Three-dimensional heat flow and solidification during the autogenous GTA welding of aluminum plates

A theoretical and experimental study of heat flow and solidification during the autogenous GTA welding of aluminum plates was carried out. The theoretical part of the study involves the development of a computer model which describes three-dimensional heat flow during welding. The model, though valid for any plate thickness, is particularly useful for moderately thick plates since both full- and partial-penetration welds can be considered. The experimental part of the study, on the other hand, involves the measurement of the thermal response of the workpiece during welding, and the examination of the configuration, grain structure, and subgrain structure of the fusion zone. The experimental results were compared with the calculated ones and the agreement was very good. With the help of the computer model, the effects of welding parameters on weld penetration in moderately thick plates were discussed. These parameters are the heat input per unit length of the weld, the thickness of the workpiece, the preheating of the workpiece, and the power-density distribution of the heat source. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA     

Welding,Glazing, and Heat Treating —A dimensional analysis of heat flow

Through a rigorous mathematic demonstration and the application of general enthalpy equations, the key dimensionless variables that characterize heat flow during welding, glazing, and heat treating of a workpiece were systematically presented. Both the 2-dimensional and the 3-dimensional heat flows due to a moving heat source were considered. The 2-dimensional heat flow during the welding of thin plates with a stationary, instantaneous heat source was also analyzed. While the primary dimensionless variables such as the dimensionless temperature, the Fourier number, and the dimensionless distances were sufficient to describe the heat flow during heat treating, additional primary dimensionless variables such as the dimensionless heat input, the Stephan number, the dimensionless thermal conductivity, and the dimensionless specific heat were found necessary to define the heat flow during welding and glazing. The validity of such a dimensional analysis was verified by existing analytical solutions. Due to the additional heat flow variables such as the size of the heat source, the size of the workpiece, the surface heat loss, and the freezing range of alloy systems, secondary dimensionless variables including the dimensionless size of the heat source, the dimensionless width and thickness of the workpiece, the Biot number, and the dimensionless liquidus temperature were presented and discussed. The results of heat flow calculations involving both the surface heat treating of a substrate with a square laser beam and the gas tungsten-arc full-penetration welding of 5052 and 2014 aluminum alloys were presented using the dimensionless variables introduced in the present study.     

Heat Flow during the Autogenous GTA Welding of Pipes

A theoretical and experimental study of heat flow during the welding of pipes was carried out. The theoretical part of the study involves the development of two finite difference computer models: one for describing steady state, 3-dimensional heat flow during seam welding, the other for describing unsteady state, 3-dimensional heat flow during girth welding. The experimental part of the study, on the other hand, includes: measurement of the thermal response of the pipe with a high speed data acquisition system, determination of the arc efficiency with a calorimeter, and examination of the fusion boundary of the resultant weld. The experimental results were compared with the calculated ones, and the agreement was excellent in the case of seam welding and reasonably good in the case of girth welding. Both the computer models and experiments confirmed that, under a constant heat input and welding speed, the size of the fusion zone remains unchanged in seam welding but continues to increase in girth welding of pipes of small diameters. It is expected that the unsteady state model developed can be used to provide optimum conditions for girth welding, so that uniform weld beads can be obtained and weld defects such as lack of fusion and sagging can be avoided.     

A Transient Thermal Model for Friction Stir Weld. Part II: Effects of Weld Conditions

In Part II of this series of articles, the transient thermal model, which was introduced in Part I, is used to explore the effects of welding conditions on the heat generation and temperature. FSW of the 6061-T651 aluminum alloy is modeled to demonstrate the model. The following two steps are adopted to study the influence of welding conditions on the heat generation and temperature. First, the thermal model is used to compute the heat generation and temperature for different welding conditions, the calculated results are compared with the reported experimental temperature, and a good agreement is observed. Second, the analytical method is used to explore the approximate functions describing the effect of welding conditions on the heat generation and temperature. Based on the computed results, we discuss the relationship between the welding conditions, heat generation, temperature, and friction coefficient, and propose a relationship map between them for the first time at the end.     

