Mendez Supplement for 10/03 zaida

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Specifications	Mendez Supplement for 10/03 zaida
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WELDING RESEARCH OCTOBER 2003-S296 ABSTRACT. An explanation for penetra- tion and defect generation in the weld pool at high

Specifications	Mendez Supplement for 10/03 zaida
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Content	WELDING RESEARCH OCTOBER 2003-S296 ABSTRACT. An explanation for penetra- tion and defect generation in the weld pool at high currents is proposed. In this regime, the arc pressure pushes the molten metal to the rear of the weld pool, creating a thin layer of liquid metal under the arc. Premature solidification of this thin layer initiates humping, split bead, parallel humping, tunnel porosity, and un- dercutting. The thin nature of the liquid layer is the cause of increased penetration at high currents. We propose a simple model to predict the onset and type of humping defect. Introduction Increases in welding productivity have a potential economic impact of several hun- dred million dollars in yearly worldwide sav- ings. Welding productivity can be improved by increasing welding speed and current; however, this strategy is limited by the ap- pearance of weld pool defects such as humping, split bead, parallel humping, tun- nel porosity, and undercutting. Humping, also called beading, pre- sents a weld bead with an irregular surface contour consisting of a series of beadlike protuberances, as shown in Fig. 1. In a split bead, the weld is split into two inde- pendent parallel seams separated by an empty channel. Parallel humping is a type of split bead in which the parallel seams show humping — Fig. 2. Tunnel porosity is a defect in which an open channel, which remains unfilled with weld metal, is formed at the root — Fig 3. Undercutting is a defect in which the weld bead has par- allel grooves at the side. The bottom left image of Fig. 1 illustrates the appearance of undercutting in a cross section. In addition to the occurrence of de- fects, at currents above 250 A, an unex- plained sudden increase in penetration with current or velocity has been reported (Refs. 1–3). The onset of this sudden change in penetration depends on whether the current or velocity is increas- ing or decreasing, resembling a hysteresis cycle, as shown in Fig. 4. Both curves in this figure correspond to gas tungsten arc welding (GTAW) of steel; the left curve shows experiments performed in partial vacuum (32 mm Hg), the right curve cor- responds to experiments performed at at- mospheric pressure. The generation of humping in some cases, such as laser welding or deposition of microdroplets (Ref. 4), can be under- stood as the consequence of a capillary in- stability in a long liquid body (Ref. 5–7). The capillary instability theory states that humping will occur only when the appar- ent contact angle, indicated as qin Fig. 5, is larger than 90%. In this situation, hump- ing will occur when the weld pool is longer than a critical value LC. Table 1 presents the expressions of LCfor different ap- proaches based on the capillary instability theory. However, capillary instability cannot account for humping observed in bead-on- plate GTA welds, where the apparent con- tact angle is close to 0 deg, and other arc welds where the weld pool is not long enough to trigger a capillary instability. The occurrence of humping in these cases is frequently associated with the large de- pression of the weld pool under the arc ob- served at high currents and velocities. Some of the literature calls this depression the “gouging region.” Yamamoto, Shi- mada et al. (Refs. 1, 2) performed re- search on bead-on-plate GTAW at low pressure and observed that at the onset of humping, the weld pool was very de- pressed and turned into a thin film under the arc. The same effect was observed by Savage et al. (Ref. 8) in bead-on-plate GTAW at atmospheric pressure. The pres- ence of a gouging region at high welding currents and velocities was also reported by others, including Ishizaki (Refs. 9, 10) and Bradstreet (Ref. 5), for bead-on-plate GMAW; Demyantsevich et al. (Ref. 11), for bead-on-plate GTAW; and Matsunawa et al., for GMAW in narrow grooves. In this work, we propose that at high currents, the occurrence of weld pool de- fects and the sudden increase in penetra- tion are a consequence of the same phe- nomenon: a change in the configuration of the weld pool between the low-current and high-current regime. At currents above 250 A, the deep depression of the free surface starts to resemble a keyhole. Weld Pool Geometry at High Current and Velocity Figure 6 shows top, cross, and longitu- dinal views of a GTA weld produced at high current and travel speed. Figure 7 shows the corresponding schematic, with its main features identified. The most salient characteristic is the deep depres- sion of the free surface, exhibiting a ”gouging region” under the arc, a “rim” of molten metal around it, and a bulk of molten metal (“trailing region”) at the rear of the weld pool. The gouging region is a very thin layer of liquid that transports molten metal to the trailing region at the rear of the weld pool. Previous studies (Refs. 13, 14) indi- cate that, in this region, the dominant dri- ving force is the aerodynamic drag of the arc and the balancing force is the viscous resistance of the molten metal. Other dri- ving forces, such as Marangoni (thermo- capillary), electromagnetic, gravity, and inertial forces play a secondary role. Char- acteristic magnitudes of the gouging re- gion are its temperature jump from melt- Penetration and Defect Formation in High-Current Arc Welding At high currents, the weld pool turns into a thin liquid film, causing humping, undercutting, and fingerlike penetration BY P. F. MENDEZ AND T. W. EAGAR P. F. MENDEZ is with Exponent, Inc., Natick, Mass. T. W. EAGAR is with Massachussetts Insti- tute of Technology, Cambridge, Mass. KEYWORDS GTAW GMAW Weld Defects Humping Porosity High-Current Arc Welding Stainless Steel
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Following Datasheets	10-2003-SOLOMON-s-1 (10 pages) 10-2003-VIANCO-s (10 pages) 10-2004-KIMAPONG-s (6 pages) 10-2004-MURUGANANTH-s (10 pages) 10-2004-WANG-s (5 pages) 10-2005-JENKINS-s (8 pages) 10-2005-POORHAYDARI-s (7 pages) 10-50TV_IPC-1752 (3 pages) 10-50TV_097_Rev_A (2 pages) 10-50TVC_IPC-1752 (3 pages)
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