The origin of weld seam defects related to metal flow in the hot extrusion of aluminium alloys EN AW-6060 and EN AW-6082

Longitudinal weld seams are an intrinsic feature in hollow extrusions produced with porthole dies. As these joins occur along the entire extruded length, it is desirable that these weld seams have a minimal impact on the structural integrity of the extrudate. In particular, defects associated with weld seam formation should be avoided. In this research, the occurrence of defects related to material flow inside the extrusion tooling is studied. In lab-scale experiments, EN AW-6060 and EN AW-6082 aluminium alloy billets are formed into strips by means of the direct hot extrusion process. By utilising model dies with an internal obstruction similar to the supports present in porthole dies, a strip with a central longitudinal weld seam is formed. The effects of different geometries of the weld-chamber and the processing conditions on the quality of the weld seam are investigated. Characterisation is performed through mechanical testing, focusing on the ability of the weld seam area to accommodate plastic deformation, and microstructural analysis provides insight into the defects related to unsound metal flow. Through computer simulations, conditions related to weld seam formation are modelled and correlated with the experimental results. The experimental results demonstrate that metal flow controlled by the die geometry causes defects leading to inferior mechanical performance of the extrudate. It is further argued that current weld seam formation criteria utilised in finite element modelling need enhancement to incorporate these flow related defects.     

Material flow and void defect formation in friction stir welding of aluminium alloys

An ingenious experimental programme by combining artificially thickened oxide layer as marker material and ‘stop-action’ welding were used to study the material flow and defect formation in friction stir welding of aluminium alloys. The results showed that material flow around the pin on the advancing side (AS) was severer than that on the retreating side (RS) and the fastest velocity of material flow in the middle stir zone (SZ) was 43.9?mm?s^?1. Moreover, the material under the RS shoulder included extruded metal only and the material under the AS shoulder included extruded and rotated metal. Lastly, instantaneous void occurrence and insufficient inflow material were reasons for the preferential formation of void defects in the top SZ on the AS.     

汽车后桥半轴挤压成形工艺数值模拟与优化

对实际生产中的锻造工艺进行了计算机数值模拟。通过虚拟地修改工艺参数，完成了汽车后桥半轴锻造工艺的优化。     

An integrated study on the effect of pre- and post-extrusion heat treatments and surface treatment on the filiform corrosion properties of an aluminium extrusion alloy

The effect of pre- and post-extrusion heat treatments on the filiform corrosion behaviour of a well-known aluminium extrusion alloy AA6063 is studied by a combination of accelerated filiform corrosion exposure tests and potentiodynamic polarisation measurements for four different surface treatments. It is shown that the post-extrusion heat treatment of this AlMgSi alloy can influence the filiform corrosion properties significantly, in particular for the milder surface treatments. In contrast, the relative effect of the pre-extrusion treatment on the filiform corrosion properties is minimal for all treatments. The alloys are most susceptible to filiform corrosion in the β′ condition. The susceptibility decreases with coarsening of the Mg₂Si particle distribution. For the post-extrusion heat and surface treated AA6063 material a clear correlation between the polarisation characteristics and the two principal filiform corrosion characteristics, i.e. the propagation rate and total area of attack after accelerated exposure, is observed. The observed correlation is attributed to a pitting corrosion mechanism with a rate depending on the (coupled) Mg₂Si precipitate size and fraction.     

Applying the finite-element method to determine the die swell phenomenon during the extrusion of glass rods with non-circular cross-sections

In the case of die-wall adhesion during the extrusion of inorganic glass melts, the emerging rod undergoes an undesired expansion, called the die-swell phenomenon, which impairs the precision of its shape. In order to predict the cross-sectional increase of the rod, a 3-dimensional finite-element model has been developed, which can replace extensive experimental preparatory trials. The results obtained with the aid of computer-based simulated flow show a high level of agreement with corresponding experimental results and confirm the suitability of the procedure presented in this paper. The analysis of the influence that the most important test parameters and some fluid properties have on the die swell, supplies valuable information for extrusion processes currently in use. Furthermore, the suitability of the finite-element method in the case of different die cross-sections as well as in the case of the pull-extrusion process is demonstrated in an exemplary manner.     

