Jean-Marie Drezet

In-situ Observation of Hot tearing Formation in Succinonitrile-Acetone

Abstract Hot tears have been induced during the solidification of a succinonitrile-acetone alloy by pulling the columnar dendrites in the transverse direction with a pulling stick. The opening of the mushy zone (hot tears) always occurred at grain boundaries. At low volume fraction of solid, the opening can be compensated by leaner-solute interdendritic liquid (i.e., “healed” hot tears). At higher volume fraction of solid, hot tears directly nucleate in the interdendritic liquid or develop from pre-existing micropores induced by solidification shrinkage. Their surface (edge) is made of secondary dendrite arms, which have not yet bridged, but a few spikes have also been observed. These later spikes formed either by the necking of solid bridges established across the grain boundaries prior to pulling, or by the sudden break-up of the liquid film during pulling. Similar spikes have been found by SEM on the hot tear surface of an aluminium–copper alloy.

Modeling of ingot distortions during direct chill casting of aluminum alloys

A comprehensive three-dimensional (3-D) mathematical model based upon the ABAQUS software has been developed for the computation of the thermomechanical state of the solidifying strand during direct chill (DC) casting of rolling sheet ingots and during subsequent cooling. Based upon a finiteelement formulation, the model determines the temperature distribution, the stresses, and the associated deformations in the metal. For that purpose, the thermomechanical properties of the alloy have been measured up to the coherency temperature using creep and indentation tests. The thermophysical properties as well as the boundary conditions associated with the lateral water spray have been determined using inverse modeling. The predicted ingot distortions, mainly, “butt curl,” “butt swell,” and lateral faces pull-in, are compared with experimental measurements performed during solidification and after complete cooling of the ingot. Particular emphasis is placed on the nonuniform contraction of the lateral faces. The influence of the mold shape and the contributions to this contraction are assessed as a function of the casting conditions.

Modeling of microsegregation in macrosegregation computations

A general framework for the calculation of micro-macrosegregation during solidification of metallic alloys is presented. In particular, the problems of back diffusion in the primary solid phase, of eutectic precipitation at the end of solidification, and of remelting are being addressed for an open system,i.e., for a small-volume element whose overall solute content is not necessarily constant. Assuming that the variations of enthalpy and of solute content are known from the solution of the macroscopic continuity equations, a model is derived which allows for the calculation of the local solidification path (i.e., cooling curve, volume fraction of solid, and concentrations in the liquid and solid phases). This general framework encompasses four microsegregation models for the diffusion in the solid phase: (1) an approximate solution based upon an internal variable approach; (2) a modification of this based upon a power-law approximation of the solute profile; (3) an approach which approximates the solute profile in the primary phase by a cubic function; and (4) a numerical solution of the diffusion equation based upon a coordinate transformation. These methods are described and compared for several situations, including solidification/remelting of a closed/open volume element whose enthalpy and solute content histories are known functions of time. It is shown that the solidification path calculated with method 2 is more accurate than using method 1, and that 2 is a very good approximation in macrosegregation calculations. Furthermore, it is shown that method 3 is almost identical to that obtained with a numerical solution of the diffusion equation (method 4). Although the presented results pertain to a simple binary alloy, the framework is general and can be extended to multicomponent systems.

Experimental investigation of thermomechanical effects during direct chill and electromagnetic casting of aluminum alloys

The deformation and the temperature field within direct chill (DC) and electromagnetic (EM) cast aluminum ingots have been measuredin situ using a simple experimental setup. The deformation of the cross section of the cold ingots has 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 electromagnetic casting (EMC) was continuous. The pull-in was maximum at the center of these faces (about 7 to 9 pct) and strongly depended upon the casting speed. Near the short sides of the ingots, the deformation was only about 2 pct and was nearly independent of the casting parameters and alloy composition. Based upon these measurements, it was concluded that the pull-in of the rolling faces was mainly due to the bending of the ingots induced by the thermal stresses. This conclusion was further supported by a simple two-dimensional thermoelastic model.

