Time-Resolved Absorptance and Melt Pool Dynamics during Intense Laser Irradiation of a Metal

Abstract

This work of the U.S. Government is not subject to U.S. copyright. I. AbstractHigh irradiance lasers incident on metal surfaces create a complex, dynamic process through which the metal can rapidly change from highly reflective to strongly absorbing. Absolute knowledge of this process underpins important industrial laser processes like laser welding, cutting, and metal additive manufacturing. Determining the time-dependent absorptance of the laser light by a material is important, not only for gaining a fundamental understanding of the light-matter interaction, but also for improving process design in manufacturing. Measurements of the dynamic optical absorptance are notoriously difficult due to the rapidly changing nature of the absorbing medium. This data is also of vital importance to process modelers whose complex simulations need reliable, accurate input data; yet, there is very little available. In this work, we measure the time-dependent, reflected light during a 10 ms laser spot weld using an integrating sphere apparatus. From this, we calculate the dynamic absorptance for 1070 nm wavelength light incident on 316L stainless steel. The time resolution of our experiment (< 1 µs) allows for the determination of the precise conditions under which several important physical phenomena occur, such as melt and keyhole formation. The average absorptances determined optically were compared to calorimetrically-determined values, and it was found that the calorimeter severely underestimated the absorbed energy due to mass lost during the spot weld. Weld nugget cross-sections are also presented in order to verify our interpretation of the optical results, as well as provide experimental data for weld model validation. II.Figure 1: a. Optical profilometry data and cross section of a sample representative of that used in this study. b. Optical microscope image of same location as used in a.

Depth Dependence and Keyhole Stability at Threshold, for Different Laser Welding Regimes

“…Only time-dependent measurement of the absorptivity, obtained for example by measuring the reflected laser power from the sample by using an integrating sphere, can be suitable to follow its evolution. This technique has been employed by Simmonds et al [36], but during a "spot welding" (SP) process, where the welding speed V w = 0, so the sample was irradiated locally in a pulsed mode in order to generate a weld nugget. In fact, one can also apply the previous scheme of KH formation by considering that there is also a threshold for generating the keyhole for these static conditions.…”

“…Experimentally, by using an integrating sphere, the time dependence of the reflected beam was measured for different incident laser powers, and a power threshold P t-SP (defined such that the melt pool depth equals the spot diameter) has been estimated [36]. As expected, for P < P t-SP , the absorptivity of the melt pool surface is constant at about 0.30, and for P > P t-SP , because of the multiple reflections inside the increasing depth of the cylindrical KH, the absorptivity increases with the incident laser power while remaining stable.…”

Bulk-Explosion-Induced Metal Spattering During Laser Processing

“…Based on the multiple reflections and Fresnel absorption [26,30,58], the ray-tracing technique is used to estimate the absorption intensity of a given keyhole shape extracted from the x-ray image frame with and without the tonguelike protrusion. First, artificial keyhole shapes are constructed based on experimental observations.…”

Measurements of thermophysical properties of solid and liquid NIST SRM 316L stainless steel

“…The National Institute of Standards and Technology (NIST) standard reference material (SRM) for 316L stainless steel (1155a) has recently been used for studies of intense laser light coupling in metal to provide data for the validation of multiphysics models of industrial laser processes like welding, cutting, and additive manufacturing [1]. In order to reduce the costs associated with empirical trial-anderror production development, manufacturers are increasingly looking to multiphysics computer simulations to more rapidly optimize process parameters.…”

“…The vast parameter space allowed by the variation in thermophysical properties allows a model to be tuned to give a satisfactory answer without ever knowing if the chosen parameters are accurate, which limits the model's ability to be predictive. The larger context of the work presented here is that our measured material properties are directly linked to experimental data useful for laser weld model validation [1], which will allow for more rigorous testing of laser weld model predictions.…”