T. A. Palmer

Problems and issues in laser-arc hybrid welding

Abstract Hybrid welding, using the combination of a laser and an electrical arc, is designed to overcome problems commonly encountered during either laser or arc welding such as cracking, brittle phase formation and porosity. When placed in close contact with each other, the two heat sources interact in such a way as to produce a single high intensity energy source. This synergistic interaction of the two heat sources has been shown to alleviate problems commonly encountered in each individual welding process. Hybrid welding allows increased gap tolerances, as compared to laser welding, while retaining the high weld speed and penetration necessary for the efficient welding of thicker workpieces. A number of simultaneously occurring physical processes have been identified as contributing to these unique properties obtained during hybrid welding. However, the physical understanding of these interactions is still evolving. This review critically analyses the recent advances in the fundamental understanding of hybrid welding processes with emphases on the physical interaction between the arc and laser and the effect of the combined arc/laser heat source on the welding process. Important areas for further research are also identified.

Heat transfer and fluid flow in additive manufacturing

In laser-based direct energy deposition additive manufacturing, process control can be achieved through a closed loop control system in which thermal sensing of the melt pool surface is used to adjust laser processing parameters to maintain a constant surface geometry. Although this process control technique takes advantage of important in-process information, the conclusions drawn about the final solidification structure and mechanical properties of the deposited material are limited. In this study, a validated heat transfer and fluid flow laser welding model are used to examine how changes in processing parameters similar to those used in direct energy deposition processes affect the relationships between top surface and subsurface temperatures and solidification parameters in Ti-6Al-4V. The similarities between the physical processes governing laser welding and laser-based additive manufacturing make the use of a laser welding model appropriate. Numerical simulations show that liquid pools with similar top surface geometries can have substantially different penetration depths and volumes. Furthermore, molten pool surface area is found to be a poor indicator of the cooling rate at different locations in the melt pool and, therefore, cannot be relied upon to achieve targeted microstructures and mechanical properties. It is also demonstrated that as the build temperature increases and the power level is changed to maintain a constant surface geometry, variations in important solidification parameters are observed, which are expected to significantly impact the final microstructure. Based on the results, it is suggested that the conclusions drawn from current experimental thermography control systems can be strengthened by incorporating analysis through mathematical modeling.

Solidification Map of a Nickel-Base Alloy

The solidification behavior of the advanced nickel-base alloys, such as Inconel® Alloy 690, is important for understanding their microstructure, properties, and eventual service behavior in nuclear power plant components. Here, an experimental and theoretical program of research is undertaken with the aim of developing a quantitative understanding of the solidification behavior under a wide range of temperature gradients and solidification growth rates. The temperature gradient and solidification rates vary spatially by several orders of magnitude during keyhole mode laser welding. Therefore, the solidification structure is experimentally characterized from microscopic examinations of the resulting fusion zones and correlated with fundamental solidification parameters to provide a widely applicable solidification map that can be employed for a broad range of solidification processes. The cell and secondary dendrite arm spacings are quantitatively correlated with cooling rates. An Alloy 690 solidification map, which illustrates the effect of temperature gradient and solidification rate on the morphology and scale of the solidification structures, is also presented.

Time resolved X-ray diffraction observations of phase transformations in transient arc welds

Abstract In situ X-ray diffraction methods have been developed at Lawrence Livermore National Laboratory for direct observation of microstructural evolution under quasi-steady state and transient welding conditions. Using intense highly collimated synchrotron radiation, the crystal structures in the weld heat affected and fusion zones are probed in real time to monitor solidification and solid state phase transformations during welding. Here the authors review recent work on the development and use of the time resolved X-ray diffraction (TRXRD) technique during transient welding and illustrate its unique capabilities to: directly observe the solidification mode; discover, in real time, the definitive sequence of phase transformations that lead to the final microstructure; and provide quantitative kinetic data of phase transformations through synthesis of TRXRD data with the temperature history obtained through heat transfer modelling. The TRXRD technique has been used to investigate welding induced phase transformations in titanium alloys, low alloy steels, and stainless steel alloys. The results show some of the first real time observations of the weld solidification mode and the evolution of equilibrium and non-equilibrium phases during rapid heating and cooling. When combined with numerical modelling, quantitative phase transformation kinetic data are obtained, allowing for its use in a wide variety of isothermal and non-isothermal processing. The potential for future applications of this and similar techniques is also addressed.

Laser-silicon interaction for selective emitter formation in photovoltaics. I. Numerical model and validation

Laser doping to form selective emitters offers an attractive method to increase the performance of silicon wafer based photovoltaics. However, the effect of processing conditions, such as laser power and travel speed, on molten zone geometry and the phosphorus dopant profile is not well understood. A mathematical model is developed to quantitatively investigate and understand how processing parameters impact the heat and mass transfer and fluid flow during laser doping using continuous wave lasers. Calculated molten zone dimensions and dopant concentration profiles are in good agreement with independent experimental data reported in the literature. The mechanisms for heat (conduction) and mass (convection) transport are examined, which lays the foundation for quantitatively understanding the effect of processing conditions on molten zone geometry and dopant concentration distribution. The validated model and insight into heat and mass transport mechanisms also provide the bases for developing process maps...

