Predicting Hot Tearing in Aluminium Castings

In many casting processes, some stages, such as the starting phase in the direct chill casting of aluminium alloys, are particularly critical because of the propensity of some alloys to develop either hot tears which initiate at non zero liquid fraction, or cold cracks which nucleate and grow exclusively in the solid metal. Figure 1 shows a cracked rolling sheet ingot: two hot tears initiated during the starting phase of casting have grown up as the ingot was further cast. For some very sensitive alloys, up to 10 % of the ingots present some cracks and have to be remelted

Direct chill and electromagnetic casting of aluminium alloys: Thermomechanical effects and solidification aspects

The tolerances of semi-continuously cast products of aluminium alloys are very critical if the scalping of the ingot faces is to be minimised before rolling. In the steady state regime of casting, the dimensions of the section of the solidified ingot are lower than those of the mould in the Direct Chill Casting (DCC) or of the inductor in the Electromagnetic Casting (EMC). The contraction of the section, several percents, is larger than the value associated with the thermal contraction of the solid and is also inhomogeneous: the short sides of the ingot contract less than the centre of the rolling faces. In order to study and to understand the mechanisms responsible for such deformation, insitu measurements and laboratory investigations have been performed whereas a thermomechanical model has been developed. The specific points of this study are as follows: measurements of the distortions undergone by the ingot during casting and after complete cooling, determination of the thermal boundary conditions corresponding to the primary cooling (contact in between the metal and the mould) and secondary cooling (water jet), using in-situ temperature measurements and inverse modelling, measurements of the thermal conductivity of two industrial alloys using inverse modelling and one dimensional casting experiments, measurements of the thermomechanical behaviour of two industrial alloys, specifically the elastic modulus by ultrasonic method, the thermal expansion coefficient by dilatometry and the creep behaviour in the solid and mushy state using tensile and indentation tests, study of the solidification path of the AA1201 alloy using a finite difference microsegregation model coupled with the Al-Fe-Si phase diagram data; the results have been validated against DTA measurements and extended to situations encountered in the DC/EM casting process, computation of the temperature, stress and strain fields in DC and EM-cast ingots with the help of the 2D and 3D thermomechanical finite element code Abaqus; the final ingot cross section for a given mould/inductor design has been calculated, comparison of the simulation results with the experiments. The mechanisms responsible for the main ingot distortions undergone during casting and subsequent cooling, notably the non-uniform contraction of the ingot cross section, have been identified. Finally, an inverse thermomechanical method for the optimisation of the mould/inductor design is proposed based upon a criterion of maximum flatness of the final ingot. Such a method should allow a reduction in the costs associated with the definition of the mould/inductor designs for new casting conditions. The present work will be extended to more complicated geometries in the BriteEuram project EMPACT (European Modelling Programme on Aluminium Casting Technology)

Validation of a new hot tearing criterion using the ring mould test

Hot tear is one of the most serious defect which a casting can suffer. It represents a major limitation to the production of foundry cast parts and to the productivity of continuous casting processes such as the direct chiil casting of aluminium alloys. As an exemple, the starting phase of the direct chili casting process remains particularly critical for some aluminium alloys because of their high propensity to develop either hot tears which initiate at non zero liquid fraction, or cold cracks which nucleate and grow exclusively in the solid metal. In order to validate a new hot tearing criterion recently proposed by Rappaz and Drezet [1), instrumented ring mould tests were carried out with aluminium alloys of different composition and further on analysed from a thermal, mechanical and microstructural point of view. The thermal field obtained in the test was determined with the help of an inverse method using five temperature histories measured at different locations and the stress build-up was computed with a transient thermomechanical model implemented in the finite element package Abaqus. Deformation in the solid was assumed to obey a viscoplastic law and the cooling conditions were those deduced by the inverse method. The new hot tearing criterion [1] based on the ability of the interdendritic flow of liquid to compensate for the thermally induced deformation of the roots of columnar dendrites, allowed the calculation of the maximum strain rate that the roots of the dendrites can undergo without initiation and/or propagation of hot tears. After implementation in the numerical FEM model of the ring mould test, the hot tearing criterion predicted the occurrence of tears precisely in the region where hot cracks were observed after the test. More generally, when implemented in thermomechanical models of casting processes, the present hot tearing criterion would be very helpful in diminishing the cracking tendency

High Apparent Creep Activation Energies in Mushy Zone Microstructures

Modelling represents an important tool in modern material processing which no longer follows the traditional trial and error route but rather represents what may be termed a right first time technology [1]. To successfully model technological solidification processes, thermodynamic and kinetic data are required. But mechanical aspects are important as well [2]: during solidification, temperature gradients or mechanical constraints imposed by the mold result in solidification stresses. These stresses must be considered for at least the following two reasons: first, they can lead to local air gap formation between metal and mold thus changing heat extraction, cooling rate and finally the cast microstructure [3]; second, at a larger scale they may influence the final product shape [4]. Moreover, they can assist in cavity formation and can produce cracking. Such stresses become important as soon as a significant amount of solid phase has formed during solidification. In principle, these stresses can be calculated using viscoelastic finite element stress analysis [5]. But, finite element calculations require as an input the constitutive law which governs the mechanical behavior. Therefore, there is an interest in mechanical data of solidifying alloys with mushy zone microstructures: Ackermann and Kurz [6] investigated the mechanical properties of a solidifying AIMg alloy perpendicular to the growth axis of the columnar crystals. The tensile behavior of solidifying AI-Cu alloys was studied by Wisniewski [7] and recently, Branswyck [8] proposed a modified indentation test which, in combination with FEM analysis, yields quantitative flow rules. Nevertheless, there is still a need for more mechanical data of solidifying alloys, especially creep data - where strain accumulates at a constant stress - only rarely exist for processing conditions

