Impact of experimental conditions on material response during forming of steel in semi-solid state

Semi-solid forming is an effective near-net-shape forming process to produce components with complex geometry and in fewer forming steps. It benefits from the complex thixotropic behaviour of semi-solids. However, the consequences of such behaviour on the flow during thixoforming, is still neither completely characterized and nor fully understood, especially for high melting point alloys. The study described in this paper investigates thixoextrusion for C38 low carbon steel material using dies at temperatures much lower than the slug temperature. Four different process parameters were studied: the initial slug temperature, the die temperature, the ram speed and the presence of a ceramic layer at the tool/material interface. The extruded parts were found to have an exact shape and a good surface state only if the temperature was below a certain value. This critical temperature is not an intrinsic material property since its value depends on die temperature and the presence of the Ceraspray©layer. Two kinds of flow were highlighted: a homogeneous flow controlled by the behaviour of the solid skeleton characterized by a positive strain rate sensitivity, and a non homogeneous flow (macro liquid/solid phase separation) dominated by the flow of the free liquid. With decreasing ram speed, heat losses increase so that the overall consistency of the material improves, leading to apparent negative strain rate sensitivity. Finally, some ways to optimise thixoforming are proposed

Forging process control: Influence of key parameters variation on product specifications deviations

Process control in forging industry is essential to ensure a better quality of the product with a lower cost at the end of the manufacturing process. To control the process, a number of key parameters must be monitored to prevent product or forging plan deviations. This paper will illustrate how a variation in a process parameter can create product specifications deviations and how key parameters influence product final state. The illustration work is done on a part obtained via hot forging. An analysis is made on product parameters such as geometry, by varying the key process parameter values previously determined from a created methodology. This later is represented as a decision support system that connects product specifications (geometry, absence of defects…) or other forging specifications (tool wear, involved energy...) to the process parameters

Sampling by blocks of measurements in compressed sensing

Various acquisition devices impose sampling blocks of measurements. A typical example is parallel magnetic resonance imaging (MRI) where several radio-frequency coils simultaneously acquire a set of Fourier modulated coefficients. We study a new random sampling approach that consists in selecting a set of blocks that are predefined by the application of interest. We provide theoretical results on the number of blocks that are required for exact sparse signal reconstruction. We finish by illustrating these results on various examples, and discuss their connection to the literature on CS

A CutFEM method for Stefan-Signorini problems with application in pulsed laser ablation

In this article, we develop a cut finite element method for one-phase Stefan problems, with applications in laser manufacturing. The geometry of the workpiece is represented implicitly via a level set function. Material above the melting/vaporisation temperature is represented by a fictitious gas phase. The moving interface between the workpiece and the fictitious gas phase may cut arbitrarily through the elements of the finite element mesh, which remains fixed throughout the simulation, thereby circumventing the need for cumbersome re-meshing operations. The primal/dual formulation of the linear one-phase Stefan problem is recast into a primal non-linear formulation using a Nitsche-type approach, which avoids the difficulty of constructing inf-sup stable primal/dual pairs. Through the careful derivation of stabilisation terms, we show that the proposed Stefan-Signorini-Nitsche CutFEM method remains stable independently of the cut location. In addition, we obtain optimal convergence with respect to space and time refinement. Several 2D and 3D examples are proposed, highlighting the robustness and flexibility of the algorithm, together with its relevance to the field of micro-manufacturing

The limb darkening of alpha Cen B: Matching 3D hydrodynamical models with interferometric measurements

For the nearby dwarf star alpha Cen B (K1 V), we present limb darkening predictions from a 3D hydro-dynamical radiative transfer model of its atmosphere. We first compare the results of this model to a standard Kurucz's atmosphere. Then we use both predictions to fit the new interferometric visibility measurements of alpha Cen B obtained with the VINCI instrument of the VLT Interferometer. Part of these new visibility measurements were obtained in the second lobe of the visibility function, that is sensitive to stellar limb darkening. The best agreement is found for the 3D atmosphere limb darkening model and a limb darkened angular diameter of theta\_3D = 6.000+-0.021 mas, corresponding to a linear radius of 0.863+-0:003Ro (assuming pi = 747.1+-1.2 mas). Ournew linear radius is in good agreement with the asteroseismic value predicted by Thevenin et al. (2002). In view of future observations of this star with the VLTI/AMBER instrument, we also present limb darkening predictions in the J, H and K bands.Comment: Accepted for publication in Astronomy & Astrophysic

Sizing safety vents for non-tempered systems (organic peroxides): a new tool at lab scale

National audienceThis paper deals with the development of a new experimental “similarity” vent sizing tool for non tempered chemical system combining the advantages of both DIERS method (laboratory scale) and UN similarity method (less overconservative). This tool is an extension of the VSP2 (Vent Sizing Package II) adiabatic calorimeter. The objective of this new vent sizing tool is twofold. The first is to provide the required A/V ratio necessary to assure a safe relief. The second is to provide measurement of mass vented during blow-down. This paper gives a description of this tool and exposes the first promising results obtained. Its main limits are also given

Influence de la vitesse de mise en forme et du cycle de chauffe sur le thixoforgeage des aciers

Analyse l’influence de la vitesse du coulisseau et de la température du lopin (fraction liquide) sur le comportement de l’acier à l’état semi-solide en utilisant un nouvel outillage de filage direct monté sur une presse hydraulique. Cette nouvelle conception évite la décélération du vérin rapide pendant la mise en forme. Il permet donc des essais à vitesse constante. Ce travail s’intéresse également à l’écoulement de la matière dans le cas du thixoforgeage de l’acier et montre l’importance d’un couplage entre la température initiale et la vitesse de déformation. Les différentes conditions permettant d’avoir un écoulement homogène et continu, avec un effort de mise en forme faible, sont étudiées et présentées.This paper analyses the influence of two parameters (tool axis speed and billet temperature) on semi-solid steel alloy behaviour. A new design of direct filling tool is proposed and adapted on hydraulic press. This concept avoids the speed decrease during forming process. This works shows thermal exchange influence in thixoforging process in function speed raw and initial temperature inside the billet by analysis forming loads and part geometries

A generic methodology to improve the control of forging process parameters

One of the common problems in forging processes is the lack of key process parameters control, as well as their identification. Certain controlled parameters exist, such as temperature or stroke length, which are usually identified and controlled through a systematic approach. Their selection depends particularly on the part to produce or on customer’s constraints, rather than a rational approach. In this paper, a methodology is proposed to select the key process parameters. There are some methodologies which already exist, such as the DMAIC, which are used to determine the control parameters and their influences on the desired specifications. However, this approach has certain drawbacks. For example, the key parameters are selected by experts, which makes each case study time consuming. The aim is to develop a generic methodology to improve the manufacturing process in the forging industry. The methodology is represented as a decision support system that connects product specifications (geometry, absence of defects…) or other forging specifications (tool wear, involved energy...) to the process parameters. The latter will be able to define the key parameters, their values and their appropriate way of control. These links will be setup using the empirical rules and physical laws