Measurement of forced surface convection in directed energy deposition additive manufacturing

The accurate modeling of thermal gradients and distortion generated by directed energy deposition additive manufacturing requires a thorough understanding of the underlying physical processes. One area that has the potential to significantly affect the accuracy of thermomechanical simulations is the complex forced convection created by the inert gas jets that are used to deliver metal powder to the melt pool and to shield the laser optics and the molten material. These jets act on part surfaces with higher temperatures than those in similar processes such as welding and consequently have a greater impact on the prevailing heat transfer mechanisms. A methodology is presented here which uses hot-film sensors and constant voltage anemometry to measure the forced convection generated during additive manufacturing processes. This methodology is then demonstrated by characterizing the convection generated by a Precitec® YC50 deposition head under conditions commonly encountered in additive manufacturing. Surface roughness, nozzle configuration, and surface orientation are shown to have the greatest impact on the convection measurements, while the impact from the flow rate is negligible.

Real time monitoring of laser beam welding keyhole depth by laser interferometry

Abstract The utility of a new laser interferometric technique, inline coherent imaging, for real time keyhole depth measurement during laser welding is demonstrated on five important engineering alloys. The keyhole depth was measured at 200 kHz with a spatial resolution of 22 μm using a probe beam, which enters the keyhole coaxially with the process beam. Keyhole fluctuations limited average weld depth determination to a resolution on the order of 100 μm. Real time keyhole depth data are compared with the weld depths measured from the corresponding metallographic cross-sections. With the exception of an aluminium alloy, the technique accurately measured the average weld depth with differences of less than 5%. The keyhole depth growth rates at the start of welding are measured and compare well with order of magnitude calculations. The method described here is recommended for the real time measurement and control of keyhole depth in at least five different alloys.

Laser-silicon interaction for selective emitter formation in photovoltaics. II. Model applications

Laser doping is an attractive way to manufacture a selective emitter in high efficiency solar cells, but the underlying phenomena, which determine performance, are not well understood. The mathematical model developed in Part I solves the equations of conservation of mass, momentum, and energy and is used here to investigate the effects of processing parameters on molten zone geometry, average phosphorus dopant concentration, dopant profile shape, and sheet resistance. The empirically calculated sheet resistance values are in good agreement with independently measured sheet resistance values reported in the literature. Process maps for output power and travel speed show that molten zone geometry and sheet resistance are more sensitive to output power than travel speed. The highest molten zone depth-to-width aspect ratios and lowest sheet resistances for 532 nm laser beams are obtained at higher laser powers (>13 W) and lower travel speeds (<2 m/s). Once the power level is set, the travel speed can be vari...

Evolution of laser-fired aluminum-silicon contact geometry in photovoltaic devices

The evolution of temperature and velocity fields during laser processing of solar cells to produce an ohmic contact between an aluminum thin film and a silicon wafer is studied using a transient numerical heat transfer and liquid metal flow model. Since small changes in pulse duration, power, and power density can result in significant damage to the substrate and, in extreme cases, expulsion of droplets from the molten zone, the selection of optimal laser processing parameters is critical. The model considers the unusually large heat of fusion of the Al-Si alloy formed during processing and the large composition-dependent two phase region. The calculated size and shape of the fusion zone were in good agreement with the corresponding experimental data, indicating the validity of the model and providing a basis for using the model to develop a better understanding of the laser-assisted fabrication of contacts for solar cell devices. The transient changes in the composition of the Al-Si molten region are fou...

Comparative Shock Response of Additively Manufactured Versus Conventionally Wrought 304L Stainless Steel.

Gas-gun experiments have probed the compression and release behavior of impact-loaded 304L stainless steel specimens that were machined from additively manufactured (AM) blocks as well as baseline ingot-derived bar stock. The AM technology permits direct fabrication of net- or near-net-shape metal parts. For the present investigation, velocity interferometer (VISAR) diagnostics provided time-resolved measurements of sample response for one-dimensional (i.e., uniaxial strain) shock compression to peak stresses ranging from 0.2 to 7.0 GPa. The acquired wave-profile data have been analyzed to determine the comparative Hugoniot Elastic Limit (HEL), Hugoniot equation of state, spall strength, and high-pressure yield strength of the AM and conventional materials. The possible contributions of various factors, such as composition, porosity, microstructure (e.g., grain size and morphology), residual stress, and/or sample axis orientation relative to the additive manufacturing deposition trajectory, are considered...