Marangoni Convection and Fragmentation in Laser Heat Treatment

Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto a single crystal substrate. It is a near net-shape process for rapid prototyping or repair engineering of single crystal high pressure/high temperature gas turbines blades. Single crystal repair using E-LMF requires controlled solidification conditions in order to prevent the nucleation and growth of crystals ahead of the columnar dendritic front, i.e., to ensure epitaxial growth and to avoid the columnar to equiaxed transition. A major limitation to the process lies in the formation of stray grains which can originate either from heterogeneous nucleation ahead of the solidification front or from remelting of dendrite arms due to local solute enriched liquid flow, .i.e fragmentation. To study this last aspect, heat and fluid flow modelling is required to establish the relationship between process parameters such as laser power, beam diameter and scanning speed, and the local solidification conditions plus the fluid flow in the vicinity of the mushy zone. Surface tension driven convection known as the Marangoni effect needs to be included in the model owing to its large influence on the development of eddies and on the shape of the liquid pool. The 3D model implemented in the FE software calcosoft® is used to compute the fluid convection within the liquid pool and to assess the risk of fragmentation using a criterion based on the local velocity field and thermal gradient. The computed results are compared with EBSD maps of laser traces carried out at EPF-Lausanne in re-melting experiments

As-Cast Residual Stresses in an Aluminum Alloy AA6063 Billet: Neutron Diffraction Measurements and Finite Element Modeling

The presence of thermally induced residual stresses, created during the industrial direct chill (DC) casting process of aluminum alloys, can cause both significant safety concerns and the formation of defects during downstream processing. Although numerical models have been previously developed to compute these residual stresses, most of the computations have been validated only against measured surface distortions. Recently, the variation in residual elastic strains in the steady-state regime of casting has been measured as a function of radial position using neutron diffraction (ND) in an AA6063 grain-refined cylindrical billet. In the present study, these measurements are used to show that a well-designed thermomechanical finite element (FE) process model can reproduce relatively well the experimental results. A sensitivity analysis is then carried out to determine the relative effect of the various mechanical parameters when computing the as-cast residual stresses in a cylindrical billet. Two model parameters have been investigated: the temperature when the alloy starts to thermally contract and the plasticity behavior. It is shown that the mechanical properties at low temperatures have a much larger influence on the residual stresses than those at high temperatures

Thermomechanical Effects during Direct Chill and Electromagnetic Casting of Aluminum Alloys. Part I : Experimental Investigation

The deformation and the temperature field evolution within direct chiil (DC) and electromagnetic (EM) cast aluminum ingots have been measured in-situ using a simple experimental set-up. The deformation of the cross section of the cold ingots bas also been characterized as a function of the casting speed, alloy composition and inoculation condition. The pull-in of the lateral rolling faces has been found to occur in two sequences for DC cast ingots whereas that associated with EMC was continuous. The pull-in vas maximum at the center of these faces (about 7-9 %) and strongly depended upon the casting speed. The present resuits constitute a basis for the validation of the model presented in part II

Experimental and numerical characterisation of heat flow during flame cutting of thick steel plates

Temperatures measurements during flame cutting of a thick steel plate and measurements of the extension of the fusion and heat affected zones and Vickers hardness after cutting have been performed. Additionally, a 3-D thermal model for simulation of flame-cutting has been developed. For the sake of simplicity, the model depends only on two parameters: i) the heat density within the flame, and ii) the heat transfer coefficient within the air gap that forms behind the cut. The results show that the model is able to properly reproduce the measured temperature curves and the heat affected zone with an input power in the same range of those reported in the literature. A process efficiency of 26.5% is found in the steady state regime of flame-cutting

Contraintes Résiduelles dans les Billettes d’Aluminium : Mesures par Diffraction de Neutrons et Simulation Thermomécanique

Les contraintes internes qui apparaissent dans les billettes d’aluminium produites par coulée semi-continue trouvent leur origine dans le différentiel du chemin thermomécanique en coeur ou en surface de la billette et posent de sérieux problèmes de sécurité lors de la découpe. Un modèle thermomécanique de la coulée semi-continue a été validé en termes de contraintes résiduelles, ces dernières étant mesurées par diffraction des neutrons. Les composantes radiales, orthoradiales et axiales du tenseur des contraintes ont été mesurées le long du rayon d’une billette de diamètre 320 mm

Numerical modelling and experimental investigation on welding residual stresses in large-scale tubular K-joints

This paper is devoted to the experimental and numerical assessment of residual stresses created by welding in the region surrounding the weld toe of tubular K-shaped joints (i.e. region most sensitive to fatigue cracking). Neutron-diffraction measurements were carried out on K-joints cut from large-scale truss beams previously subjected to high cycle fatigue. Tri-axial residual stresses in the transverse, longitudinal and radial direction were obtained from the weld toe as a function of the depth in the thickness of the tube wall. In addition, thermomechanical analyses were performed in three-dimensional using ABAQUS and MORFEO finite element codes. Experimental and numerical results show that, at and near the weld-toe surface, the highest residual stresses are critically oriented perpendicularly to the weld direction, and combined with the highest externally applied stresses. Based on a systematic study on geometric parameters, analytical residual stress distribution equations with depth are proposed