Showing papers on "Heat-affected zone published in 1997"

Weld microstructure refinement in a 1441 grade aluminium-lithium alloy

Abstract: Clad 2 mm thick sheets of Russian 1441 grade Al-Li alloys were welded using a gas tungsten arc welding process (GTAW). Comparisons were made between the weld beads obtained under (i) continuous current (CC), (ii) pulsed current (PC), and (iii) arc oscillation (AO) conditions for their macro- and microstructural details. In the case of CC GTAW, sound welds could be produced only under a narrow range of welding parameters. Centre line cracks, which occurred in CC GTAW welds under certain conditions, were halted by switching to PC or AO conditions while the welding was in progress. Microstructural refinement was significant in the case of PC and AO GTA welding.

Joining of nickel base superalloy single crystals

Abstract: The welding and weldability characteristics of single crystal nickel base superalloy PWA 1480 were investigated over a range of welding conditions and orientations using electron beam and laser welding processes. Using differential thermal analysis, the freezing range for this alloy was established and the formation of the γ/γ′ eutectic during the last stages of solidification was identified. Both electron beam and laser welds showed extensive fusion zone cracking, and the cracks were identified as solidification cracks, i.e. hot cracks. Crack free welds could be made over a very narrow range of welding conditions with preheat. Most of the microstructural features observed in the weld are similar to those previously observed in Fe–Ni–Cr alloy single crystal welds. The welds contain misoriented stray grains which playa critical role in the promotion of hot cracks in the welds. The origin of these grains is explained in terms of constitutional supercooling and the growth conditions. Detailed atom pr...

An experimental and theoretical study of heat-affected zone austenite reformation in three duplex stainless steels

Abstract: Three duplex grades, one molybdenum-free, one 22Cr type, and one super duplex grade, have been subjected to weld simulation treatments, and the resulting microstructures have been quantified by automatic image analysis techniques. Substantial differences between the duplex grades were observed with an increased ability to reform austenite with increased alloying content. A theoretical model has been applied, based upon the paraequilibrium concept elaborated by Hillert, and the paraequilibrium compositions of individual phases were calculated as a function of temperature using the THERMOCALC database. A model based on Cahns theory of grain boundary nucleated reactions has also been utilized to calculate the kinetics of the reaction. By using this model, the grain size effects could be included in the treatment. The results of the calculations were compared with experimental data, and the experimental results were reproduced using the same parameter set for the three materials, with the exception of the diffusion coefficient values which had to be adjusted. This adjustment has in a later study been verified experimentally. The results validate the model used and the physical relevance of using the paraequilibrium model. The appropriateness of a paraequilibrium approach is also supported by experimental evidence from weld metal compositions. It is shown that the nitrogen content of the alloys plays an important role, and a higher nitrogen content results in more efficient austenite reformation. This implies that the alloy nitrogen compositions should lie close to the upper specification limits for these materials and nitrogen losses should be avoided on welding since the material properties, both mechanical and corrosive, are strongly related to the austenite-ferrite phase ratio.

Mathematical modeling of three-dimensional heat and fluid flow in a moving gas metal arc weld pool

Abstract: Mathematical models capable of accurate prediction of the weld bead and weld pool geometry in gas metal arc (GMA) welding processes would be valuable for rapid development of welding procedures and empirical equations for control algorithms in automated welding applications. This article introduces a three-dimensional (3-D) model for heat and fluid flow in a moving GMA weld pool. The model takes the mass, momentum, and heat transfer of filler metal droplets into consideration and quantitatively analyzes their effects on the weld bead shape and weld pool geometry. The algorithm for calculating the weld reinforcement and weld pool surface deformation has been proved to be effective. Difficulties associated with the irregular shape of the weld bead and weld pool surface have been successfully overcome by adopting a boundary-fitted nonorthogonal coordinate system. It is found that the size and profile of the weld pool are strongly influenced by the volume of molten wire, impact of droplets, and heat content of droplets. Good agreement is demonstrated between predicted weld dimensions and experimently measured ones for bead-on-plate GMA welds on mild steel plate.

