Asymmetrical solidification structures and fluid flow in GTA welds of 7004 aluminum alloy sheets

Metallurgical and Materials Transactions B - Weld solidification structures in aluminum alloys and other materials have been studied by several investigators. In results reported to date, the welds...     

Asymmetrical solidification structures and fluid flow in GTA welds of 7004 aluminum alloy sheets

Weld solidification structures in aluminum alloys and other materials have been studied by several investigators. In results reported to date, the welds were symmetrical about their center lines. While studying solidification structures of Gas-Tungsten Arc (GTA) welds in 7004 aluminum alloy sheet, however, some asymmetrical welds were produced. Formerly graduate student at the University of Waterloo.     

Casting-chill interface heat transfer during solidification of an aluminum alloy

Unidirectional solidification tests on an aluminum alloy were conducted with a computer-controlled instrumented rig. The alloys employed in this study were poured into isolated ingot molds (made of recrystallized alumina and covered with ceramic fiber) placed on top of a steel plate, coated either with a graphite- or ceramic-based paint in order to avoid sticking of the material. Thermal evolution during the test was captured by type-K thermocouples placed at different positions in both the ingot and the plate. The bottom surface of the plate was either cooled with water or left to cool in air. The heat-transfer coefficients across the aluminum-steel interface were evaluated by means of a finite-difference model. It was concluded that the heat-transfer rate depends on the conditions at the interface, such as the type of coating used to protect the plate, and the solidification reactions occurring on the aluminum during its solidification.     

On the use of analytic approximations for describing the macroscopic heat flow during solidification

In this paper both conceptual and numerical analyses of the integral profile method as applied to solidification problems are conducted. Two important examples are considered,viz. a case where the nonsteady state exact solution of the thermal field is known and a thermal analysis of continuous casting. It is shown that the method as it was generalized by Hills may lead in the latter case to serious contradictions with basic thermodynamic principles. It is also shown that the method developed by Tien using a different set of boundary conditions is unsuitable for treating general solidification problems. It is thus concluded that approximate solutions of the thermal field during solidification must be evaluated under rather severe boundary conditions.     

Contraction of aluminum alloys during and after solidification

A technique for measuring the linear contraction during and after solidification of aluminum alloys was improved and used for examination of binary and commercial alloys. The effect of experimental parameters, e.g., the length of the mold and the melt level, on the contraction was studied. The correlation between the compositional dependences of the linear contraction in the solidification range and the hot tearing susceptibility was shown for binary Al-Cu and Al-Mg alloys and used for the estimation of hot tearing susceptibility of 6XXX series alloys with copper. The linear thermal contraction coefficients for binary and commercial alloys showed complex behavior at subsolidus temperatures. The technique allows estimation of the contraction coefficient of commercial alloys in a wide range of temperatures and could be helpful for computer simulations of geometrical distortions during directchill (DC) casting.     

Simulation of metal flow and heat transfer during hot rolling of bimetal plates

Coupled thermomechanical finite element model is applied to simulate metal flow and temperature distribution during hot rolling of bimetal plates. Calculations are carried out for various reductions and dimensions of the plates. Two possible versions of the boundary conditions are assumed. The first considers the case when special guides or tensions prevent bending of the sample before entering and after exit from the roll gap. The second allows free bending of the plate on both sides of the mill. Results of calculations include distributions of strain rates, strains, roll pressure, friction forces and temperatures during rolling. Calculated rolling force and torque are compared with the experimental data. Several conclusions regarding the metal flow and heat transfer in various rolling conditions are drawn.     

Numerical simulation of three-dimensional heat transfer and plastic flow during friction stir welding

Three-dimensional visco-plastic flow of metals and the temperature fields in friction stir welding have been modeled based on the previous work on thermomechanical processing of metals. The equations of conservation of mass, momentum, and energy were solved in three dimensions using spatially variable thermophysical properties and non-Newtonian viscosity. The framework for the numerical solution of fluid flow and heat transfer was adapted from decades of previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate, temperature, and temperature-dependent material properties. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing. The heat and mass flow during welding was found to be strongly three-dimensional. Significant asymmetry of heat and mass flow, which increased with welding speed and rotational speed, was observed. Convective transport of heat was an important mechanism of heat transfer near the tool surface. The numerically simulated temperature fields, cooling rates, and the geometry of the thermomechanically affected zone agreed well with independently determined experimental values.     

