For this, first, the effects of GMAW-P parameters on the various types of droplet detachments were studied and classified in different zones on the basis of type of droplet detachment. Secondly, the average current per pulse cycle time was measured from the each waveform for the various types of droplet detachments and the values of mean, standard deviation, and coefficient of variation were calculated. The optimum GMAW-P was selected by choosing the pulse parameter which resulted in the least standard deviation and the coefficient of variation, to produce cost-effective, spatter free, high quality welds. Among four types of droplet detachments studied, one droplet detachment during peak duration was considered to be the best for providing a more uniform average current than the other types of droplet detachments. Thus processintegrated quality assurance of the weld was possible on the basis of more uniform average

current.

Statistical analysis of fluctuations in average current per pulse cycle time using a personal computer was carried out for the quantitative assessment of welding arc stability. The computer analysis represents a powerful means of measuring the degree of arc stability and filler metal melting uniformity. This technique facilitates research into process-integrated quality assurance and to improve the design of GMAW-P equipment to ensure that preferred welding parameters are used and to research the cause of variable performance of GMAW-P equipment in actual practice. The study can also be extended to monitor the manual skill of welding machine operators during their training or actual work. Importantly during underwater welding, the instructor cannot find out the weld quality because of poor visibility of the prevailing or existing condition.

CONCLUSION

1. Statistical analyses of average current from the pulsed current waveform using a computer provide a powerful means of directly assessing the arc stability. This can be very helpful for monitoring and improving the GMAW-P process and for optimizing the parameters of the GMAW-P process, optimizing filler metal, welding equipment, and power sources.

2. Every variation in the melting and detachment of filler metal can be seen in the waveforms of pulsed voltage and pulsed current. This study helped to obtain optimum arc behavior in terms of arc stability, spatter level, and droplet detachment in order to ensure optimum weld quality.

3. The peak duration is critical, for if it is too long or too short it results in the generations of excessive spatter.

4. The zone 'C' is preferable among all the zones to carryout welding. In this zone, only the one droplet detachment during peak duration was desirable because the standard deviation and coefficient of variation of average current were found to be the lowest, among from the four types of droplet detachments.

5. The peak energy required for one droplet detachment during background duration was the lowest. Furthermore, one droplet detachment during peak duration, two droplets detachment during peak duration, and three droplets detachment during peak duration

require higher peak energies, increasing in the order given. The highest peak energy of 40.76 J was observed in the case of three droplets detachment during peak duration.

6. The most uniform average current was observed for the welds made with one droplet detachment during peak duration (weld no. EL-1) compared with the average currents for one droplet detachment during background duration, two droplets detachment during peak duration, and three droplets detachment during peak duration. Because, among from the four types of droplet detachments, the lowest standard deviation (0.9228) and the lowest coefficient of variation (0.8361) were found for the weld no. EL-1. Based on this, the following parameters of the pulsed current were found to be more suitable than the other combinations of pulsed current: Ip= 196.8 A; Tp=5.0 ms; I = 20 A; TB = 6 ms; WF = 100 mm/s; IAV = 110.3718 A; and Ws = 10.0 mm/s. Thus process-integrated quality assurance of the weld was possible on the basis of more uniform average current. This study can be very much useful for fabricators to derive the fullest benefits from GMAWP in high quality or automatic or robotic welding.

7. The value of Ip should be set above the spray current level and Tp should be so adjusted that the filler metal tip is melted to a proper size and one droplet is detached appropriately to the base metal during peak duration.

8. Control parameters that characterize the arc stability include average current, peak energy, and droplet detachment time. Monitoring and analyzing these parameters can make sure that the GMAW-P process will be carried out in a optimum droplet detachment type, thus ensuring the best welding conditions for the optimum droplet detachment.

REFERENCES

1. Rajasekaran, S.; Kulkarni, S.D.; Mallya, U.D.; Chaturvedi, R. C. 1998. Droplet detachment and plate fusion characteristics in pulsed current gas metal arc welding. Welding Journal 77 (6): 254-s to 269-s.

