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Brief Summary of Papers and Conference Presentations by the Author During the Course of this Research

Case 4 – Horizontal T-joint fillet welds with gap

2.16 Brief Summary of Papers and Conference Presentations by the Author During the Course of this Research

I presented a paper in an automotive Al session at the ASM Fall Conference in Columbus in 2001 (Harris, 2001).

I presented a paper at the Sheet Metal Welding Conference X in Detroit in 2002, (Harris. 2002) based on weld sizing and procedural tolerances for robust production welding with GMAW-P and GMAW-VP.

As a result of work on hydroformed gas tanks, I was invited to an AISI/SASFT meeting defining and discussing alternative joining technologies for ‘next generation steel gas tanks, held in Detroit in July 2002.

EWI Members Reports were completed in early 2004 on EWI Project 44634IRP ‘Weld Sizing and Gap Weldability for Arc Welding Production Robustness in Sheet Metal’ (Boring and Harris, 2004) and in 2005 on EWI Project 47412GTO ‘Hybrid Laser-GMAW Welding for Sheet Metal and Fill Passes on Plate’ (Harris et al., 2005). OSU was funded to conduct some of the latter work on behalf of EWI, directed and managed by the present author.

Assemble Twist & clamp

Weld Fillet Break off tab Weld slot

Assemble Twist & clamp

A paper on LBW/GMAW hybrid welding with the LBW aimed at the weld toes for increased wetting, based on the results of my research, was presented at the AWS Annual Convention in April 2005 (Harris, 2005). This led to three commercially sponsored projects worth over $100,000 with the heavy fabrication industry for high- speed hybrid LBW/GMAW of steel products.

2.17 Summary of the Literature Review and Work Scope in the Thesis

Research

A summary of known items from the literature review is as follows:  Weld bead humping limits welding travel speed

 Existence of weld bead humping is documented

 Several theories exist but there is a lack of current practical solutions in the literature

 Modeling of the humping event has been conducted, but this, at best, is based on bead on plate work and does not identify the key event between one hump and the next.

A study of weld bead humping was first documented by Bradstreet (1968) but little work was done in the following 25 years. In studying the formation of the humping defect, the observations of (Schiaffino and Sonin,1997 (2)) were interesting in that they noted that Gratzke et al. (1992) rejected the thermocapillary mechanism and proposed that humping was caused by Rayleigh instability, i.e., the breakup of a liquid cylinder by the action of surface and gravity forces. Gratzke concluded that (i), the width/length ratio of the weld pool was the most important factor ,and that (ii) the surface tension does not affect the onset of humping, only the kinetic behaviour. Mills et al. observed that this latter conclusion seems to be inconsistent with the observation that humping is prevalent in high sulphur casts of base metal. Based on the competing findings and opinions of Schiaffino and Sonin, Gratzke, and Mills et al, is seems most likely that surface tension is indeed one of the dominant forces

involved in the mechanism of humping. Gratzke’s proposed Rayleigh instability theory has some merit, but mostly as it involves the effect of surface tension, rather than gravity. Surface tension affects wetting angles both in brazing and welding. It seems likely that anything in the welding process that promotes good wetting and higher contact angles between the base metal and the weld will work to reduce the humping defect. Therefore, although the welding fraternity does not understand the phenomenon, the answer would seem to lie in further study of weld geometry, viscosity, and weld toe wetting. This is the basis for the area of study conducted in this dissertation.

There is evidence (Choi et al) that use of lasers along with GMAW-P can increase the wetting angle of a spot weld, and that some increase in toe wetting angle can be achieved in large fillet welds at slow to moderate travel speeds. However, there has been no work to evaluate the effect of lasers directed at the weld toes for small fillet welds in sheet materials. It seems reasonable that, with sufficient power density, this can be achieved.

 Weld sizing for sheet metal to minimize over-welding and excessive distortion  Examination, using high-speed video (HSV), of weld bead humping

mechanisms at high travel speeds that explain the event occurring between the end of one bead hump and the beginning of the next

 Practical solutions for high-speed welding of sheet metal without humping are needed.

Even fairly recent research (Harwig, 2003) has continued to discuss the limitations of CV GMAW and GMAW-P for high-speed welding of sheet products, characterizing it as between 0.5 and 1 m/min. Based on this characterization, GMAW-VP has been reported to have productivity twice that of GMAW-P (Harwig, 2003). This limitation needs to be evaluated experimentally, and the results achieved with GMAW-P need to be applied to laser/GMAW-P hybrid welding for high-speed fillet welding of lap and T-butt joints.

