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International audience; Wire arc additive manufacturing (WAAM) is emerging as the main additive manufacturing (AM) technology used to produce medium-to-large-sized thin-walled parts (order of magnitude: 1 m) at lower cost. To manufacture a part with this technology, the path planning strategy used is the 2.5D. This strategy consists in slicing a 3D model into different planar layers parallel to each other. The use of this strategy limits the complexity of the topologies achievable in WAAM, especially those with large variations in curvature. It also involves several start/stops of the arc as it passes from one layer to another, which induces transient phenomena in which the control of the supply of energy and matter is complex. In this article, a new manufacturing strategy to minimize the start/stop phases of the arc to one unique cycle is presented. The goal of this strategy, called "Continuous Three-dimensional Path Planning" (CTPP) is to generate a continuous trajectory in spiral form for closed-loop thin parts. An adaptive wire speed coupled with a constant travel speed allow a modulation of the deposition geometry that ensures a continuous supply of energy and material throughout the manufacturing process. Using the 5-axis strategy coupled with CTPP allows the manufacture of closed parts with a procedure to determine the optimum closing area and parts on non-planar substrates useful for adding functionalities to an existing structure. Two geometries based on continuous manufacturing with WAAM technology are presented to validate this approach. The manufacturing of these parts with CTPP and several numerical evaluations have shown the reliability of this strategy and its capacity to produce complex new shapes with a good geometrical restitution, difficult or impossible to reach today using 2.5D with WAAM technology.

International audience; A thermo-mechanical simulation of the wire + arc additive manufacturing (WAAM) process is presented in this work. The simulation consists in the deposition of 5 successive layers of 316 L stainless steel on a 316 L base plate. The thermo-mechanical analysis is solved in two dimensions under plane stress assumption. Nonetheless, the metal addition is taking into account in this numerical analysis. An increment of material is added at each time step. This numerical approach allows reducing the computational time. The temperature and residual stress fields are computed at each time step. Two patterns of deposition strategy are also investigated. It is shown that the longitudinal stress varies mainly along the vertical axis. A sample with 5 overlaid layers has been scanned with neutron diffraction technique in order to measure the final residual stresses. Both numerical and measured residual stresses are in good agreement. The Aster finite element software is employed for the numerical analysis.

International audience; The purpose of this article is to investigate the influence of growth kinetic models and nucleation models on the grain structure predicted by cellular automata (CA) during bead on Cu30Ni thin plate. Temperature field is obtained with the computation of two-dimensional finite element model (FE). Based on these temperatures, a CA model simulates the evolution of the envelope of grains along the solidification front. It is shown that for the same temperature field, the grains structure is modified if the growth model changes. The influence of the nucleation law parameters is also discussed from a modelling point of view. Experimental size of the weld pool are compared with thermal results and EBSD maps are compared with grain structure prediction obtained with the CA algorithm.

International audience; Wire Arc Additive Manufacturing (WAAM) has the possibility to build metallic structures in 3D space. WAAM system is based on welding process to deposit metallic material and on a robot that moves the welding torch to add material at a given position. For large skeleton structures, it was chosen to deposit material point by point. Welding process induces uctuations.To be fully scalable, two main features must be taken into account. First, monitoring of the process is necessary. Local control on the geometry of the deposition must be used to reach the nal shape. Secondly, some deposition strategies must be implemented to manage branch intersections. To reach these two objectives, an adaptive and modular slicer and a process manager have been developed in order to implement this control. It allows us, if an error occurs during the deposition, to change the position of the eector. To obtain the desired geometry, the CAM software have to be able to, (i) do a slicing during the additive process of the part with a variable deposit height in order to take into account variation of the deposition process and (ii) manage the deposition strategy at intersection to output the position of the torch.

International audience; The relation between fluid flows, solidification mechanisms and welding parameters during GTA weld­ ing of a Cu30Ni plate is investigated. An experimental study, based on in-situ observations of the trailing edge of a fully penetrated weld pool is carried out. The experimental setup includes: (i) a high-speed camera used to observe the solidification front and the fluid flow at microscale, (ii) two cameras placed over and below the plate used to observe the entire weld pool on the top side and back side respectively (iii) and an infrared camera used to monitor the tempe rature field on the back side. Ali these observation devices are employed to do measurements that allow us to explore and better understand the influences of the welding parameters on the solidification and the fluid flow behaviours, as well as between the different physical phenomena being involved.