A new technique for three-dimensional transient heat transfer computations of autogenous arc welding

Three-dimensional (3-D) transient temperature variations during autogenous gas tungsten arc welding are determined. The heat diffusion equation is solved using an efficient semidiscrete technique. The model employs a combination of unequally spaced grids concentrated near the moving torch in order to minimize the total number of nodes. Finite differencing is used for the spatial terms. The resulting ordinary differential equations for the transient evolution of thermal transport are solved using the fourth-order Runge-Kutta technique. The temperaturedependent thermal properties and latent heats of phase transformations are accounted for. Computations are carried out for a rectangular parallelepiped with convective and radiative surface thermal conditions. Sample results are presented first for the evolution of thermal profiles during ideal welding conditions. These are next compared with variations obtained due to defects, such as weld track misalignment and inclusions. The potential use of this model in the development of an expert welding system using infrared imagery is indicated.     

Dendrite coherency during equiaxed solidification in binary aluminum alloys

Dendrite coherency, or dendrite impingement, is important to the formation of the solidification structure and castability of alloys. Dendrite coherency in the systems Al-xMn, Al-xCu, Al-xFe, and Al-xSi(x = 0 to 5 wt pct) has been studied by continuous torque measurement in solidifying samples. The fraction solid at the dendrite coherency point, fs*, varies with the alloy system and the solute concentration in the alloy, from 18 to 56 pct for the present alloys investigated. An increase in solute concentration decreases the coherency fraction solid,fs*. An alloy system with a large slope of the liquidus line has a high coherency fraction solid. A theoretical approach has been developed to account for the effects of the alloy system and solute concentration on the dendrite coherency in the alloy. The grain sizes of the alloys were evaluated using the parameters at coherency point.     

The use of heat flow modeling to explore solidification phenomena

Some of the basic solidification characteristics of alloys which freeze over a finite temperature range are examined with the help of an explicit finite difference model. Comparison is made between observed and predicted changes in local cooling conditions during directional solidification and deductions made about thermal features and the growth behavior. Some general conclusions are outlined regarding the speed-up of dendrite tip and root isotherms (and other end effect phenomena), elimination of melt superheat by convection currents, and the effect of isothermal latent heat evolution (as in eutectic formation). Some comments are made about the relevance of these considerations to real solidification processes and the importance of numerical modeling in future developments.     

Microstructural evolution of 6063 aluminum during friction-stir welding

The microstructural distribution associated with a hardness profile in a friction-stir-welded, age-hardenable 6063 aluminum alloy has been characterized by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). The friction-stir process produces a softened region in the 6063 Al weld. Frictional heating and plastic flow during friction-stir welding create fine recrystallized grains in the weld zone and recovered grains in the thermomechanically affected zone. The hardness profile depends greatly on the precipitate distribution and only slightly on the grain size. The softened region is characterized by dissolution and growth of the precipitates during the welding. Simulated weld thermal cycles with different peak temperatures have shown that the precipitates are dissolved at temperatures higher than 675 K and that the density of the strengthening precipitate was reduced by thermal cycles lower than 675 K. A comparison between the thermal cycles and isothermal aging has suggested precipitation sequences in the softened region during friction-stir welding.     

GTA welding of AISI 316L metal injection moulded components

Abstract

The thin sections and residual porosity typical of metal injection moulded (MIM) components pose challenges when using welding as a joining technology, but if welding can be successfully applied the potential commercial benefits are considerable. Process development of gas tungsten arc (GTA) welding for MIM parts to tubing and wrought components are described and the suitability of MIM 316L products in welded applications, particularly fluid system components, is discussed.     

