Optical Investigation of Transport Phenomena During Solidification Process

1999 ◽  
pp. 65-78
Author(s):  
Yau-Ming Chen ◽  
Yuan-Jen Chang ◽  
Yeong-Tsuen Hsieh
Keyword(s):  
Transport Phenomena ◽  
Solidification Process ◽  
Optical Investigation

Transport Phenomena and the Associated Humping Formation in Laser Welding

2005 ◽  
Author(s):  
J. Zhou ◽  
H. L. Tsai ◽  
P. C. Wang
Keyword(s):  
Heat Transfer ◽  
Liquid Metal ◽  
Laser Welding ◽  
High Speed ◽  
Solidification Rate ◽  
Transport Phenomena ◽  
Metal Flow ◽  
Welding Process ◽  
Solidification Process ◽  
Melt Flow

Humping is a frequently observed welding defect in laser welding which is caused when the welding speed exceeds a certain limit while the other welding conditions remain unchanged. Humping is characterized by the appearance of unsmooth and discontinuity of humps at the surface of the weld. The formation of humping is generally understood to be caused by the complex heat transfer and melt flow in a high speed welding process. However, so far the fundamental mechanisms causing humping are not fully understood, and research on determining the onset of humping has been based on the “trial-and-error” procedure. In this paper, mathematical models previously developed by the authors for the transport phenomena in laser welding have been extended to investigate the formation of the humping defect. In this study, the transient heat transfer and melt flow in the weld pool during the keyhole formation and collapse, and melt solidification are calculated for a 3-D moving laser welding. Different humping patterns have been predicted by the present study in different laser power levels and welding speeds. From the present study, it was found that the formation of humping in laser welding is caused by the interplay between two important factors: a) the strong liquid metal flow in the real part of the keyhole induced mainly by the laser recoil pressure and b) the rapid solidification rate of the liquid metal. The humping pattern can be well explained by the calculated melt flow and the solidification process.

Transport Phenomena and Defect Formation in Laser Welding of Zinc-Coated Steels

2004 ◽  
Author(s):  
J. Zhou ◽  
H. L. Tsai ◽  
P. C. Wang
Keyword(s):  
Laser Welding ◽  
Weld Pool ◽  
Defect Formation ◽  
Solid Phase ◽  
Transport Phenomena ◽  
Solidification Process ◽  
Zinc Vapor ◽  
Pulse Profile ◽  
Weld Defects ◽  
Steel Sheets

Zinc-coated steels are used extensively in the auto industry because they are inexpensive, durable and have high corrosion resistance. Lasers are being used to weld zinc-coated steels due to high welding speed, small seam and narrow heat affected zone. However, it is difficult to laser weld lap-joint zinc-coated steel sheets under a very small gap condition between the metal interfaces since there is a considerable amount of zinc vapor generated. This vapor must be vented out; otherwise it will be trapped in the weld pool leading to different welding defects, such as large voids at the tip of the weld and porosities in the form of small bubbles in the weld. These defects can significantly decrease the strength of the weld. In this paper, a mathematical model and the associated numerical techniques have been developed to study the transport phenomena in laser welding of zinc-coated steels. The volume-of-fluid (VOF) method is employed to track free surfaces. The continuum model is used to handle the liquid phase, solid phase and mushy zone of the metal. The enthalpy method is employed to account for the latent heat during melting and solidification. The transient heat transfer and melt flow in the weld pool during the keyhole formation and collapse processes are calculated. The escape of zinc vapor through the keyhole and the interaction between zinc vapor and weld pool are studied. The aforementioned weld defects are found to be caused by the combined effects of zinc vapor-melt interactions, keyhole collapse and solidification process. By controlling the laser pulse profile, it is found that the keyhole collapse and solidification process can be delayed, allowing the zinc vapor to escape, which results in the reduction or elimination of weld defects.

The effect of back-melting on the continuity of crystallographic orientation and composition in HgZnTe

1992 ◽  
Vol 50 (2) ◽  
pp. 1696-1697
Author(s):  
H.J. Zuo ◽  
M.W. Price ◽  
R.D. Griffin ◽  
R.A. Andrews ◽  
G.M. Janowski
Keyword(s):  
Directional Solidification ◽  
Space Shuttle ◽  
Solidification Process ◽  
Space Experiment ◽  
Infrared Detection ◽  
Directionally Solidified ◽  
Crystallographic Defects ◽  
Semiconducting Alloys ◽  
Uniform Composition ◽  
Growth Experiments

The II-VI semiconducting alloys, such as mercury zinc telluride (MZT), have become the materials of choice for numerous infrared detection applications. However, compositional inhomogeneities and crystallographic imperfections adversly affect the performance of MZT infrared detectors. One source of imperfections in MZT is gravity-induced convection during directional solidification. Crystal growth experiments conducted in space should minimize gravity-induced convection and thereby the density of related crystallographic defects. The limited amount of time available during Space Shuttle experiments and the need for a sample of uniform composition requires the elimination of the initial composition transient which occurs in directionally solidified alloys. One method of eluding this initial transient involves directionally solidifying a portion of the sample and then quenching the remainder prior to the space experiment. During the space experiment, the MZT sample is back-melted to exactly the point at which directional solidification was stopped on earth. The directional solidification process then continues.

Interfacial structures of dissimilar materials joined by capacitor-discharge welding

1994 ◽  
Vol 52 ◽  
pp. 534-535
Author(s):  
C. P. Doğan ◽  
R. D. Wilson ◽  
J. A. Hawk
Keyword(s):  
Rapid Solidification ◽  
Fusion Zone ◽  
Dissimilar Materials ◽  
Solidification Process ◽  
Weld Interface ◽  
Capacitor Discharge ◽  
Fine Grained ◽  
Iron Aluminum ◽  
Conductive Ceramics ◽  
Capacitor Discharge Welding

Capacitor Discharge Welding is a rapid solidification technique for joining conductive materials that results in a narrow fusion zone and almost no heat affected zone. As a result, the microstructures and properties of the bulk materials are essentially continuous across the weld interface. During the joining process, one of the materials to be joined acts as the anode and the other acts as the cathode. The anode and cathode are brought together with a concomitant discharge of a capacitor bank, creating an arc which melts the materials at the joining surfaces and welds them together (Fig. 1). As the electrodes impact, the arc is extinguished, and the molten interface cools at rates that can exceed 106 K/s. This process results in reduced porosity in the fusion zone, a fine-grained weldment, and a reduced tendency for hot cracking.At the U.S. Bureau of Mines, we are currently examining the possibilities of using capacitor discharge welding to join dissimilar metals, metals to intermetallics, and metals to conductive ceramics. In this particular study, we will examine the microstructural characteristics of iron-aluminum welds in detail, focussing our attention primarily on interfaces produced during the rapid solidification process.

Advanced Transport Phenomena

2014 ◽  
Author(s):  
P. A. Ramachandran
Keyword(s):  
Transport Phenomena

Network analysis of transport phenomena in flow fields

2014 ◽  
Vol 2 ◽  
pp. 314-317
Author(s):  
Naoya Fujiwara
Keyword(s):  
Network Analysis ◽  
Transport Phenomena ◽  
Flow Fields
Sign in / Sign up

Export Citation Format

Share Document