FE analysis of metal flow and weld seam formation in a porthole die during the extrusion of a magnesium alloy into a square tube and the effect of ram speed on weld strength

The present communication concerns a detailed analysis of metal flow into a porthole extrusion die to produce a thin-walled square magnesium tube by means of three-dimensional FE simulation in both the transient state and steady state. The research was aimed to get an insight into the longitudinal weld seam formation during extrusion through porthole dies and to evaluate the factors determining the quality of the weld seams. FE simulation revealed distinctive stages at the beginning of an extrusion cycle, corresponding to the changes in extrusion pressure during the process. It showed that the commonly observed defect at the extrudate head was due to entrapped air under the bridges in the upper part of the welding chamber. The dead metal zones existed at the corners between the container and die face and between the bottom and sidewall of the welding chamber. Because of the friction at the die bearing, the metal flow through the die bearing resembled laminar flow. Only the virgin metal from the interior of the billet flowed along the bridges and formed the welding seams. As ram speed increased, the mean stresses and temperatures on the welding plane in the welding chamber increased, which was reflected in the increases in extrusion pressure and extrudate temperature, being beneficial to the solid-state bonding at the weld seams. Tensile tests confirmed that extrusion at a higher ram speed led to enhanced transverse tensile strength and strain of the extruded square tube, as a result of improved bonding at the longitudinal weld seams.     

Behaviour of different bare and anodised aluminium alloys in the atmosphere

Exposure tests in natural atmospheres are an indispensable means for determining the behaviour and durability of metallic materials in the atmosphere. The corrosion behaviour of bare aluminium and anodised aluminium with three different coating thicknesses has been evaluated for two years' exposure in two natural atmospheres of very different corrosivities: one urban and the other marine. Several techniques have been used to evaluate changes in the specimens during exposure, but special attention is paid to the direct measurement of corrosion by gravimetry and its indirect estimation by the comparatively much more sensitive electrochemical impedance spectroscopy (EIS) technique. The results show that if no demands are placed on the conservation of its appearance, aluminium may be used without protection even in atmospheres of medium or high corrosivity. The anodising and sealing of aluminium alloys, above an ill-defined minimum thickness threshold, is an appropriate solution to prevent localised corrosion of aluminium and to conserve its appearance, even in aggressive atmospheres.     

Incorporation of transition metal ions and oxygen generation during anodizing of aluminium alloys

Enrichment of nickel at the alloy/film interface and incorporation of nickel species into the anodic film have been examined for a sputtering-deposited Al-1.2at.%Ni alloy in order to assist understanding of oxygen generation in barrier anodic alumina films. Anodizing of the alloy proceeds in two stages similarly to other dilute aluminium alloys, for example Al-Cr and Al-Cu alloys, where the Gibbs free energies per equivalent for formation of alloying element oxide exceeds the value for alumina. In the first stage, a nickel-free alumina film is formed, with nickel enriching in an alloy layer, 2 nm thick, immediately beneath the anodic oxide film. In the second stage, nickel atoms are oxidized together with aluminium, with oxygen generation forming gas bubbles within the anodic oxide film. This stage commences after accumulation of about 5.4 × 10¹⁵ nickel atoms cm⁻² in the enriched alloy layer. Oxygen generation also occurs when a thin layer of the alloy, containing about 2.0 × 10¹⁹ nickel atoms m⁻², on electropolished aluminium, is completely anodized, contrasting with thin Al-Cr and Al-Cu alloy layers on electropolished aluminium, for which oxygen generation is essentially absent. A mechanism of oxygen generation, based on electron impurity levels of amorphous alumina and local oxide compositions, is discussed in order to explain the observations.     

