Papers by Keyword: Thermal History

Abstract: Bobbin friction stir welding (BT-FSW) is a variant of the conventional friction stir welding (C-FSW). This method has been applied of welding high density polyethylene (HDPE) plates; where a rotating symmetrical tool causes a fully penetrated bond, it can weld the upper and lower surface of the work-piece in the same pass. BT-FSW process involves complex heat generation and HDPE flow, which directly affects on the weld area and on mechanical properties of welded joint. Heat generation and material flow during BT-FSW are significantly affected by the tool design features, process parameters and mechanical behavior of work piece materials. Studying the temperature of polyethylene sheets welded by BT-FSW can help in analyzing the mechanism of weld formation and also can provide theoretical guidance for the tool design, process parameter selection and even new process development. In the unique work described in this paper, the 11.4 mm-thick HDPE plates were welded successfully by bobbin-tool friction stir welding. Measurements of the material temperatures were performed by thermocouples which are placed near and at the weld seam. The weld quality was determined in terms of no defects in the stir zone and the tensile strength of the joint. It was found that considerable melting occurred between the rotating shoulders and on the trailing side of the rotating pin. Movement of the molten material by the rotating tool created a very black band in the stir zone. Thermocouples measurements indicated that the temperatures were higher on the advancing side (AS) compared to those on the retreating side (RS). Tensile tests and hardness measurements were performed on welded and seamless sheet samples. The results were analyzed to compare the mechanical properties. To demonstrate the variation in micromechanical properties between welded and seamless sheet samples, micro hardness (HV) testing was used to explain the difference. The HV of the HDPE plates weld by BT-FSW, were relatively symmetrical with respect to the parting line. The maximum hardness levels were reached in the weld bead at around 66 HV in the welded nugget; there was a rise in the level of hardness, in particular at retreating side (RS) and at advancing side (AS) where the value reached 68.60 HV.

Abstract: Simulating powder bed fusion processes (PBF) can reveal temperature evolution in transient mode. Accurate temperature prediction using finite element (FE) method demands both mesh and timeincrements to be very small; thus, requiring a high computational cost. To avoid this, in part-scale simulation, coarse meshes representing multiple powder layers added at once, are usually used which results in fast solving of FE models. Powder layers and time increments are lumped in such a configuration, which results in a deviation of the temperature history. This research proposes a methodology to predict the nodal temperature (NT) due to the combined effect of space and time lumping for part-scale FE thermal simulation for PBF processes. It shows its effects in predicting both the local temperature history and the average far-field temperature.

Abstract: The mechanic properties mainly the room temperature bending strength and the creep of the Al₂O₃ ceramic specimens which were prepared by cold isostatic pressing and had undergone different thermal histories have been measured and compared. The results suggest that the bending strength of the ceramic increases after high temperature soaking. The ceramic can withstand cyclic rapid cooling with an initial temperature of 200 °C. The sample has shown a very good dimensional stability after a long service time (50h) at 1550 °C.

Abstract: A numerical analysis of thermal phenomena occurring during lined-pipe welding is presented in this paper. Numerical models of surfaces and volumetric heat sources were used to predict the time evolution of the temperature field both in a corrosion-resistance-alloy (CRA) liner, made of SUS304 stainless steel (SS), and for the single-pass girth welding of a carbon-manganese (C-Mn) steel pipe. Using the finite-element code ABAQUS, three-dimensional non-liner heat-transfer analyses was carried out to simulate the gas-tungsten-arc (GTA) welding process used in liner welding and the metal-inert-gas (MIG) welding process consumed in C-Mn steel backing welding. FORTRAN user subroutines were coded to implement the movable welding heat source and heat transfer coefficient models. The thermal history was numerically computed at locations where circumferential angles from the welding start/atop position are 90°, 180° and 270° with respect to axial distances from the weld centerline (WC). Keywords: Finite element analysis FEA, CRA Liner, C-Mn steel backing, Heat source, Thermal history.