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.     

焊接速度对机器人搅拌摩擦焊AA7B04铝合金接头组织和力学性能的影响

针对熔化焊在焊接AA7B04铝合金时易在焊缝中出现孔洞等缺陷,且接头性能下降明显、焊后变形大,以及采用铆接等机械连接方式会增加连接件的重量等问题,采用集成了搅拌摩擦焊末端执行器的KUKA Titan机器人对2 mm厚AA7B04高强铝合金进行了焊接,在转速为800 r·min-1的条件下,研究了焊度对焊接过程中搅拌头3个方向的受力F_x、F_y和F_z的影响.研究发现,F_z受焊速的影响显著,随焊速的增加而降低.利用光学显微镜、透射电子显微镜、拉伸试验、三点弯曲试验和硬度测试等方法,研究了不同焊速下AA7B04铝合金接头的微观组织和力学性能.结果表明:当焊速为100 mm·min-1时,接头的抗拉强度最高为447 MPa,可达母材的80%,且所有接头的正弯和背弯180°均无裂纹;接头横截面的硬度分布呈W型,硬度最低点出现在热力影响区和焊核区的交界处,焊速不同会导致不同的焊接热循环,且随着焊速的增加接头的硬度随之增加;焊核区组织发生了动态再结晶,生成了细小的等轴晶粒,前进侧和后退侧热力影响区的晶粒均发生了明显的变形;前进侧热影响区析出η'相,后退侧热影响区因温度较高析出η'相和尺寸较大的η相.      

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.     

A new finite element model for welding heat sources

A mathematical model for weld heat sources based on a Gaussian distribution of power density in space is presented. In particular a double ellipsoidal geometry is proposed so that the size and shape of the heat source can be easily changed to model both the shallow penetration arc welding processes and the deeper penetration laser and electron beam processes. In addition, it has the versatility and flexibility to handle non-axisymmetric cases such as strip electrodes or dissimilar metal joining. Previous models assumed circular or spherical symmetry. The computations are performed with ASGARD, a nonlinear transient finite element (FEM) heat flow program developed for the thermal stress analysis of welds.* Computed temperature distributions for submerged arc welds in thick workpieces are compared to the measured values reported by Christensen¹ and the FEM calculated values (surface heat source model) of Krutz and Segerlind.² In addition the computed thermal history of deep penetration electron beam welds are compared to measured values reported by Chong.³ The agreement between the computed and measured values is shown to be excellent.     

Effect of arc oscillations on the temperature distribution and microstructure in GTA tantalum welds

A two-dimensional mathematical model was developed to calculate the thermal history in thin tantalum sheets, GTA welded with arc oscillations. The model, based on the finite difference solution of the unsteady heat flow equation, was employed to calculate the temperature distribution for several arc oscillation conditions. The obtained results were used to explain experimentally observed improved microstructures in the fusion zone achieved by welding with arc oscillations. Indications are given for using the mathematical model in choosing the optimum arc oscillation conditions required for improving the microstructure of the weld. He is presently on Sabbatical leave at Lewis Research Center Cleveland, OH 44135.     

Microstructure and Hardness of T250 Maraging Steel in Heat Affected Zone

 Electron beam (EB) welding was used in T250 maraging steel, microstructures of both base material and heat affected zone (HAZ) were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and microhardness was tested. The results showed that during EB welding, the HAZ of T250 maraging steel exhibited a continuous gradient structure. The microstructure of the entire HAZ, from fusion line, could be divided into four zones: fusion zone, overheated zone, transition zone, and hardened zone. The microhardness showed a distinct regularity in each area. The softest region was the fusion zone, whereas the hardest was the hardened zone. In the overheated zone, the hardness increased as the grain size decreased. Furthermore, in the transition zone, the hardness level dropped noticeably. The peak temperature during the thermal cycle had a great influence on the formation of reverted austenite and dissolution of the precipitated particles, which contributed a lot to the microstructure and hardness of this material.     