Welding of nickel-base superalloys having a nil-ductility range

Abstract: An article made of a nickel-base superalloy having a nil-ductility range from the solidus temperature of the alloy to about 600° F. below the solidus temperature is welded, as for example in the weld repair of surface cracks, by removing foreign matter from the area to be welded, first stress relieving the article, adjusting the temperature of the article to a welding temperature of from about 1800° F. to about 2100° F., welding a preselected area in an inert atmosphere at the welding temperature, and second stress relieving the article. Welding is preferably accomplished by striking an arc in the preselected area so as to locally melt the alloy in the preselected area, providing a filler metal having the same composition as the nickel-based superalloy of the article, and feeding the filler metal into the arc so that the filler metal is melted and fused with the article to form a weldment upon solidification.

Friction stir welding with simultaneous cooling

Abstract: A method and apparatus for friction stir welding that produces a weld of significantly reduced surface roughness at significantly higher welding rates, in materials that are difficult to weld, such as non-extrudable aluminum alloys. The method includes cooling the stir welding tool during the welding process, thereby reducing the tendency of softened metal to adhere to the rotating pin and shoulder of the tool. The apparatus includes a tool with internal spaces or an external jacket, through which coolant can be pumped to remove heat and cool the tool during welding operations. In another embodiment, the apparatus includes a device for spraying a coolant onto exterior surfaces of the distal end of the welding tool to thereby remove heat from the tool, and the surrounding workpiece, during welding.

Mechanical and thermal response of a nickel-base superalloy upon grinding with high removal rates

Abstract: The formation of a plastically deformed and a heat affected zone during grinding of superalloy IN738LC with a high depth of cut but slow work speed (creep feed grinding) was investigated. In order to obtain a better understanding of material removal mechanisms, the alloy strength and contact temperatures under typical machining conditions were evaluated. Temperature calculations comprised both the stationary equilibrium between global heat input and heat transfer to the coolant, as well as flash-like pulses at small contact spots. As a result, it was found that local melting at contact spots seems to be a rather common mechanism during grinding of superalloys, leading to so-called white layers which can easily be observed on metallographic cross sections. Grinding wheel topography has a significant influence on the maximum contact temperature and the width of the white layer. Sufficient cooling is very essential in limiting the width of the molten zone to few micrometers, which then is not critical in respect to mechanical properties. On the other hand severe damage was observed after creep feed grinding without coolant.

Resistance Spot Weld Failure Loads and Modes in Overload Conditions

Abstract: The failure modes of two single-weld specimens, the coach-peel and the tensile-shear specimens, were studied in detail. Weld overload experiments, along with optical and scanning electron microscopy, revealed that the coach-peel specimen failed by microvoid coalescence (ductile fracture) near the weld nugget/heat affected zone (HAZ) boundary and that the tensile-shear specimen failed predominately by localized necking (shear localization) near the HAZ/base metal boundary. Empirical data extracted from measurements performed on metallurgical cross sections of interrupted coach-peel and tensile-shear specimens established the deformed characteristic material distance (coach-peel) and the existence of a critical thickness strain for localized necking (tensile-shear). These quantities were used to predict weld failure via finite element analysis as described in Ref I. The work presented here is the first step in a larger project that is focused on developing a methodology for predicting spot weld overload failure in detailed finite element simulations of spot-welded joints. This methodology is based upon the failure phenomena (as reported here) and detailed characterization of the HAZ (as reported in Ref I). The main requirement of this predictive methodology is that it be adaptable to any combination of joint configuration and loading direction. This predictive methodology will serve as the basis for the final step of developing a model of resistance spot weld failure based upon a simpler representation of the spot weld that can be used in car crash simulation models.

Microwave welding of high density polyethylene using intrinsically conductive polyaniline

Abstract: The use of intrinsically conductive polymers in welding of plastics and composites offers the possibility of developing new welding methods. Intrinsically conductive polyaniline (PANI) composite gaskets were used to microwave weld high density polyethylene (HDPE) bars. Two composite gaskets were made from a mixture of HDPE and PANI powders in different proportions. Adiabatic heating experiments were used to estimate the internal heat generation and electric field strength in the gasket. During welding, the effects of heating time, heating pressure and welding pressure were evaluated. It was found that increasing the heating time and the welding pressure increased the joint strength. The maximum tensile joint strength was achieved using a 60 wt% PANI gasket with a heating time of 60 sec and a welding pressure of 0.9 MPa; this resulted in a tensile weld strength of 24.79 ± 0.34 MPa, which equals the tensile strength of the bulk HDPE.

