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Plasma Monitoring of Laser Beam Welds

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Plasma Monitoring of Laser Beam Welds Plasma Monitoring of Laser Beam Welds Light and sound intensity measurements can be used for the monitoring of plasma initiation and propagation during laser beam welding BY G. K. LEWIS AND R. D. DIXON ABSTRACT. Experimental and theoretical depth of joint penetration for any one set with industrial CO2 lasers. studies using high power density laser of laser conditions, particularly on high Although the same plasma theory pulses (greater than 109 W/cm2) at pulse reflectivity materials. The data, along with explained in the text applies to the higher lengths less than 1 microsecond have the appreciation that more must be multikilowatt power levels, the CO2 proven the existence of laser supported known about the process, led to experi­ lasers produce different plasma waves absorption waves. In our ND:YAG laser ments using acoustic emission as a moni­ with different characteristics than the LSC beam welding power and pulse regimes toring technique. These studies showed waves experienced in the lower power (400 watts maximum average power with significant acoustic energy generation but regime of the Nd:YAG The type of pulse durations of 0.5-8 ms), high speed no apparent correlation with weld mor­ plasma wave produced depends on the photography, microphone, and light phology. The affect of the plasma was power density of the laser beam. Hence, intensity measurements show that multi­ not considered in these studies. As a the laser-plasma interaction may change ple laser supported waves form during a result, a literature review and experimen­ as power is increased. Other effects such single pulse and produce enhanced cou­ tal study were performed to determine as radiation trapping during deep joint pling with the target. the laser light-plasma-material interaction penetration keyhole welding are not sig­ process and the affect on welding. nificant at the power levels discussed here. Introduction Theoretical and experimental studies, primarily at power densities much greater The use of lasers for welding and 9 2 than 10 W/cm and pulse lengths of less Laser-Plasma-Material Interaction materials processing has become an than 1 microsecond (ps) duration, have important manufacturing process. The attempted to characterize the laser-plas­ Absorption of laser energy depends Nd:YAG (1.06 pm)* and C02 (10.6 pm)* ma-target interaction (Refs. 1-25). These initially on the intrinsic absorptivity of the lasers have evolved into primary tools for studies have shown that the interaction target material. Absorptivity generally laser beam welding operations. Typical involves many variables such as the type increases with decreasing wavelength; it Nd:YAG systems are pulsed with average of plasma formed, laser light intensity, may increase because of a change in powers up to 400 watts, while CO2 wavelength, interaction time, energy spa­ metal temperature, base metal oxidation systems may be pulsed or continuous tial and temporal distribution, environ­ state or other surface reaction. wave with average powers up to 20 ment above the target, surface condi­ Figure 1 shows the spectral reflectance kilowatts. These systems are used on a tions, target composition, and material (reflectance equals 1-absorptivity) for wide range of metals for low heat input, physical properties, among others. various elements at room temperature. high precision welds. Although far removed from the laser At the Nd:YAG and CO2 wavelengths 4 9 At the Los Alamos National Laboratory, beam welding power regime of 10 — 10 (1.06 and 10.6 pm, respectively), less than 2 a significant effort is in progress to devel­ W/cm and pulse durations of several 10% of the light is absorbed by Al and less op techniques for controlling and certify­ milliseconds to continuous wave, these than 5% by Cu, Ag and Au. Absorption of ing the laser beam welding process. Initial studies have provided information appli­ the laser radiation typically does not weld morphology vs. weld variable cable to the plasma effects occurring account for the amount of material studies indicated a large variation in during laser beam welding. melted during the welding process, par­ This paper deals with the laser-plasma- ticularly for highly reflective materials. material interaction as experienced with a Instead, an increase in coupling efficiency * Ifim (micrometer) = 0.00004 in. Raytheon Nd:YAG laser in the pulsed is noted as laser intensities are increased mode at average power levels up to 400 above plasma initiation threshold intensi­ ties. This phenomenon is referred to as Based on a paper sponsored by the DOE watts. In this power regime, we are con­ "enhanced coupling" (Ref. 18) and is Interagency Group for presentation at the 64th cerned primarily with low joint penetra­ Annual A WS