Overview
DEEPWELD contributes to strengthening the competitiveness of the aeronautical industry in Europe. Friction stir welding (FSW) is a new technique that could revolutionise the way aircraft are built by replacing riveting with welding. The benefits of FSW include the ability to join materials that are difficult to fusion weld. It is a simple, robust process that involves no consumables. When handled properly, FSW results in a defect-free weld with superior properties. However, there is still a lack of knowledge on its applicability to aircraft structures. Although there is still a significant need for experiments, an advanced simulation tool is required to understand further the effect of the various welding parameters on the properties of the welds.
The ultimate aim of the DEEPWELD project was the development of a new multi-physics and multi-scale numerical software tool for the accurate modelling of the Friction stir welding (FSW) process. This tool would help shorten the design cycle and decrease cost by reducing the number of experimental prototypes by replacing them by virtual prototypes. The new tool would be a large step forward compared to current solutions, as it would be equipped with a thermo-fluid module in order to simulate the important material flow around it and an advanced metallurgy model in order to predict the evolution of microstructures. Specifically instrumented experiments would be conducted in order to define accurate thermally varying friction laws, material constitutive laws and data in order to validate the new numerical tool.
The software simulation introduced was based on a multi-scale approach in which a new advanced finite element solver, based on a velocity-pressure formulation, solves the material flow and thermal effects around the tool, taking into account complex thermally varying friction laws. The material flow solver at the lowest scale was coupled with State-of-the-Art industrial software to compute the complete process from the starting transitory phase to a steady phase and eventually to the final phase of the process. New metallurgy models were implemented in the industrial code in order to take into account the changes in micro-structure due to the stirring and cooling of the metal. Attention was paid to the applicability of the new technique to an industrial basis.
The work was organised into four technical Work Packages:
- Work Package 1: Detailed specifications of industrial target applications: The development of a numerical tool to simulate the FSW process, within the DEEPWELD consortium, would be carried out following specifications of the industrial end-users. This Work Package defined these specifications in terms of materials, applications, performance, software and experiments.
- Work Package 2: Physics and Metallurgy: The first objective was to provide quantitative information to be introduced as input in the numerical codes to be developed in the DEEPWELD Project. The second objective was to provide sufficient information for a better understanding of the physical phenomena occurring during FSW. This was required to select appropriate modelling assumptions for the different models or modules (thermo-fluid, thermo-mechanical, and metallurgical).
- Work Package 3: Multi-physics simulation tool development: Development of a general, numerical tool incorporating the coupling of the following fields: mechanical, thermal, metallurgy and flow calculation. The development of this general and multi-physics model would allow for predictive FSW simulations by: 1) eliminating the equivalent heat flux determined from experiments and replacing it by a thermo-fluid calculation, which would predict the amount of heat generated through plastic work and friction; 2) taking into account the changes in mechanical properties due to transformations in the micro-structure of the material.
- Work Package 4: Validation and Applications: The objectives of this Work Package were: 1) an experimental validation campaign with a wide range of operating conditions;
Funding
Results
The project introduced the following eight innovations:
- Multi-scale multi-physics solution based on industrial software
DEEPWELD did not aim to re-invent existing, proven, industrial finite element solvers, but to complement them with sufficiently advanced features in order to simulate a specific process such as friction stir welding. The objective was to perform analysis at different scales. First, a thermo-fluid simulation was performed at the scale of the flow material around the tool, advanced friction models and metallurgy were implemented in this new solver. Information such as thermal fields, strain, strain rates of the material flow region were then passed onto the industrial thermo-mechanical solver in order to compute a global scale analysis for the whole process. The thermo-mechanical solver was equipped with metallurgy modules capable of accurately predicting the evolution of microstructures during the process. Coupling between local scale thermo-fluid analysis and global process thermo-mechanical modelling was achieved. - Material flow visualisation
Experiments were conducted to visualise material flow. Those data were used to validate the thermo-fluid finite element tool and to calibrate the analytical flow model, as well. Techniques used to visualise the material flow laid on the introduction of elements such as very small balls, very thin wires or sheets that are used as tracers of the material flow. Also, an innovative method was proposed which consisted of using a number of tracers with known low-melting temperatures, for instance solder alloys to be selected on the basis of their melting temperature. By using such tracers, it was unfortunately not possible to obtain information either on the tracer particles displacements during FSW or on the maximum temperatures attained during their displacement, in particular in the nugget where thermocouples and thermographic measurements are impossible. Therefore, this method was insufficiently demonstrated during the project. - No experimentally calibrated heat flux
DEEPWELD focused on eliminating equivalent heat flux used in current State-of-the-Art thermo-mechanical simulation of FSW process. Indeed, in the latter, an equivalent heat flux should be determined on a case by case basis from experiments measuring heat input and tool loads and tuned to obtain good correlations between simulation and measurements. Hence, this methodology was a major burden for any optimisation of welding