensitivity analysis, the local/global sensitivity analysis of parameters for the modeling of thermal transport in aquifer under special groundwater flow and heat source conditions were carried out with the summer running of Groundwater Source Heat Pump (GSHP) system as a study case, by taking the temperature of typical observation points and thermal affected zone (TAZ) of injecting water at the end of summer as the model outputs. The results showed that the sensitivity of each modeling output was various for the different parameter in local analysis. The orders of local sensitivity coefficients of the nine parameters for two modeling outputs were almost coincident. In global sensitivity analysis, the sensitivity of the parameter being investigated was influenced by the different values of other parameters. The two modeling outputs almost had the same variation tendency of global sensitivity with different parameter combinations.

Development of electric discharge equipment for small specimen sampling

We have developed the on-site electric discharge sampling equipment that can effectively take samples such as small specimens from the surface portion of the plant components. Compared with the conventional sampling equipment, our sampling equipment can take samples that are thinner in depth and larger in area. In addition, the affection to the equipment can be held down to the minimum, and the thermally-affected zone of the material due to electric discharge is small, which is to be ignored. Therefore, our equipment is excellent in taking samples for various tests such as residual life evaluation.

Fracture analysis of collapsed heavy-duty pulley in a long-distance continuous conveyors application

In this paper we present a case study of a heavy-duty pulley that failed after 2.1 × 10 7 cycles. The pulley was installed as a take up on a continuous conveyor under a load of 350 kN transmitted by the counterweight tower. The failure was analyzed using (a) finite element analysis by Ansys, (b) field data analysis, and (c) fracture macro examination. The prior FEA analysis showed that the pulley was designed according to its field application and moreover when re-simulated, by applying higher loads, revealed high potential towards its safety factor. The field examination showed that the conveyor did not suffer any overload that would contribute to the pulley collapse. The macro failure analysis revealed stress discontinuities and fatigue symptoms such as beach marks and even chevron marks within the welding region. In the meantime the welding region did not show evidence of thermal affected zone (TAZ), which is the key to its mechanical strength. The case analysis showed that the pulley was designed in accordance with British Standard [British Standards Institution, BS 5400:1980. Steel, concrete and composite bridges: Part 10: code of practice for fatigue; 1980] but a welding failure generated unexpected stresses and weakness on the welding region which generated fatigue and the final fracture.

Thermal affected zone obtained in machining steel XC42 by high-power continuous CO 2 laser

A high-power continuous CO 2 laser (4 kW) can provide energy capable of causing melting or even, with a special treatment of the surface, vaporization of an XC42-steel sample. The laser–metal interaction causes an energetic machining mechanism, which takes place according to the assumption that the melting front precedes the laser beam, such that the laser beam interacts with a preheated surface whose temperature is near the melting point. The proposed model, obtained from the energy balance during the interaction time, concerns the case of machining with an inert gas jet and permits the calculation of the characteristic parameters of the groove according to the characteristic laser parameters (absorbed laser energy and impact diameter of the laser beam) and allows the estimation of the quantity of the energy causing the thermal affected zone (TAZ). This energy is equivalent to the heat quantity that must be injected in the heat propagation equation. In the case of a semi-infinite medium with fusion temperature at the surface, the resolution of the heat propagation equation gives access to the width of the TAZ.

Measurement equipment for investigation of the influence of viscosity of dielectric working fluids on spark erosion

In this paper, measurement equipment for investigation of electrical discharges in dielectric working fluids is presented. Special features of these discharges are determined through a small distance between electrodes of 5 to 150 /spl mu/m and the ignition voltage up to 600 V. With the measuring system, a time-co-ordinated measurement of all electrical and optical processes of the discharging process can be accomplished. For using spark discharges is a machining process analyzing the influence of the viscosity of the work liquid on the ignition conditions is of great interest. Parameters measured at the plasma channel are starting parameters for a simulation of the thermal-affected zone of electrodes surfaces. The conditions for ignition and recovery strength of the work liquid are important characteristics for a machining process up to 4 million discharges per second.

Co-occurrence of photochemical and thermal effects during laser polymer ablation via a 248-nm excimer laser

Polypropylene (PP) was ablated using a 248-nm KrF excimer laser with a fluence in the range of 0.4–2 J/cm 2 and at a repetition rate of 10 Hz. It has been found that both photochemical and thermal effects are co-operative during laser ablation process, evidenced by the existence of thermal affected zone (TAZ), together with ablative zone (AZ) and unaffected zone (UAZ) in the ablated crater. Increment of both fluence and the number of pulses leads to a more striking thermal effect, demonstrated by the spatial expansion of rim for PP. Relative importance of photochemical and thermal effects was theoretically discussed.

Microstructural evaluation and interpretation of the mechanically and thermally affected zone under railway wheel flats

During a wheel–rail slide, the temperature is high enough to austenitise the material close to the contact surface. The material is rapidly cooled by heat conduction into the rest of the wheel when the wheelset starts rolling again and thus martensite can form in the surface layer. In the present investigation, the mechanically and thermally affected zones (TAZ) under railway wheel flats have been studied by optical microscopy, scanning electron microscopy and hardness testing. The wheel flat samples were generated in full-scale experiments on the last axle of a train consisting of a locomotive pulling three carriages. The axle load, train speed and locking time of the last axle were varied to simulate several different situations that arise in revenue traffic. The entire population of samples has earlier been studied, but the present investigation goes more into detail on a limited number of samples. Several discoveries are made and suggestions of mechanisms are presented. Among the most important findings are stretched MnS inclusions that act as crack nucleation sites, limited grain growth indicating low austenitisation temperatures and reformation of ferrite and pearlite in the deformed surface layers.

Analytical 1D model for analysis of the thermally affected zone formed during railway wheel skid

During sliding between a railway wheel and rail, frictional heat is generated that causes a temperature increase in the surface layer high enough to austenitise it. Due to the rapid cooling that follows when the wheel starts to roll again, martensite often forms in the surface layer. In the present paper, an analytical model of heat conduction in the thermally affected zone (TAZ) is established and evaluated. The model is based on metallurgical observations made after full-scale experiments on wheel skid. These observations indicate that the heat flow is close to one-dimensional (1D) in relevant material volumes, and that the contact surface temperature of the wheel rapidly rises and then saturates at a certain value. Comparison between model output and experimental results supports the choice of boundary condition. Evaluation of the model by the aid of experimental results of TAZ thickness indicates saturation surface temperatures of around 900±100°C. Another interesting result is that the cooling rate in temperature ranges relevant for phase transformation is almost independent of depth.

Characterization of zones resulting from the action of carbon dioxide laser impacts on dentinal hydroxyapatite

In spite of the reasonably large number of works relating to CO 2 laser irradiation of teeth, the effects of power level and impact time on dentine have not yet been analysed in detail, in particular the effects of power level on the development of microcracks and the blocking phenomenon of tubules. For this purpose, thermal effects were investigated by single and multiple impacts on dentinal hydroxyapatite. Results obtained from numerous samples show that laser irradiation may produce deep craters, the size of which depends on the power level and duration of the laser beam. The craters thus formed are composed of three distinct heat-affected zones. Formation of the craters is accompanied by the development of microcracks in the heat-affected zones as well as in the non-affected region surrounding the craters. Compositional, phase and structural changes are observed. An increase in hardness and the blocking of dentinal tubules in the thermal-affected zone were detected.