Convention held in Philadelphia, tion welds on the order of 1.5-3.0 mm related to the plasma formation and sub­ Pennsylvania, during April 24-29, 1983. (0.06-0.12 in.) penetration. The results of sequent radiation heating of the base metal by the plasma. G. K. LEWIS and R. D. DIXON are with the our studies are applicable to this welding Materials Science and Technology Division, regime, and we have not extended the The significance of the radiant heat flux Los Alamos National Laboratory, Los Alamos, same experimental tests to the multikilo- from the plasma to the base metal is New Mexico. watt deep joint penetration welds made shown in Fig. 2 and equation (1). As the WELDING RESEARCH SUPPLEMENT 149-s A :: ::: 0.8 - ' 4r^^^^^ ^^ ^ 0.6 " // ' A c/> 0.4 ~ 2^r^ ,000 0.2 i i i i i i II 0.2 0.4 0.6 LO 2.0 4.0 6.0 10.0 5000 10,000 5,000 20.000& Wavelength , pm \— Fig. 1 — Reflectance as a function of wavelength Fig. 2 — Blackbody energy flux emitted as a function of wave­ length and temperature temperature of the plasma increases, the 1). This is a significant improvement target leaving a shocked region for the maximum energy flux emitted increases over the 1-10% absorbed without the absorption wave to propagate into. This by the fourth power of temperature and plasma. process is shown schematically in Fig. 3 shifts towards shorter wavelengths. The Because the plasma formation signifi­ (from Ref. 1) with the regions for the short wavelengths are more readily cantly affects the welding process, initia­ relevant plasma variables: pressure (P), absorbed than the infrared laser radiation tion and propagation mechanisms must density (p), particle velocity (U), tempera­ as shown in Fig. 1. The total energy be understood. Incident laser light partial­ ture (T), enthalpy (h) with spatial coordi­ emitted, assuming the plasma radiates as ly absorbed by the target and surround­ nates X and L. The subscript S refers to a black body, is given by the Stefan- ing medium causes heating, vaporization the shock region and the subscript co Boltzmann law: of asperites at high energy density sites, refers to ambient conditions. Note the Wp- = o-T4 (1) electron emission and ion emission (Ref. laser is incident from the left and that the where rr = 5669 X 10"5 ers cm"2 s"1 k~4 20-25). The electron, ion, and neutral absorption wave has been identified as a and T = absolute temperature. atoms constitute a plasma that ignites and Laser Supported Combustion (LSC) wave with velocity (v ). Laser plasmas typically reach tempera­ absorbs the incident laser energy when w tures of 5000-20,000 K (Ref. 1) and thus the plasma temperature and density Laser-target interactions produce two can contribute a significant amount of become high enough. Additional mecha­ distinct types of absorption waves: the heat to the part. However, calculation of nisms contribute to the plasma formation previously mentioned LSC wave and a the heat flux from plasma to target must for low power laser systems. The most laser supported detonation (LSD) wave. account for changing plasma tempera­ dominant are: thermoelectron emission, Both waves contribute to enhanced cou­ ture due to plasma density, velocity, photo-electron emission and inverse pling, but the LSC wave is optimal. Pirri, volume and other factors that are difficult "bremstrallung" in the medium above Kemp, Root and Wu (Ref. 1), among to measure. An upper limit of such a the target. other investigators, have modeled the calculation assumes that the laser light is The ignition is signalled by the creation plasma wave formations and characteris­ totally absorbed by the plasma and yields of a laser supported absorption (LSA) tics. Some of their results are discussed an absorption of energy by the material wave and is preceded by a pressure below, and their models are shown in of 50% of the incident laser energy (Ref. pulse that propagates away from the Figs. 3-5. Precursor Surface - Surface Shock x* L Laser Intensity y *s I, LSC Wave r .... / a) fc. Shock > JI /_Wave VelocitVeto y Equals Particle \ ^ Shocked |" 's Velocity Behind Shock Air | II C- •IB piasma ^Surface Loser I > ^ Laser Intensity I >I, Weak LSD Wove Flux l( 1 1 <> 2 Shock ! 1 b) •> KP | s | • '/ Wave Velocity > Particle Velocity HI 1> Poo u ; i Surface K " x> Laser Intensity shoc|( •~-;J >ck 13**2 LSD Wove X c) 1\ Vw = Abso lute LSC Wave Velocity -Exp. Fan Fig. 3 —Schematic representation of a LSC wave (Ref. 1) Fig. 4 — Transition from LSC to LSD wave (Ref. 1) 50-s | FEBRUARY 1985 Laser Beam Laser Beam Shock Wave Thin Laser •LSD Wave Absorption Zone ±*Hot , High Pressure Air T Blast Wave Expansion FansV ! i: fr Radiation To Surface vyy^yys^iyyyyy/ V7pfrs^77y Conduction Boundary Conduction Layer Fig. 5— Two-dimensional LSC and LSD waves (Ref. 1) After plasma initiation the absorption waves; it shows some of the more salient without an aperture in the beam path.
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