Material flow patterns and cavity model in friction-stir welding of aluminum alloys

Friction-stir welding (FSW) of 3-mm-thick plates of 6061 Al and LF6 Al was conducted and the materials’ flow patterns in the weld nugget along three perpendicular planes were analyzed. The onion structure viewed on any cross section normal to the travel direction is independent of weld position. The weld morphology was examined along its length by considering planes of different depths parallel to the surface. These showed semicircle streaks whose shapes depended on the depth of the observation plane. It is determined that the weld nugget is composed of a series of identical half ellipsoid regions. A tentative simplified cavity model is presented to explain the mass flow pattern and formation of defects in the weld nugget. This model is based on the assumption that only the metal between the pin surface and the last maximum circle created by the pin rotation is in a plasticized state. From this model, it is shown that the location and size of the cavity formed during the rotation of the pin changes cyclically and it is related to the position of the pin’s center. The holes or slots left in the weld nugget center or near the advancing side are directly related to the size of the cavity. The welding parameters or weld pitch affects the volume of the cavity, and consequently influence the weld defects. A large weld pitch will cause holes to be formed in the weld nugget because of the large cavity. The flow patterns, which show that the plasticized material flows from both advancing and retreating sides to the weld center behind the pin, can be easily explained with this cavity model.     

Grain refinement in magnetically stirred GTA welds of aluminum alloys

The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.     

Effects of flow on morphological stability during directional solidification

Research involving the interaction of flow with morphological instability during directional solidification of binary alloys is reviewed. In general, flow may arise during the solidification process from thermal and solutal buoyancy, changes in density upon solidification, thermocapillary forces at free boundaries, or external forcing of the system. We focus primarily on the last of these, giving details of the influence of various forced flows on the critical conditions for morphological instability. These flows include the asymptotic suction profile, stagnation-point flow, and periodically driven shear flows. Parallel shear flows are unable to stabilize morphological instabilities in three dimensions but may lead to new long-wave, traveling instabilities. Flow-induced, long-wave instabilities are also encountered in the presence of both steady and modulated stagnation-point flows. Unsteady, nonparallel shear flows may stabilize morphological instability if the flow parameters are adjusted properly. This article is based on a presentation made at the “Analysis and Modeling of Solidification” symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2–6, 1994, under the auspices of the TMS Solidification Committee.     

Dimensionless maps for heat flow analyses in fusion welding

In the present investigation, attempts have been made to bridge the various analytical heat flow models (i.e. the thick plate, the medium thick plate, and the thin plate solutions) by establishing dimensionless maps for a general outline of the quasi-stationary temperature distribution pertaining to a moving heat source on plates of different thickness and thermal properties. The accuracy of the maps has been tested against extensive experimental data, as obtained from in situ thermocouple measurements and numerical analysis of a large number of stringer bead welds. It is shown that the thermal programme within the HAZ (heat affected zone) of such weldments can be adequately predicted from the medium thick plate solution for a wide range in operational conditions (including aluminium and steel welding).     

The effect of solidification time and heat treatment on the fatigue properties of a cast 319 aluminum alloy

Solidification time and heat treatment are known to have a large effect on the microstructure of cast aluminum alloys. This study was conducted to quantify how the fatigue properties of a 319-type aluminum alloy are affected by solidification time and heat treatment. Both porosity-containing (non-hot isostatically pressed (HIP)) and porosity-free (HIP) samples in the T6 (“peak aged”) or T7 (“overaged”) heattreated conditions were tested. As the solidification time increased, the average initiating pore diameter increased and stress-controlled fatigue life decreased. Heat treatment was observed to have a large effect on fatigue properties of the HIP samples. However, in the non-HIP fatigue samples, heat treatment did not significantly change the fatigue life or fatigue strength of the cast 319-type alloy. The absence of an influence of heat treatment on fatigue response is attributed to the predominance of the microporosity in fatigue crack initiation in cast aluminum.