Electron and laser beam welding of high strain rate superplastic Al-6061/SiC composites

The welding characteristics of a fine-grained 6061 Al and three 6061/1, 5, and 20 pct SiC composites under high energy electron beam welding (EBW) and laser beam welding (LBW) were examined. The three composites exhibited high strain rate superplasticity (HSRS). The 6061 Al was more readily welded by EBW than by LBW, and the situation was reversed in the reinforced composites. In the reinforced composites, the fusion zone contained the once fully melted matrix and fully reacted SiC, and the heat affected zone (HAZ) contained the partially melted matrix and nearly unreacted SiC. This effect was particularly apparent in the 20 pct SiC composite. With increasing SiC content from 0 to 20 pct, the reflection of the laser beam decreased, and the melt viscosity increased due to the increasing amount of Al₄C₃ compounds. For the HSRS fine-grained 6061/20 pct SiC composite, there formed a sharp V-notch under EBW. The high viscosity or low fluidity of the melt inside the fusion zone of 6061/20 pct SiC resulted in incomplete backfill and notch formation. The postweld mechanical performance and joint efficiency both became seriously degraded. The original fine structures in the HSRS composites could not be restored after welding.     

Effect of Welding Speed on Gas Metal Arc Weld Pool in Commercially Pure Aluminum: Theoretically and Experimentally

Temperature and velocity fields, and weld pool geometry during gas metal arc welding (GMAW) of commercially pure aluminum were predicted by solving equations of conservation of mass, energy and momentum in a three-dimensional transient model. Influence of welding speed was studied. In order to validate the model, welding experiments were conducted under the similar conditions. The calculated geometry of the weld pool were in good agreement with the corresponding experimental results. It was found that an increase in the welding speed results in a decrease peak temperature and maximum velocity in the weld pool, weld pool dimensions and width of the heat-affected zone (HAZ). Dimensionless analyses were employed to understand the importance of heat transfer by convection and the roles of various driving forces in the weld pool. According to dimensionless analyses droplet driving force strongly affected fluid flow in the weld pool.     

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.     

国产A6N01铝合金型材MIG焊接头的微观组织与力学性能

利用金相观察、显微硬度测定、拉伸和弯曲性能测试等方法研究了A6N01-T5铝合金型材MIG焊接接头的显微组织和力学性能.结果表明：焊接接头焊缝中心金属为明显的激冷形成的铸态组织,呈等轴晶状;熔合区靠近焊缝侧的结晶形态为沿散热方向排列的柱状晶,邻近熔合区的热影响区晶粒粗化.焊缝中心处具有较高的显微硬度,在距离焊缝中心10~12 mm处的热影响区显微硬度值最低.国产A6N01-T5铝合金型材焊接接头抗拉强度达到欧洲标准DIN EN 288-4的要求.     

Effect of Backing Plate Thermal Property on Friction Stir Welding of 25-mm-Thick AA6061

By using backing plates made out of materials with widely varying thermal diffusivity this work seeks to elucidate the effects of the root side thermal boundary condition on weld process variables and resulting joint properties. Welds were made in 25.4-mm-thick AA6061 using ceramic, titanium, steel, and aluminum as backing plate (BP) material. Welds were also made using a “composite backing plate” consisting of longitudinal narrow strip of low diffusivity material at the center and two side plates of high diffusivity aluminum. Stir zone temperature during the welding was measured using two thermocouples spot welded at the core of the probe: one at the midplane height and another near the tip of the probe corresponding to the root of the weld. Steady state midplane probe temperatures for all the BPs used were found to be very similar. Near root peak temperature, however, varied significantly among weld made with different BPs all other things being equal. Whereas the near root and midplane temperature were the same in the case of ceramic backing plate, the root peak temperature was 318 K (45 °C) less than the midplane temperature in the case of aluminum BP. The trends of nugget hardness and grain size in through thickness direction were in agreement with the measured probe temperatures. Hardness and tensile test results show that the use of composite BP results in stronger joint compared to monolithic steel BP.     