The Computation and Measurement of Residual Stresses in Laser Deposited Layers

Laser metal forming is an attractive process for rapid prototyping or the rebuilding of worn parts. However, large tensile stress may arise in layers deposited by laser melting of powder. A potential solution is to preheat the substrate before and during deposition of layers to introduce sufficient contraction during cooling in the substrate to modify the residual stress distribution in the deposited layers. To demonstrate the value of this approach, specimens were prepared by depositing stellite F on a stainless steel substrate with and without preheating. Residual stresses were computed by numerical simulation and measured using the crack compliance method. For non-preheated specimens simulation and experiment agreed well and showed that extremely high residual tensile stresses were present in the laser melted material. By contrast, pre-heated specimens show high compressive stresses in the clad material. However, in this case the numerical simulation and experimental measurement showed very different stress distribution. This is attributed to out of plane deformation due to the high compressive stresses which are not permitted in the numerical simulation. A ‘‘strength of materials’’ analysis of the effect of out of plane deformation was used to correct the simulation, Agreement with experimental results was then satisfactory.

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.

Early precipitation during cooling of an Al-Zn-Mg-Cu alloy revealed by in situ small angle X-ray scattering

Early subnanometre cluster formation during quenching of a high-strength AA7449 aluminium alloy was investigated using in situ small angle X-ray scattering. Fast quench cooling was obtained by using a laser-based heating system. The size and number density of homogeneous nucleated clusters were found to be strongly dependent on the cooling rate, while the volume fraction of cluster formation is independent of the cooling rate. Heterogeneous larger precipitation starts at higher temperatures in volume fractions that depend on the cooling rate.

Stress-Strain Predictions of Semisolid Al-Mg-Mn Alloys During Direct Chill Casting: Effects of Microstructure and Process Variables

The occurrence of hot tearing during the industrial direct chill (DC) casting process results in significant quality issues and a reduction in productivity. In order to investigate their occurrence, a new semisolid constitutive law (Phillion et al.) for AA5182 that takes into account cooling rate, grain size, and porosity has been incorporated within a DC casting finite element process model for round billets. A hot tearing index was calculated from the semisolid strain predictions from the model. This hot tearing index, along with semisolid stress–strain predictions from the model, was used to perform a sensitivity analysis on the relative effects of microstructural features (e.g., grain size, coalescence temperature) as well as process parameters (e.g., casting speed) on hot tearing. It was found that grain refinement plays an important role in the formation of hot cracks. In addition, the combination of slow casting speeds and a low temperature for mechanical coalescence was found to improve hot tearing resistance.

Fracture prediction during sawing of DC cast high strength aluminium alloy rolling slabs

Abstract The semicontinuous direct chill (DC) casting of large cross-section rolling sheet ingots of high strength aluminium alloys (2xxx and 7xxx series) gives birth to high residual (internal) stresses generated by a non-uniform cooling. These stresses must be relieved by a thermal treatment in order to be able to safely saw both ingot butt and head. Otherwise, saw pinching or blocking might occur due to the compressive residual stresses, or cut parts might be brutally released by erratic propagation of a crack ahead of the saw groove thus injuring people or damaging equipment. As these high added value ingots must be produced in secure conditions, a better control of the sawing procedure is required, which could allow the suppression of the thermal treatment and therefore save time and energy. By studying the stress build-up during casting and cooling then the stress relief during sawing operations of rolling sheet ingots, key parameters for the control and optimisation of the processing steps can be derived. To do so, the DC casting of the high strength AA2024 alloy is modelled with ABAQUS 6·5 with special attention to the thermomechanical properties of the alloy. The sawing operation is then simulated by removing mesh elements such as to reproduce the progression of the saw in the ingot. Preliminary results showing the stress relief during sawing accompanied by the risk of saw blocking due to compression or initiating a crack ahead of the saw are analysed with an approach based on the rate of strain energy release.

A new interpretation of bulge test measurements using numerical simulation

Analysis of the deflection of a circular membrane under differential pressure (bulge test) is a well-known method of determining the elastic properties of thin films. However, analytical models always suffer from simplifying hypotheses. In this study we present a new approach, based on numerical modeling, to interpret pressure-deflection curves. By adjusting Young’s modulus and Poisson’s ratio in the simulation program, it is possible to reproduce the experimental curves faithfully. The method was successfully tested with two different materials (silicon and aluminium) with known elastic properties and was then used to determine biaxial Young’s moduli of CVD diamond thin films for three different microstructures. The values of E varied from 565 to 620 GPa (assuming a Poisson ratio of 0.1). Grain boundaries are thought to be responsible for the relatively low values of Young’s moduli. Uncertainties in E are relatively large (lo%-15%) because the method is highly sensitive to experimental parameters such as thickness or membrane diameter and to the initial residual stress state which is known only approximately.