An in situ process control monitor is presented by way of experimental results and simulations, which utilizes a pulsed laser ultrasonic source as a probe and an optical heterodyne displacement meter as a sensor. The intent is for a process control system that operates in near real time, is nonintrusive, and in situ: A necessary requirement for a serial manufacturing technology such as additive manufacturing (AM). We show that the diagnostic approach has utility in characterizing the local temperature, the area of the heat-affected zone, and the surface roughness (Ra ∼ 0.4 μm). We further demonstrate that it can be used to identify solitary defects (i.e., holes) on the order of 10 to 20 μm in diameter. Moreover, the technique shows promise in measuring properties of materials with features that have a small radius of curvature. We present results for a thin wire of ∼650 μm in diameter. By applying multiple pairs of probe–sensor systems, the diagnostic could also measure the local cooling rate on the scale of 1 μs. Finally, while an obvious application is used in AM technology, then all optical diagnostics could be applied to other manufacturing technologies.
A method is presented that combines pulsed laser ultrasonics with laser heterodyne interferometry for possible application as an in-situ process control for additive manufacturing. The method implements two lasers that are focused near the build area (i.e. heat affected zone : 1) a pulsed laser that excites a surface acoustic wave (the probe signal) at or near the build area and 2) a CW laser heterodyne interferometer operating as a sensor that measures the time resolved features of the propagated waves. We have conducted investigations on the utility of this type of in situ probe-sensor system for real time measurement of the local temperature, surface defects, surface roughness, and grain boundary (grain size) determination. The all-optical method allows for remote in-situ process control that can be tailored for different build situations and materials. The current setup utilizes a UV (355 nm) or visible (532 nm) pulse laser and a narrow band 488 nm CW laser. By measuring the surface displacements with sub nm accuracy and by conducting analyses on the arrival time of the signal and frequency, the interferometric technique can characterize materials akin to non-destructive evaluation (NDE). Prior, we have presented evidence on the utility of the technique to measure local temperature, we now present evidence for surface defects/roughness and grain boundary identification. In addition, we are now exploring the utility of laser ultrasound to monitor changes in residual stress.
The change in properties of a propagating ultrasonic wave has been a mainstay characterization tool of the nondestructive evaluation (NDE) industry for identifying subsurface defects (e.g. damage). A variant of this concept could be applicable to 3D additive manufacturing where the existence of defects (e.g. pores) within a sub-layer could mark a product as non-qualifying. We have been exploring the utility of pulsed laser ultrasonic excitation coupled with CW laser heterodyne detection as an all optical scheme for characterizing sub surface layer properties. The all-optical approach permits a straight forward integration into a laser additive processing tool. To test the concept, we have developed an experimental system that generates pulsed ultrasonic waves (the probe) with high bandwidth (<<10MHz) and a surface displacement sensor that can capture the ultrasonic "return" signal with bandwidth close to 300 MHz. The use of high frequencies enables the detection of smaller defect sites. The technique is time resolved with the sensor and probe as point (>>30-200 microns) beams. Current tests include characterizing properties of spot weld joints between two thin stainless steel plates. The long term objective is to transition the technique into a laser additive manufacturing tool.
Fluorescence microscopy is used to explore the efficacy of surface acoustic waves (SAWs), as generated by a pulsed laser “tapping” a surface, to enhance the surface mobility of molecular adsorbates. The candidate adsorbate system under investigation comprises a series of gold clusters that are directly prepared on a silicon (111) surface by laser ablation of nanoparticles. Within the debris field the gold cluster Au8 is tracked because the fluorescence spectrum is known. The gold cluster is distinguished by using band-pass filters and tracked by fluorescence. The SAW source is a pulsed UV (355nm) laser operating at a repetition rate of 100Hz, where the laser fluence is set below the damage threshold of silicon. The experiment measures the location of the emitted light for a particular cluster through a high magnification (100X) imaging microscope that is integrated with a water-cooled (512x512 pixel) EMCCD camera. Image processing algorithms are used to track the light emission. Initial results show that the cluster Au8 moves approximately 0.5 Angstroms/s (when the excitation source is approximately 0.65 cm away). Diffusion and displacement data of adsorbed atoms and molecules on surfaces is sparse, though this value is similar for displacements of gold clusters on polymer films at 440 K, some metal-on-metal adatoms at room temperatures, and other nonmetal-metal interactions. The experiment also includes a laser heterodyne probe which measures the frequency distribution of the surface displacement induced by the pulsed SAWs. Results show that even at a source-to-probe distance of 1.8cm, frequency components up to 120MHz are present. These results suggest that “growing”/synthesizing thin films via surface aggregation of cluster compounds may be feasible.
A CW laser heterodyne spectrometer has been assembled with time resolved data acquisition for probing surface acoustic waves (SAWs) generated by the interaction of a pulsed laser and surface. Literature suggests that SAWs can enhance chemical catalysis, nucleation and surface chemical mobility. Pulsed lasers are known to induce SAWs with bandwidth that is inversely proportional to the pulse width. The goal of this experiment is to apply laser heterodyne spectroscopy to understand the photophysical interactions that promote the formation of laser induced SAWs. The experiment explores the effects produced by a 100 Hz repetition rate UV (355nm) laser with a 6 ns pulse width. Silicon (111) is used as the substrate material, making it is less likely for propagating non linear waves to experience anisotropy. The development of the time-resolved heterodyne spectrometer includes the development of specific data acquisition and software analysis tools to monitor sub-nanometer surface displacements. In addition, to insure that the pulsed laser irradiated material remains within the thermoelastic regime as opposed to ablation, a 2D laser thermal heating model is used to define the duty cycle of the repetitive pulsed laser. Results show that it is possible to measure and analyze laser induced SAWs many centimeters away from the source and substrate dispersion affects the spectral properties of the propagating SAWs. Under controlled conditions, we have measured surface vertical displacements approaching 0.1nm.
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