International audience; The paper presents an apparatus to measure surface temperature distribution of liquid metals during fusion processes. The apparatus is based on dual wavelength radiation thermometry and is designed to measure temperature from 1500 to 3000 K. The pyrometer is based on standard optical parts and industrial CCD cameras. Uncertainties are analysed on the base of the radiometric equations. To insure relative precision in the measurement, a calibration procedure is conducted on tungsten ribbon with known emissivity in the temperature range. This procedure is also used to discuss the influence of wavelength couples. To conclude, experimental measurements on pure laser melted materials are conducted to assess the validity of the apparatus and methodology.

International audience; Feed rate effect on scallop height in complex surface milling by torus-end mill is rarely studied. In a previous paper, an analytical predictive model of scallop height based on transverse step over distance has been established. However, this model doesn’t take feed rate effect into consideration. In the present work an analytical expression of scallop height, including feed rate effect, is detailed in order to quantify feed rate effect and thus to estimate more precisely the surface quality. Then, an experimental validation is conducted, comparing the presented model predictions with experimental results. Actually, the share of the scallop height due to feed effect is highly dependent on the machining configuration. However, most of time, the feed effect on total scallop height values is far from being negligible.

International audience; A two step modeling for predicting the grain morphology − columnar or equiaxed − of the melting zone in welding is proposed. The first step consists in numerically compute, using the Finite Element Method, the thermal fields around the weld pool boundary, according to the welding parameters. The second step consists in a post-treatment of the thermal field in order to compute the undercooling in the liquid all along the solidification front of the columnar grains, and finally to compute the fraction of equiaxed grains formed in the undercooling zone. The modeling takes into account the effect of the convection phenomena on the undercooling, and is based on an heterogeneous nucleation model specially adapted to steels inoculated with titanium. The nucleation model involves the size distribution of Ti-rich refractory compounds formed in the weld pool, that act as heterogeneous nucleation sites. A method to identify this distribution is also presented. The comparison of the modeling with experimental results of welding with a Gas Tungsten Arc Welding process has proved its ability to predict the columnar to equiaxed transition of a ferritic stainless steel inoculated with Ti.

International audience; A two-scale in situ observation setup has been used to investigate the physical phenomena around the solid/liquid interface in welding. A spot gas tungsten arc welding process is used to observe weld pool solidification phenomenon after arc extinction. The solidification front and the fluid flow in the weld pool have been observed at a microscale by a high speed camera. The whole weld pool and the surrounding base metal have been filmed at a macroscale by an infrared camera to estimate temperature fields around the weld pool and bead shape. A qualitative and quantitative dataset was extracted from these observations that involve useful results for further improvements of the understanding of weld solidification mechanisms and to enhance solidification models.

International audience; An experimental setup is presented in order to obtain experimental data during solidification of a static weld pool after arc extinction with a GTAW process. Several devices have been setup to extract three kinds of measurements: (i) solidification front velocity (ii) fluid flow velocity at the vicinity of the front (iii) temperature field in the solid part. A high-speed camera is used to film the interface during welding at microscopic level and an infra-red in order to take thetemperature field around the weld pool in the solid part. After processing and calibration of the videos, the experimental results are compared to theoritical results founded on an adapted model from the KGT [1] and from the one of Gandin et al. [2]. All the tests are done thin plate of Cu-30wt.%Ni.

International audience; Hot cracking phenomena in welds are still a major issue to choose suitable welding conditions. Solidification cracking in welds depends on microstructure morphology, thermal and mechanical loadings. It is still difficult to discriminate the influence of the different phenomena because they are all connected. To investigate solidification cracking, numerical modelling that integrates most of these phenomena is developed in this paper. Thermal and mechanical fields are computed with a finite element model at the process scale. The thermal field is combined with a cellular automata (CA) to predict microstruc-ture at a meso scale. Based, on this prediction, the intergranular network is extracted. Finally, the pressure in the intergranular fluid network is computed during solidification. The fluid flow model along grain boundaries predicts an eventual nucleation of pore/void in the remaining liquid by investigating the maximum pressure drop in the solidifying microstructure. The numerical model is able to describe some hot cracking phenomena observed during experimental tests.

International audience

Liquid droplet deposition on solid surfaces has an important role in the industrial and research activities. The behaviour of such deposit is influenced by volume and interfacial phenomena and involves a large number of mechanisms such as gravity effect, mass transfer, capillary forces, and wetting. For the case of metal deposition the analysis of the problem is more complex because of the importance of thermal effects, involving steep gradients and phase changes. A unique experimental approach is presented in order to study the evolution of the spreading of a large drop of liquid metal called “macro-drop”. The objective of this work is to supply qualitative and quantitative information during the deposit of liquid metal in relation with process parameters. The overall shape of the macro-drop, especially its spreading and contact angles are studied in detail. The gradual spreading of the macro-drop is mainly governed by mass and heat transfers. The initial rapid spreading is due to kinetic energy of depositing droplets and direct arc heating on the solid target. All experimental results are analysed in the light of process parameters to identify the physical mechanisms involved and appreciate their effects on the behaviour of such a macro-drop.