The productivity of GMAW, GMAW-VP, GMAW-P, TGMAW, and LBW/GMAW hybrid processes have been reviewed here as a context and benchmark for the present research. There is contradictory evidence on the relative productivity of GMAW, GMAW-VP, GMAW-P as noted above, depending on whether one is welding on joints with significant gaps, or on joints with good fit-up. Certainly GMAW-VP has good gap bridging capabilities, but the typical 200-250 A current limit for these power sources makes them of limited use for high-speed GMAW.

Much work has been conducted over the years in the field of welded design for plate structures in terms of fillet weld size requirements. By comparison, little effort, or published work exists in weld sizing for sheet metal structures. Part of the reason for the latter is that sheet products are generally characterized as having thicknesses of 3 mm or less, and that GMAW of fillet sizes smaller than 3 mm is relatively difficult, even for mechanized welding. The area of interest in terms of material thickness is 2 mm and less, as this thickness range is less well suited for welding by conventional processes at high speeds.

In the past, the tendency of over-welding fillet sizes has been less of an important issue, but with the trend toward thinner and higher strength steels, and the continuing emphasis on productivity and global competitiveness, higher welding speeds and reduced distortion are ever more important to reduce cost while meeting the required welding quality standards.

Based on this literature and industry review, the intent was to test and develop the thesis that modification of fluid flow within the GMAW-P weld pool using laser beams can suppress the onset of bead humping by increasing the wetting angle at the bead toes, thus increasing the maximum speed and productivity for production of fillet welds in GMAW-P. It is anticipated that the laser power required to achieve this will be lower than that typically used in ‘conventional’ LBW/GMAW as the laser energy will be directed at the weld toes, rather than in alignment with the axis of the welding arc. A study using HSV recording, to examine the humping defect formation in GMAW-P is needed to gain further insight into the mechanisms and use this in conjunction with weld bead toe wetting with lasers to suppress the humping defect to higher welding speeds.

Although there has been significant work on weld pool fluid flow and modeling of bead humping over the years, almost all of it has been conducted with GTAW. In contrast, there has been very little study of humping in GMAW. The work by

Bradstreet in 1968 was the first to study this in any detail. However, little significant further study is reported until that recently reported by Nguyen et al. in 2005.

Although this work used a LaserStrobe technique to examine metal transfer, it involved little direct viewing of the weld pool dynamics and fluid flow. Hence, a study of the mechanisms for formation of bead humping using HSV is needed to investigate this.

Looking beyond mechanisms, little work has been done to provide practical welding solutions and welding parameters establishing higher welding speeds. Thus, this was also an area requiring more work and in which new knowledge has been added to this field.

Work reported to date has been done with bead on plate only and has not progressed to look at practical welded joints. The work scope presented in this thesis addresses this often overlooked area, addressing differences in weld pool behaviour in this situation.

Work reported to date has been done on plate, and there is no work found in the literature on humping in sheet metal for GMAW. As such, this thesis research

involved conducting trials to generate and evaluate weld bead humping in sheet metal joints where the weld pool depth is, by necessity, shallow. Deeper weld pools in sheet products lead to GMAW ‘cutting’ as the weld penetrates completely through the sheet. The balance between adequate penetration while avoiding cutting is quite fine when welding on sheet metal.

The bead humping defect has been responsible for the limitation of GMAW and GMAW-P to 1-m/min TS for the welding industry at large. There is a need to demonstrate that the process is capable of higher TS using modifications to the welding setup and parameters that have not been adequately developed or demonstrated to date. This work was also planned to develop and also publish suitable welding parameters that can achieve twice the welding speed, 2 m/min, typically attributed to the GMAW process.

A lot of studies of pool dynamics have been carried out, especially in GTAW, but little work has been conducted with GMAW. The situation for arc physics, process modeling, and other aspects is complicated considerably when molten metal is added to the scenario in the form of metal droplets crossing the arc and impacting the liquid weld pool. In the past, modeling in particular has been confined almost completely to GTAW based on the complexity and computational power needed to address GMAW. This has changed in the last few years based on high processing power of modern computers outside of the supercomputers.

3.0 RESEARCH AIMS AND OBJECTIVES