Electron beam welding of a Ti–45Al–2Nb–2Mn+0.8 vol.% TiB2 XD alloy

Abstract: The feasibility of using conventional electron beam welding to join a Ti–45Al–2Nb–2Mn+0.8% TiB 2 XD alloy was assessed. The alloy showed no evidence of hot cracking. However, a susceptibility to solid-state cracking was observed when weldments cooled at high cooling rates. The microstructural features of weldments were found to be a function of welding speed, heat input, preheating temperature, and thus cooling rate. Effects of cooling rates on microstructural evolution was evaluated with thermal simulation in a Gleeble 1500 system and it was found that cracking occurred when high cooling rates resulted in suppression of the α → γ phase transformation. It was observed that sound welds could be made when welding conditions were chosen so that the cooling rate at the fusion zone boundary was 250 K s −1 or less.

Plasma transferred arc repair welding of the nickel-base superalloy IN-738LC

Abstract: Plasma transferred arc welding (PTA) has been considered a promising process to restore worn areas of land-based gas turbine blades and vanes.

Friction welding of polyamides

Abstract: Results from different experiments on friction welding are used to characterize the behavior of polyamide over a wide range of welding conditions Several types and grades of polyamide were joined using the vibration and spin welding processes The quality of the welds was evaluated by short time tensile tests and microscopy In addition to the geometry of the parts being joined, the process parameters and the material were found to affect the quality of the weld, so that associated with each application is a different set of optimum welding parameters

Method of welding wallpaper alloy and arc welder modified to practice same

Abstract: A method of and welder for welding a corrosion resistant, wallpaper alloy to the inside surface of a vessel wall formed from a corrosion susceptible steel sheet after the wallpaper alloy has been affixed to the inside to provide an exposed seam of wallpaper alloy extending in a given path wherein the method and welder comprising moving a welding wire toward the seam, melting and depositing the welding wire onto the seam along the path by a short circuit arc welding process of the type having a welding cycle with a short condition and an arcing condition, which arcing condition constitutes a plasma boost portion with a set peak current level followed by a plasma portion with a current decreasing from said peak current level toward a set background current level with a given time between the plasma boost portion and the short condition andsetting the length of time of the plasma portion of the arcing condition to a value greater than 25% of the given time or greater than 2.0 ms.

CO2 laser beam welding of 6061-T6 aluminum alloy thin plate

Abstract: Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO2 laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening β″ (Mg2Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.

Dual pass weld overlay method and apparatus

Abstract: A method is disclosed for applying a weld overlay to a tube Using a first weld head, a bead of overlay material is applied onto a tube by melting the overlay material In the process of applying the weld bead to the tube, a heat affected zone is created within the tube Thereafter, a second weld head is employed to apply sufficient heat to the exterior of the tube to raise the temperature within the heat-affected zone to a temperature higher than its tempering temperature but lower than its Ae1 temperature In this manner the heat affected zone is eliminated without creating a new heat affected zone in the process In the disclosed embodiment, the tube is rotated with respect to the weld heads, while the weld heads move along the longitudinal axis of the tube According to the disclosed embodiment, the first weld head employs a gas-metal arc welding process, and the second weld head employs a gas-tungsten arc welding process

Creep Continuum Damage Constitutive Equations for the Base, Weld and Heat-Affected Zone Materials of a Service-Aged {1 over 2}cr{1 over 2}mo{1 over 4}v:2{1 over 4}cr1mo Multipass Weld at 640 °c

Abstract: Creep continuum damage constitutive equations have been determined for the base, weld and heat-affected zone materials of a service-aged 1/2Cr1/2Mo1/4V:21/4Cr1Mo multipass weld at 640°C. The base and weld material properties were determined from uniaxial creep and notched bar creep rupture test results. The properties of the heat-affected zone material were determined from impression creep tests, waisted cross-weld specimen tests and notched cross-weld specimen tests. At the test temperature of 640 °C, surface oxidation occurs. For the uniaxial creep and notched bar creep tests, this oxidation was considered not too significant. However, for the low-load impression creep tests, the oxidation depth was comparable to the indentation depth.

