Laser : Laser Processes

Since the invention of laser 50 years ago by T.Maiman, and the first industrial developments in the early 80's, laser has become a mature and performing tool for materials' transformation. It allows, among many other potential applications, to remove matter by cutting or drilling, to weld up to several cm metal thickness, to manufacture complex shapes par powder melting, to harden or texture surfaces at the µm or mm scale.

In a first step, the use of all those processes requires understanding precisely physical mechanisms underlying in the different laser-material interaction regimes, by the use of either dedicated experimental diagnostics (very high cameras up to 100 kHz, IR thermal camera, Doppler velocimetry, spectroscopy) or numerical modeling more or less simplified. In a second step, we need to be able to analyze laser-transformed materials, through the characterization of their surface finish, microstructure, mechanical state, in order to be able to correlate those states to the thermal, thermo-hydraulic or thermo-mechanical cycles experienced by the matter.

The LASER group, directly derived from the former LALP laboratory, and located in the "Centre de Paris" of Arts et Métiers ParisTech since February 2009, has two main objectives: (1) understanding and improving the use of laser processes, (2) controlling the effects induced in metallic (most of our applications), ceramic or polymer materials. For this purpose, we own recent and dedicated laser sources (for instance a 10 kW Yb:YAG laser bought in 2009, or a new pulsed laser for shock generation, expected for the end of 2013 through the Hephaistos SESAME project).

In recent years, many important results have been obtained on all laser processes investigated so far.

We can mention first a complete description concerning the physics of key-hole and molten pool during laser welding. Coupled with analytical and numerical modeling, this has considerably improved our technical approach of laser welding. Different industrial solutions have been carried out like, for instance, the use of micro gas -nozzles for enlarging and stabilizing key-holes. Following this, investigations have concerned the optimization of MIG-laser hybrid welding (PhD doctorate of E. Le Guen, 2009, Cooperation with LIMATB Lorient), the development of a phenomenological description of the formation of ripples during deep penetration cutting (PhD doctorate of K. Hirano, 2012, cooperation with Nippon Steel), or the Aluminum-Titanium dissimilar welding (Attila Carnot project, in cooperation with LTM-ICB le Creusot)

 

Figure 1: Laser cutting kerfs with different interaction régimes depending on cutting speeds (a) V < 2 m/min, unstable, (b) 2 < V < 6 m/min, stable  front et and instable edges, (c) V> 6 m/min, stable

 

Among the most recognized themes of the laser group, we must also mention investigations concerning laser-shock waves (LSW), for which our group is known for its pioneer work, since the beginning of the 90's. Various applications are envisaged for LSW: (1) the use as an adhesion test for interfaces (époxy-carbon composites : PhD doctorate of E.Gay, 2011), (2) as a dynamic loading for investigating high strain rate behavior of metals (ANR SIPRODYN, PhD doctorate A.Nifa, 2013) or (3) the the well-known laser-peening applications (ANR CAPSUL Project).  For all these applications, we have the possibility (rather unique in the world) to carry out laser-shock tests, to characterize laser-induced plasmas or LSW propagation by using VISAR velocimetry, and to combine experimental determinations with numerical simulations using finite element codes (Abaqus). Our strong cooperation with other labs (LULI – Ecole Polytechnique) also allows us to work with other interaction regimes (shorter pulses: ps, fs) allowing to reach much higher pressures (several Mbars).

 

Figure 2: Velocity profile obtained on a 100 µm-thick aluminium target par a 600 TW/cm² laser shock. Generation of a spall (expérience versus simulation with a use of a Kanel damage criterion)

 

Our investigations on laser additive manufacturing (LAM) by powder melting, are focused on different aspects. First, we try to optimize surface finish, and to understand the physical mechanisms responsible for the deleterious surface aspects, in order to improve the process, by the use of practical and easy-to-use solutions. In this area, we cooperate with LIMATB-UBS, on the numerical calculation of fusion zones (FZ) shapes and dimensional stability (ANR ASPECT, PhD Doctorate M.Gharbi).

The LAM process also allows generating innovative or graded materials such as metal matrix composites, for instance Titanium matrix composites for which a Ph doctorate (S.Pouzet) has just started.

Last, in the frame of the FUI FADIPLAST project involving various industrial partners (Dassault, MBDA), we have started working on the selective laser melting of polymer powders (cooperation with the ARPE group). A fully original instrumented SLM prototype has been designed and manufactured for this purpose. It allows investigating CO2 laser / polymer interaction, and measuring physical data such as local temperature or grain bonding kinetics.

 

Figure 3: Surface finish obtained by LAM on a titanium alloy. Benefits from the use of high lineic energies (P/V) and small layer heights (a) P=400 W, V=0.2 m/min, (b) P=500 W, V=0.4 m/min (3D profilometry)

The laser drilling, applied for instance to aeronautical blades, is a highly concurrential process. Our specific contribution is: (1) to establish simplified analytical models of the semi-confined interaction regime allowing to optimize and predict hole geometries, (2) to optimize the laser drilling of complex structures such as thermally coated metals, and evaluate the influence of laser holes on the static and cyclic resistance of structures (cooperation with COMET group, ANR ULTRA). A numerical description of the percussionnal laser drilling, using the C-Nem method developed by the DYSCO group from PIMM, is also carried out, through the PhD doctorate of J.Girardot.

Even though most of the previously mentioned studies directly address laser processes, it's also possible to use laser sources only like a well-controlled (energy, spatial distribution) heat source allowing to investigate other physical issues.

For instance the PhD doctorate of M.Muller (2013) on laser assisted combustion uses a laser energy deposit to initiate and propagate an exothermal combustion phenomenon under O2 in order to simulate incidents occurring in high O2 pressure pipes.

Currently, the LASER group is formed by 14 personnes including 8 permanent workers, 5 pHD students, and a research engineer under medium time contract). Through the GIS GEPLI, we have systematized a strong partnership with industrials (Air Liquide, Arcelor, Peugeot-Citroën, Safran et Thales) which contributes to the orientation of our scientific areas. We also have many contacts with the french scientific community (PPRIME-Poitiers, Ecole des Mines de Paris, University of South Brittany, INSA Strasbourg, University of Burgondy …), and with international labs (ILT Aächen, CNRC Canada …).