钛基非晶合金电子束焊接热力耦合模拟及非晶化

根据电子束焊接焊缝形貌特征及其深宽比大等特点,选用复合热源作为热源模型.通过线性插值等方式估计材料热力学参数随温度变化,模拟Ti基非晶合金电子束焊接温度场.模拟结果与实际焊缝取得良好的吻合,验证了热源模型的准确性.获得一定变量参数下电子束焊接钛基非晶合金温度场及热循环曲线.在温度场的基础上再进行焊接应力场的模拟,获得残余应力分布曲线.实验验证整个焊件没有晶化相析出,验证了该焊接工艺的可行性.      

预热对铍环激光束钎焊过程的影响研究

研究预热对铍环激光束钎焊过程温度场和应力场分布的影响。采用轴对称模型和热力解耦的有限元方法，并假定沉积到钎缝表面的激光束能量为Gauss分布，预热通过在焊接加热前添加一个能量密度低、有效加热半径大的单独工况实现。结果表明，预热使镀环钎缝外表面焊接最高温度增加，温度梯度减小，但焊深明显增加；采用预热工况焊接后，钎缝附近塑性变形区焊接残余应力明显减小，而热影响区残余应力增大。从整体分布来看，预热使铍环外表面焊接残余应力分布均匀化。对铍环外表面钎缝附近焊接残余应力进行X射线应力测试，并与有限元分析结果对比，二者应力变化趋势基本一致。     

Solidification Microstructure of Aluminum AA 6016-T4 Resistance Spot Welds Using a Short-Pulse Technique

The influence of the duration of the current pulse on the solidification microstructure of resistance spot welded (RSW) samples of aluminum alloy 6016-T4 with the short-pulse technique was investigated through experiments and numerical modeling. Microstructure was analyzed in terms of morphology and size, and primary and secondary dendrite arm spacing were measured on experimental samples. A reduction in pulse width resulted in a fine, columnar-dendritic microstructure in the outer regions of the fusion zone as well as a larger equiaxed-dendritic zone in the fusion zone center. A two-dimensional, axisymmetric finite element model of the spot welding process with new methods for calculation of the solidification parameters G (thermal gradient in the solid behind the solid–liquid interface) and R (velocity of the solid–liquid interface) was used for investigation of influence of pulse time on solidification microstructure and comparison to experimental results. Morphological trends in the solidification structure showed good agreement between experiments and simulations, and the influence of the pulse duration on the solidification parameters evolved because of changing heat transfer conditions. Simulated solidification data suggest that the solidification of aluminum during RSW falls in the regime of rapid solidification.

     

Computer modeling of heat flow in welds

This paper summarizes progress in the development of methods, models, and software for analyzing or simulating the flow of heat in welds as realistically and accurately as possible. First the fundamental equations for heat transfer are presented and then a formulation for a nonlinear transient finite element analysis (FEA) to solve them is described. Next the magnetohydrodynamics of the arc and the fluid mechanics of the weld pool are approximated by a flux or power density distribution selected to predict the temperature field as accurately as possible. To assess the accuracy of a model, the computed and experimentally determined fusion zone boundaries are compared. For arc welds, accurate results are obtained with a power density distribution in which surfaces of constant power density are ellipsoids and on radial lines the power density obeys a Gaussian distribution. Three dimensional, in-plane and cross-sectional kinematic models for heat flow are defined. Guidelines for spatial and time discretization are discussed. The FEA computed and experimentally measured temperature field,T(x, y, z, t), for several welding situations is used to demonstrate the effect of temperature dependent thermal properties, radiation, convection, and the distribution of energy in the arc.