2. Rajasekaran, S. 1999. Weld bead characteristics in pulsed GMA welding of Al-Mg alloys. Welding Journal 78 (12): 397-s to 407-s.

3. Rajasekaran, S. 2000. Weld surface undulation characteristics in the pulsed GMA welding process. The Ninth International Conference on Computer Technology in Welding, September 28-30,1999, National Institute of Standards and Technology, United States Department of Commerce, Special Publication No. 949, May 2000, 349-360.

4. Rajasekaran, S.; Kulkarni, S.D.; Mallya, U.D.; Chaturvedi, R. C. 1995. Molten droplet detachment characteristics in steady and pulsed current GMA welding Al-Mg alloys. Proceedings of the Sixth International Conference on Aluminum Weldments (INALCO '95), April 3-5, 1995, Cleveland, Ohio, U.S.A. American Welding Society, Miami, Fla, 207-224.

5. Rajasekaran, S.; Kulkarni, S.D.; Mallya, U.D.; Chaturvedi, R. C. 1994. Droplet detachment and plate fusion characteristics in pulsed current gas metal arc welding. Proceedings of the Advanced Joining Technologies for New materials II, March 2-4, 1994, The Cocoa Beach Hilton, Florida, U.S.A, American Welding Society, Miami, Fla, 172-187.

DROPLET OSCILLATION AND WELD POOL IMAGING

USING COMPUTER-CONTROLLED COMPOSITE PULSE CURRENT

B. Zheng

ABSTRACT

This paper presents a unique solution to real-time monitoring of both droplet detachment and weld pool during a pulsed gas metal arc welding process: a composite pulse current consisting of a square wave form followed by a sine wave form was designed. The instant for initiating a constant base current adaptively started at the instant droplet detachment was sensed. During the period of the base current, a flag signal was generated to trigger the imaging of a weld pool. The approach makes droplet detachment and image acquisition be proceeded without outside intervention. This provides the possibility for real time quantifiable monitoring and control for both droplet transfer and weld pool penetration.

INTRODUCTION

The results of the previous researches showed that metal transfer and weld pool penetration both have a large influence on the generation of the defects such as undercut, burn through, insufficient melting, spatter, gas pore, and even weld cracks during a gas metal arc welding (GMAW) process {Ref. 1-3). Hence, in-process monitoring and control for metal transfer and weld pool penetration are crucial in order to minimize the cost of post-weld inspection and repair. Currently, a droplet/arc oriented control strategy is often used even though it is inefficient in some applications. Since maintaining a consistent weld pool has a dramatically direct impact on weld quality, a weld pool oriented control strategy has been largely demanded. In a GMAW process with argon-rich shielding, the different metal transfer modes of shortcircuiting, globular, and spray can be observed in sequence as the welding current is increased (Ref. 2-5) when steel electrode wire is used. The critical welding currents at which the metal transfer mode changes are defined as transition currents, one of which is the spray transition current at which globular transfer becomes spray transfer (Ref. 2-3). The level of the spray transition current mainly depends on many factors such as wire diameter and composition of shielding gas. Quality welds can be achieved using the projected spray transfer mode of one droplet per pulse (ODPP). To ease the flexible selection of ODPP and simplify the adjustment of process parameters, pulsed current welding (GMAW-P) is a preferred process (Ref. 3 and 6-9). However, most of GMAW-P processes with ODPP mode use open loop control that regulates only the metal transfer mode and pay little attention to the weld pool (even though some researches have been pursued on sensing and closed-loop control of droplet transfer) (Ref. 4-21). The partial reason for this is the difficulty in measuring characteristic signals and control compatibility of droplet detachment with weld pool penetration.

Besides through-arc sensing of a weld pool (Ref. 22-28), machine vision systems have been used to sense the weld pool and control the full penetration during a PAW, or GTAW, or GMAW process (Ref. 29-45). These vision systems can be characterized into band-pass arc light filtering,

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