Method of welding

Abstract: A process for buttwelding metal workpieces (12) having bevelled joint preparations using an automatic GTAW welder (20) using filler wire (32) includes preparing the bevelled workpieces (12) with bevelled joint areas having minimal land thickness at the root extremities; placing the prepared workpiece joint sections together with an open gap between their adjacent root extremities, the gap having a minimum dimension that avoids harmful compression stress between the workpieces due to weld shrinkage and a maximum dimension that avoids filler wire penetration of the gap; fusion welding the open root area of the adjacent workpieces (12) with a root pass weld using an automatic GTAW welder (20) supplied with filler wire (32) and a shield gas including 1 to 10 % hydrogen and the balance inert gas; and then promptly overlaying the root pass weld with at least one additional filler weld pass using an automatic GTAW welder (20) supplied with filler wire (32) and hydrogen-free shield gas.

Laser beam processing of a SiC particulate reinforced 6061 aluminium metal matrix composite

Abstract: SiC particulate reinforced 6061 Al metal matrix composites were laser beam cut using a 3kW continuous wave CO2 laser. The influence of laser processing parameters such as cutting speed, laser power, and shielding gas on the quality of the cuts were investigated. Optical microscopy, scanning electron microscopy and X-ray diffraction were used to analyse the laser treated zone. Experimental results show that 6061 Al metal matrix composites can cut be successfully using laser. A number of Al4C3/Al4SiC4 plates were formed in the heat affected zones due to a chemical reaction between Si and Al that occurred during the laser processing.

A mathematical model of gas tungsten arc welding considering the cathode and the free surface of the weld pool

Abstract: A two-dimensional axisymmetric numerical model, including the influence of the cathode and the free surface of the weld pool, is developed to describe the heat transfer and fluid flow in gas tungsten arc (GTA) welding. In the model, a boundary-fitted coordinate system is adopted to precisely describe the cathode shape and deformed weld-pool surface. The current continuity equation has been solved with the combined arc plasma-cathode system, independent of the assumption of current density distribution on the cathode surface, which was essential in the previous studies of arc plasma. It has been shown that the temperature profile, the current, and the heat flux to the anode show good agreement with the experimental data. Moreover, the current and the heat-flux distributions may be affected by the shape of the cathode and the free surface of the weld pool.

Method of welding in the horizontal position and welding apparatus therefor

Abstract: The object of this invention is to prevent the occurrence of welding defects such as undercuts, overlaps and faulty angles of fusion, and to increase the efficiency of welding in the horizontal position. Auxiliary wires 5a and 5b are inserted into molten pool 12, and a unidirectional current is made to flow between them. A magnetic field 9 is induced which is approximately orthogonal to the surface of the parent material so as to generate an upward Lorentz force (opposite the pull of gravity) in molten pool 12. This force supports the molten metal and prevents it from dripping, resulting in a better-shaped bead.

Absorption and transport of hydrogen during gas metal arc welding of low alloy steel

Abstract: Although hydrogen induced cracking remains a major problem in the welding of steels, the present methods of managing hydrogen in the weldment are mostly empirical in nature. In recent years, numerical modelling of heat transfer and fluid flow has provided detailed insight into the physical processes in welding. However, very little effort has been made in the past to use these transport phenomena based calculations to understand the dissolution of hydrogen in the weld metal and its subsequent transport in the liquid and solid regions. The aim of the present work was to address this important need. Heat transfer, fluid flow, and hydrogen transport calculations in transient, three-dimensional form are used to predict the spatial distribution of hydrogen concentration in the weld metal during gas metal arc welding of mild steels for different welding conditions. The enhanced hydrogen solubility in the weld metal above that predicted by Sieverts law was determined from a model for the partitioning of ...