High-speed X-ray Transmission Research Articles

Simultaneous high-speed x-ray transmission imaging and absolute dynamic absorptance measurements during high-power laser-metal processing

During high-power laser metal processing, the absorbed light is intimately related to the molten metal cavity shape. For the first time, this relationship is observed directly and simultaneously by implementing state-of-the-art techniques of high-speed x-ray imaging and integrating-sphere radiometry. Experiments were performed on Ti-6Al-4V solid and powder under single spot laser illumination for laser conditions that cause keyhole formation and collapse. The data from x-ray imaging corroborates that a dramatic rise in laser absorption is due to keyhole formation. We also find that the keyhole area correlates most strongly with energy absorption followed closely by keyhole depth. Furthermore, time synchronization reveals correlations between keyhole fluctuations and sinusoidal variations in energy absorption that occur during nominally “stable” keyhole conditions. Absorption data show a 24 % periodic change in absorbed laser power at a 50 kHz frequency. The absorption peaks correlate to relatively large, open keyholes, whereas images taken at the troughs reveal keyholes with substantial undulations of the keyhole sidewalls. These observations give crucial, quantitative data for computational modeling whose aim is to predict porosity and defect formation.

High-Speed X-Ray Transmission and Numerical Study of Melt Flows inside the Molten Pool during Laser Welding of Aluminum Alloy

By using the X-ray transmission imaging system, melt flows inside a molten pool were studied during laser welding of aluminum alloy at different welding speeds. Then, the correlation between temperature gradients along the direction of weld penetration and melt flows in the rear part of a molten pool was analyzed by using a three-dimensional numerical method. And the presented model was verified by experimental results. The corresponding investigation was carried out to further study the correlation between temperature gradient and melt flow behavior of the molten pool in the plate heated by preheating temperature. The results indicated that, in the rear part of the molten pool, the maximum flow velocity was located at the bottom of the molten pool. The melt metal in the rear molten pool caused by different welding speeds had significantly different flow trends. As the welding speed increased, the absorbed intensity on the keyhole front wall also increased as well as the recoil pressure that could maintain the keyhole opened. Consequently, the increase of the welding speed was more beneficial to improving the stability of the molten pool.

High power disk laser-metal active gas arc hybrid welding of thick high tensile strength steel plates

In this study, welding with hybrid heat sources combined with a high power disk laser and a metal active gas (MAG) arc was carried out on 780 MPa high strength steel plates of 12 mm in thickness. The effects of respective welding conditions, such as laser power, root gap and welding speed on the penetration, geometry, and defects of weld beads, were investigated. The results showed that the process window of welding conditions for the production of good welded joints was narrow. Also, the laser keyhole behavior, the molten pool geometry, and the melt flows inside the molten pool were observed by the high-speed x-ray transmission real-time imaging system. It was confirmed that the melt flows inside the molten pool during hybrid welding were different between humping and good weld beads. The melt flowed from the bottom tip of a keyhole to the humping portion and did not flow forward to the keyhole bottom tip, resulting in the formation of the humping defect. On the other hand, in the case of good weld beads, the tungsten particle flowed to the back part of the molten pool but then flowed to the forward keyhole. It was clarified that a good weld bead without humping nor underfilling could be produced due to the forward melt flow.

X-ray investigation of melt flow behavior under magnetic stirring regime in laser beam welding of aluminum

The use of magnetic fields to influence weld bead shape and dilution in laser welding of aluminum alloys was recently suggested. It was already demonstrated for the case of laser welding of hot-cracking sensitive aluminum alloys with silicon-containing filler wire that applying alternating magnetic fields has an impact on the dilution of silicon in the melt pool, yielding a sufficient silicon content throughout the weld and allowing to suppress hot-cracking. However, the interaction mechanisms between the aluminum melt and the magnetic field are still subject of current investigations and are not fully revealed yet. The behavior of the melt flow under influence of an external magnetic field can be visualized by microfocused high-speed x-ray transmission imaging. To do so, high density tracer materials such as tin (Sn) and tungsten (W) particles that follow the melt flow are introduced into the base material. It can be seen that the additionally induced forces of the magnetic field cause higher velocities in the melt pool. Moreover, the flow of molten liquid perpendicular to the magnetic field is modified. The experimental results are discussed in light of general theoretical assumptions concerning a liquid metal flow under the influence of an external magnetic field. It is established that the effect of the alternating magnetic field can be explained as an anisotropic pulsating electromagnetic force brake that causes a specific deflection of the liquid metal flow.

Laser welding of die-cast AZ91D magnesium alloy

It is well known in welding of die-cast magnesium alloys that porosity is easily formed and degrades the performance of the welded joints. The porosity produced inevitably during laser deep-penetration welding has been studied through metallographic observation, gas content analyses and a high-speed X-ray transmission real-time imaging system. Porosity formation was chiefly attributed to the phenomena that the gases in the pre-existing pores in the base material were swelled to form bubbles during welding and these bubbles had few chances to escape from the weld molten pool because of the vigorous melt flows and the rapid solidification associated with high welding speeds. It was also observed that keyhole instability contributed to porosity formation by generating bubbles from a keyhole tip or a bulged keyhole. Moreover, the insertion of an extruded porosity-free alloy sheet between the die-cast alloy parts resulted in 50% reduction in the weld metal porosity.

High-speed X-ray transmission tomography for detonation investigation

This article describes the procedure and results of high-speed X-ray tomography application to reconstruct density distribution of detonation products of condensed explosives from the results of measurements with synchrotron radiation. Data obtained for a cylindrical charge (12.5 mm in diameter) of pressed TNT of density of 1.6 g/cm 3 are compared with the results of solution of the corresponding gas-dynamic problem of stationary detonation. Qualitative and satisfactory quantitative compliance of the experimental and calculated results is established. Particularities of the procedure and the distribution obtained are analyzed. It is noted that the procedure on the whole and the results can be used to define the known equations of state of detonation products more accurately and to construct new ones.