Abstract: Characteristics of silica optical material largely depend on its thermal history. In this paper, formation of thermally induced defects in silica optical material is studied. The formation process of defect is analyzed in detail. The results show that there is an obvious difference in defect formation induced by heating treatment when the composition of silica optical material changes. Defect formation mainly displays as the produce process when the initial defects of the silica material are zero. However, defect formation expresses as the produce and annealing process when the initial defects of the silica material are not zero. The initial defect concentration can be decreased significantly when the silica material is heated in high temperature. At the same time, the new defect is also produced. These theoretic results are consistent with the previous experimental ones.

Abstract: Mechanical testing is foremost a means to measure material performance, however it provides a probe into the complex elastic, viscoelastic and viscoplastic behavior of polymer morphologies. The techniques in this work utilize variables of time/frequency, temperature, stress and strain with emphasis on dynamic and modulated implementation. Several instruments were used since a particular instrument does not provide all of the capabilities. The material response is complex and it has been resolved into typically instantaneous and time-dependent components. Some of the techniques are widely used and these have been extended, while other techniques introduce control over alternate variables. Polycarbonate was chosen as the main example with support from similar polymers, though the techniques are applied to many polymer types.

Abstract: For the manufacture of safe, lightweight vehicles, the demand for ultra-high-strength steel in the automotive industry is increasing. Although transformation-induced plasticity (TRIP) and dual-phase (DP) steels have a strength of under 1 GPa, boron-alloyed steel produced using the hot press forming process has a strength of more than 1500 MPa. Laser and resistance spot welding processes are used to join press-hardened steel, but the characteristics of the resulting weldments are not yet fully understood. In this study, the thermal cycles for both welding processes were investigated using finite element (FE) analysis. Resistance spot welding was analyzed using a combination of thermal, electric, and mechanical models, whereas the thermal behavior of laser welding was predicted using only a thermal model. The calculated bead shapes were compared with experimentally measured ones to validate the simulation models. The mechanical and metallurgical characteristics of the weldments were explained using the thermal history of each welding process.

Abstract: In order to theoretically simulate the welding process of complex structure with large quantities of welding spots, a simplified method for analyzing a single spot welding should be developed firstly. In this paper, a 2D axisymmetric model of thermoelectric Finite Element Method (FEM) is developed to analyze the transient thermal behavior of Resistance Spot Welding (RSW) process using ANSYS. The determination of the contact resistance at the faying surface is moderately simplified to reduce the calculating time, while the temperature dependent material properties, phase change and convectional boundary conditions are taken into account for the improvement of the calculated accuracy. The thermal history of the whole process and temperature distributions for any position in the weldment is obtained through the analysis. The model can also predict the weld nugget size and the width of the Heat Affected Zone (HAZ).

Abstract: Basicity of glass is still a challenge in spite of various attempts to measure or calculate it. The values assigned for basicity of glasses, either calculated or experimentally determined, are not always in full agreement with actual facts, and discrepancies among the theoretical ones are not unusual. For instance, SiO2 is described by a single basicity value even if the polymorphs of SiO2 are quite different. Only few attempts were made to face this challenge. Present paper deals with a novel approach on theoretical ionicity / basicity based on electronic energy levels or band structure of solids. Another major adjustment takes into account the possibility of decomposing ionicity of complex chemical bondings into a sum of binary bondings. Considering the distribution of the interbonding angles specific for vitreous systems, it is possible to estimate both a local ionicity (basicity) of bondings and a global (mean) basicity of glass. The variation of basicity (ionicity) with temperature is also presented, this finding being able to open a new view on thermal history of glass considered through chemical character of bondings.

Abstract: High-strength Mg-Y-Zn alloy plate was obtained by friction stir processing (FSP) after casting. In this study, the effect of processing order on the strength of processed specimens was discussed. It was revealed that the microstructures and strength of doubly stirred zone depended on the direction of overlapping pass. In the area of the doubly stirred zone that was affected by the thermal history of Retreating-side, the hardness was 121Hv. It was twice as hard as Base Metal, and harder than the doubly stirred zone that was affected by thermal history of Advancing-side.