A universal formulation for the extrusion of sections with no axis of symmetry

The subject of theoretical analysis for the extrusion of profiled sections has been heavily worked on by many researchers. However most of these have concentrated on the extrusion of sections with some axes of symmetry. This paper aims to provide a new formulation for the analysis of the extrusion process for non symmetric profiled sections with no axis of symmetry. The upper bound theorem has been used to obtain an approximate solution. Both flat faced dies and bilinear dies were used in the present work. In fact this formulation could be applied to the extrusion process of hollow sections as well as solid parts. The advantage of the proposed method is the new way of discretizing the deforming region in the extrusion of shaped sections based on which a generalized kinematically admissible velocity field was formulated. For a given extruded profiled section this method employs fewer number of sub-divisions than before, giving a lower upper bound value for the extrusion power. The generality, easy application to different sections, short computing time and fairly realistic outcomes of the analysis could be regarded as other strengths of the present method. The lack of incorporation of temperature effects for hot extrusion processes could be mentioned as one of the disadvantages of the present method. The position of the die cavity was also optimized using the new method. A design map for the extrusion die geometry was also given as a graph of extrusion pressure versus the optimum die length for the bilinear dies with different frictional conditions and reductions of area. The extrusion of L-shape profiled section was chosen as a sample since it had no axis of symmetry. Extrusion dies were also manufactured and experiments carried out to verify the theoretical results. Finite element simulation for the extrusion of U, T and I-shape profiled sections was also carried out using ABAQUS\CAE\Explicit commercial software.     

Numerical modelling of the galvanic coupling in aluminium alloys: A discussion on the application of local probe techniques

A discussion is proposed on the determination of the input values and the experimental validation of finite element modelling of the galvanic coupling in aluminium alloys by local probe techniques such as the Scanning Vibrating Electrode Technique (SVET) and the microcapillary electrochemical cell (microcell). Polarization curves obtained by the microcell were introduced as input conditions in the model based on Laplace or Nernst-Planck equation. SVET measurements were performed to determine the coupling current distribution on an Al/Al4%Cu bimetallic system. Agreement was found between simulated and experimental current distributions depending on the input conditions and the solved equation.     

Sensitivity analysis of a finite element model for the simulation of stainless steel tube extrusion

In this work, a sensitivity analysis has been performed on a finite element model of glass-lubricated extrusion of stainless steel tubes. Fifteen model parameters, including ram speed, billet and tool temperatures, friction coefficients and heat transfer coefficients, were considered. The aim of the study was to determine the parameters that are most important for the response of the extrusion force. The relationship between the model parameters and the responses was analyzed by a calculation of two different regression models: one linear polynomial model and one model that includes interaction terms. Additional simulations were then carried out to validate the regression models. The results show that the initial billet temperature is the factor that has the strongest impact on the extrusion force within the parameter ranges studied in this work. The goodness of prediction and goodness of fit are very good for both regression models.     

A theoretical and experimental analysis of variances in weld bead morphologies

Predictable and reproducible weld bead morphologies and dimensions are a major concern in welding. In bead-on-plate welding, the heat flow is controlled by the heat source parameters (power, speed, and radius) and the physical properties and dimensions of the workpiece, especially its thickness. Complex models that account for weld pool circulation have been developed to quantify welds. However, to some extent, fluctuations in weld dimensions can be explained with conduction models of moving Gaussian heat sources. In early investigations with point and line heat sources, relationships between process parameters and plate thickness were derived to differentiate between two- and three-dimensional heat flow. To date, the heat source radius (R) has not been taken into account. The dimensionless ratio (D*) of the plate thickness (D) to the heat source radius (R) is actually a variable to consider. With the introduction of additional dimensionless parameters (*)—speed (v*), power(q*)— relationships among governing variables, heat flow dimension, and weld bead dimensional fluctuations can be derived. Weld bead fluctuations are found to depend on dimensionless variables (v*, q*, D* ) and occur when the heat flow dimension is intermediate between two- and three-dimensional. Occasionally, experimental data exhibit trends that differ from predictions. This article presents a dimensionless version of a heat flow model and discusses the discrepancies between experimental and theoretical results. Formerly with ECN.     

The use of macroscopic modelling of intermetallic phases in aluminium alloys in the study of ferricyanide accelerated chromate conversion coatings

Intermetallic (IM) second phases of FeAl₃, Cu₂FeAl₇, and CuAl₂, were coupled to aluminium for the macroscopic study of the deposition of chromate conversion coatings. Characterisation of the coating deposition was done using X-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, Auger electron spectroscopy, scanning electron microscopy with X-ray analysis, and Fourier transform infrared spectroscopy. The coatings covering the IM phases were only one-tenth the thickness of the matrix, and contained higher levels of F, Al, and O. Cr, O, Fe, and N, indicative of a chromate conversion coating, were detected over the matrix. Over IM phases, decomposition of [Fe(CN)₆]3−/4−, and fluoride ion attack were found to be responsible for reduced rates of deposition.     

