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Advanced Simulation Solutions Applied to Quality Control of Laser Deposited Metals

Periodic Reporting for period 1 - ASSALA (Advanced Simulation Solutions Applied to Quality Control of Laser Deposited Metals)

Reporting period: 2019-04-01 to 2020-06-30

Productive industries of transport sector, such as aerospace, have realized that some geometrical deficiencies and high manufacturing costs could be overcome if critical components were manufactured by Additive Manufacturing (AM) methods. Aerospace sector pursues the cost-effective manufacturing of mission critical components, which are machined from previously wrought or forged preforms, scrapping high amounts of expensive materials such as superalloys, which are not still allowed to be re-used/recycled for the same structural applications. The buy-to-fly ratio for a part machined from forged billet is typically 10-20 and can potentially drop nearly to 1 with AM methods for the most favourable cases.
Laser based direct energy deposition processes offer the possibility of sorting out the geometrical, cost and process related issues. Depositing near-net-shape geometries, heat treating and machining them yield more efficient productive results than traditional processes in specific materials and markets. The main challenge to attain with AM technologies is to ensure structural integrity of fatigue loads through robust processes where simulation has a vital role.

Structural components of specific sectors have been inherently linked to the concept of catastrophic failure as a consequence of the dynamic loads (fatigue) acting on them. Failure has enormous repercussions on aspects such as personal safety, environment or economic cost. ASSALA project will develop, test and integrate the necessary simulation technologies to limit and control defect generation in laser metal deposited components.

Due to the fact that the material internal structure is being generated in the melting process, pores, crack, residual stresses, strains and component distortions are also inherent to additive manufacturing processes. Actually, the lack of predictive tools is overcome with acquired process experience and time consuming trial-error setup processes. However, the use of thermo-mechanical simulation tools fed with statistical failure probability models can lead to notorious time and cost reductions in laser based AM process set-up and subsequent optimization and certification processes.

Summarizing, ASSALA project is focused on the development and introduction of a new manufacturing process on a highly standardized sector such as aerospace because clear advantages over traditional manufacturing processes have been envisaged. To this end, it is proposed to work on the development of digital simulation tools, both for the process and for the productive means, thus, enabling the achievement of more robust and reliable laser wire deposition (LWD0p rocess because there is a real and growing demand from the aerospace sector.
During this reporting period, several activities have been developed under the framework of four different technical workpackages (WPs):

Wok performed in WP2 was related to the definition of specifications and requirements of the additive manufacturing process and demonstrator geometry. Besides this, a thorough document compiling the state of the art of all the technologies to be developed in ASSALA was completed. This document also specifies the planned equipment to carry out the activities.

WP3 is where the bulk of the activity was focused, since it deals with the development of robot and process models and also with supporting digital tools, such as process specific CAM and monitoring and control strategies. First, the performance of both the rotary table and robot were assessed, to verify both their kinematic and dynamic responses, based on an external metrology framework.

Once this was completed, the kinematic model of the system was tackled. The results of this kinematic calibration are the real Denanvit-Hatemberg (DH) parameters that correspond to the physical robot model. Based on elastic body assumption, robot performance can be improved approximately 10 times.

To finish the robot related developments, the dynamic model was completed. The model includes the most important features to accurately represent the dynamic performance of the robots: setpoint generation, control loops, sensors, and the robot (mechanical part). The model allows the simulation of both joint and cartesian coordinate movements and provides setpoints about joint (position, velocity, acceleration, motor torque) and TCP performance (position, velocity, acceleration).

Also, in the framework of WP3, thermo-mechanical process models were developed. These efforts rely on an approach which considers several time and space scales, in an increasing order of complexity: track -> layer -> component, which culminate in a user-friendly app, which provides component distortions as a function of process parameters and deposition trajectories. Due to the reduced order modelling strategy, results are provided in almost real time, so the end user can evaluate a complex parameter space in a reasonable time.

Regarding the supporting digital tools, the following can be highlighted:
- Development of AM specific CAM tool. The toolpath defines the deposition build sequence for the manufacture of 3D components with Laser Metal Deposition. It is based on the sequential deposition of beads that are overlapped to generate layers that are, in turn, stackedd vertically to form a 3D structure. A tool has been developed based on MATLAB programming algorithms to automatically calculate the deposition sequence from some process parameters and a 3D geometry specification.

- Monitoring and control strategies, based on three different technologies:
1. Structured light 3D scanner for geometry measurement and part growth control.
2. Side camera with laser illumination for the wire movement monitoring and its interaction with the melt pool.
3. Coaxial camera for melt pool monitoring.

In WP4, the logic for the global architecture of the solution has been defined, as tool which can perform a global optimization procedure by calling meta-models of the previously developed robot and process models.

And finally, in WP5, the following activities must be addressed:

- A DoE for defects has been defined and executed. This DoE has the goal of relating several quantities of interest (QoI) to the process inputs, by using a data – based approach.

- Development of sensitivity analysis for process models. Since the parameters are included as extra coordinates to the problem in a separated form, the sensitivities are readily available with respect to the parameters. Therefore, the sensibilities are computed on the fly.
Main exploitable results identified by the partners at this stage of ASSALA project are mainly related to:

- Fast & precise thermo-physical AM process simulation
- Deposition trajectory generation software
- Dynamic robot modelling and compensation metrology service.
- Geometrical in-process control system
- Thermal in-process control system