Laser welding of thermoplastic polymers is a non-contact process especially efficient for joining thermoplastic polymers. This innovative technology is already used for industrial series production in different sectors (automobile, packaging,...). The majority of the basic research concerns the weld strength depending on polymer nature, optical properties, butt design and process parameters. Nevertheless, a lack of knowledge concerning the influence of thermal history of the weld seam on morphology of semicrystalline polymer still exists, when this parameter strongly influences the strength of the weld. Actual results of diode laser transmission welding (LTW) experiments on polypropylene, a semicrystalline polymer widely used in industry, could contribute to a better understanding of the process itself and to success in practical applications.
The modifications included on PET by an excimer laser radiation or a low pressure plasma as well as their ability to improve A1-PET adhesion were investigated. For this purpose, surface roughness, chemical composition, surface wettability and adhesion properties of PET were studied depending on the process parameters. Both treatments can significantly enhance the adhesion but the surface change responsible for the improvement was different for each pretreatment.
The modifications induced by an excimer laser irradiation at 193 nm or 248 nm on organic surfaces, below their ablation threshold, were investigated for different kinds of polymers such as: PEEK (Polyether Etherketone), PC (Polycarbonate), PET (Polyethylene terephtalate). Treatments were carried under argon or air at different laser fluences. Treated surfaces were characterized by XPS (X-rays Photoelectron Spectroscopy) and surface wettability measurements. For, all the studied polymers, the results shows that the surface modifications first depended on the laser wavelength. Surface oxidation occurred at 193 nm, leading to the formation of polar groups (carbonyls, carboxyls, hydroxyls) and inducing an increase of the surface energy. Treatments at 248 nm never induced any oxygen enrichment of the surface. This is due to the loss of oxygen by CO or CO2 desorption at this wavelength.
Chemical transformations on stainless steel surfaces (304 AISI) are obtained by melting with an excimer laser. The very high quenching rate allows to create a thin continuous metastable gradient in composition and structure by opposition to other melting techniques which promote multiphase layer. The mechanical properties of the bulk are maintained and the external layer is chemically modified for a better resistance to environmental aggressivity. Molybdenum is used for corrosion protection and ruthenium for cathodic and anodic properties. The composition is analyzed on cross-sections obtained by ultramicrotomy. A continuous gradient approximately 500 nm (nanometer) of Mo or Ru with decreasing grain size (< 0,3 micrometer compared to 10 - 20 micrometers for 304 AISI) is observed. Mo and Ru surface alloys show an important beneficial effect from cathodic and transpassive potential range.
The growth and morphology of nitrided layers formed during the solid phase nitriding of pure titanium by CO2 laser were investigated. In the case of a laser treatment carried out under isothermal conditions, it was shown that CO2 photons irradiation of the substrate does not produce any specific assisted nitride growth: nitriding kinetics, nitride composition and structure were similar to those obtained after nitriding using a classical heating system. From the nitriding kinetics, nitrogen diffusion coefficients were determined using an analytical solution of Fick's equation. This allows to plot the evolution of the nitrogen concentration with respect to depth and to compare calculated profiles to those determined experimentally by Nuclear Reaction Microanalysis.
Surface treatment with restricted thermal effects can be achieved using pulsed excimer lasers. Therefore these tools are appropriate to thermally sensitive materials such as polymer. The present work deals with the possibility of using the excimer laser to achieve a prebonding treatment on the PEEK and to study the influence of the wavelength on the efficiency of the treatment in terms of bonded joint strength. Regarding the mechanical test results, it appears that the adhesive properties of the polymer are mainly dependent on the laser wavelength. Without any pretreatment, the bonded joint strength does not exceed 5 J/m2 and the failure occurs at the interface between the PEEK and the adhesive. After a 193 nm irradiation, below or above the ablation threshold, a cohesive failure in the adhesive at 700 J/m2 is obtained whereas at 248 nm the laser treatment leads to an adhesive failure at the PEEK-adhesive interface at 330 J/m2. In order to understand why the wavelength have such a strong influence on the adhesive properties, the modifications induced on the surface after treatment are investigated.
Due to its important chemical stability, polyether- etherketone (PEEK) thermoplastic presents poor adhesive bonding properties. The possibilities of enhancing the PEEK adhesive properties by excimer laser pretreatments are investigated. Surface modifications are characterized, depending on the experimental working conditions, using SEM, profilometry, XPS, wettability and mechanical tests. In the case of an excimer laser pretreatment, the induced surface modifications are dependent on the energetic density of the incoming laser beam: (1) for E > E ablation threshold, surface ablation occurs. The treated surface is characterized by an considerable roughness increase, a carbon content enrichment and a decrease of wettability. The adhesive bounding properties are very significantly increased. (2) For e < E ablation threshold, the treatment induces superficial thermal and chemical modifications. Surface amorphization, formation of new 0-C equals O bonding, decrease of the C-O and C equals O bonding and a decrease of the shake up signal. The increase of the water wettability is more pronounced when the treatment gas contains oxygen. A cohesive failure in the adhesive was obtained when treatment is achieved using argon gas whereas an adhesive failure at the interface may be evidenced when air is used. Lap shear strength values show that excimer laser irradiation improve PEEK adhesion bonding properties for all treatment conditions.
Solid state nitriding of titanium under cw CO2 laser was investigated. The nitriding kinetics were studied in the case of CO2 laser treatment in isothermal conditions, and were compared to those obtained by using a conventional heating system. Identical kinetics rate constants and activation energy were obtained; therefore CO2 irradiation of the titanium surface did not influence nitride growth. Studying nitriding treatment in non isothermal conditions showed that high heating rates induced modifications of the nitride morphology. At last, we investigated the evolution of the CO2-surface coupling during the nitriding treatment. It was found that it depended only on the surface temperature and the substrate surface roughness since the growth of a few micrometers which nitride layer did not modify the coupling.
Liquid phase nitridation of TA6V by a high power homogenized CO2 laser had been studied depending on the experimental parameters. Nitrided layers thickness and hardness as well as cracks concentrations and wear resistance were taken into account for the evaluation of experimental optimum parameters.
In order to control CO2 laser treatments, a precise knowledge of the CO2 radiation- surface coupling is required. For this purpose, numerical identifications of the absorption coefficient, either with temporal resolution in non stationary 1D configuration or with spatial resolution in a stationary 2D configuration, were achieved from thermal cycles measurements and simulations. Evolutions of the coupling during laser treatment were studied both in the case of coated steel hardening (graphite and manganese phosphate coatings) and in the case of solid state nitridation of titanium alloys. Moreover, the determination of the CO2-surface coupling for those thermodiffusional solid state laser processes had been used to correlate experimental treated depths to those obtained by a simple thermodiffusional model taking into account carbon or nitrogen diffusion under a calculated thermal cycle.
In order to control CO2 laser surface treatments, a precise knowledge of the CO2 radiation-surface coupling is required. For this purpose, numerical identification of the absorption coefficient, either with temporal resolution in non stationary 1D configuration or with spatial resolution in a stationary 2D configuration, were achieved from thermal cycles measurements and simulations. Evolutions of the coupling during laser treatment were studied both in the case of coated steel hardening (graphite and manganese phosphate coatings) and in the case of solid state nitridation of titanium alloys. Results were interpreted in terms of chemical evolutions of the surface; moreover, the influence of a reflective shielding gaz device covering the substrate was also pointed out.
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