Real-time corrosion monitoring of aluminium alloys under chloride-contaminated atmospheric conditions

In this study, the use of electrical resistance (ER) sensors to monitor the corrosion of Al₉₄Cu₆ alloy is assessed and compared with 2024-T3 coupons. Under uniform corrosion, a good correlation was found between the ER sensors and mass loss on coupons. Three different chloride depositions are studied: (i) pre-contamination with dry/wet cycles, (ii) Volvo standard accelerated corrosion test and (iii) neutral salt spray test. The obtained results show good reproducibility of the ER sensors under all tested conditions. This suggests that ER sensors more levelled the effect of localised corrosion through a large surface evaluation compared with cross-sections. The corrosion thickness obtained with the ER sensors does not correspond to the mean depth obtained by cross-sections. This can be explained by the distribution and size of the localised corrosion events according to a finite element model proposed. The ER method allows obtaining useful real-time corrosion data for the understanding of the corrosion mechanisms and the development of accelerated tests. The chloride concentration, the frequency of salt application and wet/dry cycles have a strong influence on the corrosion rate of aluminium alloys.     

基于DEFORM的药筒坯料选择数值模拟分析

通过采用先进的有限元成形分析软件DEFORM-3D,对某旋压药筒热成形的坯料形状选择进行数值模拟研究,并在此基础上进行实验验证,将毛坯尺寸定为覬80mm×25,减少前期实际试验,节约成本,缩短工艺开发周期,为解决旋压药筒热成形工艺设计和实际生产问题提供依据。     

Measurement and modelling of the nanoindentation response of shape memory alloys

A nickel–titanium shape memory alloy was subjected to nanoindentation over a range of temperature (up to 200 °C), such that the starting material was either predominantly martensitic or largely composed of the parent phase. The load–displacement data were interpreted to give information about whether the imposed strain was being at least partly accommodated by the martensitic phase transformation, i.e. whether superelastic deformation was taking place. This interpretation was assisted by finite element simulation of the evolving strain field under an indenter, with or without the superelastic deformation mechanism being operative. It is concluded that the nanoindentation response can be used to determine whether the material is capable of exhibiting superelastic deformation, provided appropriate procedures are employed. Spherical indenters are more suitable than sharp tips. A relatively low value for the remnant indent depth ratio (depth after unloading/depth at peak load) is indicative that superelasticity is occurring. The procedure was found to be viable with a small radius (10 μm) spherical indenter, so it can be employed to explore local variations in superelastic response.     

基于HyperXtrude的ZK60镁合金型材挤压成形数值模拟和实验验证

应用Hyper Xtrude软件对ZK60镁合金空心型材的挤压成形过程进行了有限元数值模拟研究,分别得到了型材在稳态挤压下的温度场、速度场、应变场和位移场,分析了金属流动情况、焊合室入口和出口处的压力。通过不同挤压速度下的稳态模拟分析,确定了合适的型材挤压速度为10 mm·s-1。在挤压温度为350℃和挤压速度为10 mm·s-1条件下进行实验验证,得到了形状外观均合格的产品。对比分析发现,模拟结果与实验结果的型材断面宏观组织形貌具有一致性,证明了应用Hyper Xtrude软件可以有效预测镁合金型材成形过程中的金属流动和焊合情况。     

液压集成块内弯曲流道流场数值计算与分析

本文对液压技术中广泛应用的液压集成块中常见的“S”型流道的流场用有限元方法进行数值计算,并将计算所获得的流场的结构以可视化的图像形式给出,为分析流道中的能量损失和优化设计流道提供了理论依据。     

3D visualization of the material flow in friction stir welding process

0IntroductionThe friction stir welding(FSW),a new solid-statewelding process invented by TWI in1991,is perhaps themost remarkable and potentially useful new welding tech-nique.In the welding process,the base metal doesn’tmelt,so great advantages can be a…     

Analysis of the cracks formation on surface of extruded magnesium rod based on numerical modeling and experimental verification

To reduce the surface cracks of extrusion rod for AZ31 magnesium caused by nonho-mogeneous metal flow in extrusion process, 3D computer finite element (FE) simulations of extruding a wrought magnesium alloy AZ31 into rods have been performed and the results have been verified in extrusion experiments under identical conditions. The tendency to generate the dead zone is decreased by employing the die angle 60° at the cone-shaped die comparing with the die angle 180°. The surface additional tensile stresses o...