International audience; Hot cracking in welding is a complex phenomena due to coupling between process, metallurgy and mechanical loading. In this paper, a coupled approach that integrates microstructural prediction based on the process simulation and criteria at the microstructural scale is proposed. The criteria is based on some development of Rappaz et al [1] and takes into account the influences of grain morphology, mechanical and thermal fields induced by welding, on hot cracking. To integrate all factors influencing hot cracking, a methodology is developed that couples process simulation, simple microstructural prediction and criteria along columnar grains. The criteria based on the microstructure behavior is able to predict crack onset location in columnar grains zones.

Owing to their low ductility at room temperature, heat resistant cast austenitic stainless steels are very sensitive to weld cracking. Cracks are formed in brittle zones of the heterogeneous microstructure constituted by the carbide rich interdendritic spaces. In order to identify the operating factors affecting the cracking propensity of such steels during welding, a three step method, based on numerical simulation, is presented. First, the macroscopic emperature and stress fields are determined by a finite element calculation, considering a homogeneous material. In a second step, a localisation criterion is defined to identify critical zones, according to the macroscopic fields determined in the first step. Then, the heterogeneous microstructure of the cast steel is modelled in a 'representative cell', and the thermomechanical history of the critical zone previously identified is applied as boundary condition, in order to determine the local stress field. The cracking is then supposed to occur when the maximal principal stress in the interdendritic zone of the representative cell reaches the cleavage stress of the carbides, determined using SEM in situ tensile tests. This method is used to predict cracking during multipass weld repair of bulk samples, under various welding conditions. A comparison is carried out between experiment and simulation to validate the method.

International audience; We propose here a model to describe the rupture of woven ply laminated structures. The validity scope of this model depends on the 'diffused damage' phase up to the first intra-laminar macro-crack only (first ply failure model). The model is based on a previous continuum damage approach [Hochard C, Aubourg PA, Charles JP. Modelling of the mechanical behaviour of woven fabric CFRP laminates up to failure. Compos Sci Technol 2001;61:221-30] and a new non-local fibre rupture criterion. This non-local criterion uses a ply scale characteristic volume of ply thickness and in plane area depending on the meso-structure of the ply. In the case of the balanced woven ply studied here, a simple circular area is chosen and identified on a test showing a stress gradient. The local shear damage and inelastic strain evolution laws and the non-local fibre rupture criterion have been integrated in Abaqus. Numerical results compared to tests for woven ply laminated structures show the efficiency of the proposed approach.

A non-linear cumulative damage model for woven ply laminates under static and fatigue loads is developed in this paper. The validity scope of this model depends on the ‘diffuse damage’ phase up to the first intra-laminar macro-crack only (first ply failure model). The model, which is based on a continuum damage approach (CDM) and a non-local fibre rupture criteria, was implemented in a FEM code. Finally, full field strain measurements are used to compare experiments and computations for a plate with an open hole submitted to a fatigue tensile load.

Inexistant

International audience

For unidirectional ply laminates, the great diversity of the damage mechanisms and their patterns of evolution make it extremely difficult to estimate the strength margins. In the case of woven ply laminates, the number of damage mechanisms is fairly small (no transverse rupture occurs and the material has a greater resistance to delamination) and the behaviour of the material is fairly simple to model up to rupture. In this study, a numerical model for woven ply laminated composite structures up to rupture is developed. The implementation is performed in a Euler Backward scheme and the consistent tangent stiffness matrix is calculated. Comparison with some experiments on structures are made and the model predicts these experiments well.

A plastic yield function for ductile materials is discussed in order to predict damage during modeling of bulk metal forming processes. The variational formulation used by Rice and Tracey [J. Mech. Phys. Solids 17 (1969) 201] is modified for a cavity in an unit cell in contact with an inclusion. Following Gurson [J. Eng. Mater. Technol. 99 (1977) 2], the influence of the plastic flow on the size and the shape of the void is analysed at the mesoscopic level. Then, the influences of the volume, of the shape of the void and the contact are introduced in a macroscopic plastic potential similar to the specific one proposed by Staub and Boyer [J. Mater. Proc. Technol. 79 (1998) 9]. This dilatational plastic potential and its corresponding plastic flow rule are implemented in a proprietary software and used for the numerical modelling of a workability test.

International audience