Metal Transfer in Gas Metal Arc Welding: Droplet Rate A study of droplet rates for various transfer modes verifies optimum operating parameters
BY S. LIU AND T. A. SIEWERT
ABSTRACT. This study reports the changes in droplet-transfer mode and rate during gas metal arc welding as the voltage is varied at a series of current levels. The droplet-transfer rate was found to be maximum (approximately 100 s_1) for the voltage/current combinations that are normally suggested by the electrode manufacturers and are considered optimum in the judgment of experienced welders. At voltages above or below the TV-wide optimum range, the transfer rate decreased by about 10 s _ 1 per V in the vicinity of the optimum condition. Furthermore, statistical analysis of the arc current and voltage data showed that during operation outside the optimum range, the welding arc was unstable and the current output was very irregular with varying cycle time between each droplet transfer. At the maximum droplet-transfer rate, the droplet-transfer cycle time was very consistent and revealed a narrow rate range, which correlated with the high stability and lower spatter at these optimum operating conditions. The possibility of using the concept of maximum droplet-transfer rate range with minimum rate fluctuation and corresponding arc current-voltage signals as a means of short-circuiting welding process control and automation is being considered. At voltages below the optimum range, high-speed video recording confirmed that the shortcircuiting transfer was very unstable and the arc reignited explosively. Above the optimum voltage, the arc became longer and the droplets became visibly larger, with mixed globular and short-circuiting transfer. The droplets, however, were no longer directed uniformly to the weld pool, resulting in increased spatter.
Introduction Metal Transfer Mode in Arc Welding Arc welding includes the major joining processes used in manufacturing industries. Examples are shielded metal arc (SMA), gas metal arc (GMA), flux cored
52-s I FEBRUARY 1989
arc (FCA), submerged arc (SA) and gas tungsten arc (GTA) welding. With the exception of GTA welding, all these processes require a consumable electrode, which has the dual function of carrying the current that heats the weld pool and providing filler metal to complete the weld joint. This dual function has long been a topic of research. Spraragen and Lengyel (Ref. 1) reviewed the basic principles of an electric arc and summarized the development of the field of welding arc physics. In particular, they concluded that in the area of liquid metal transfer from the electrode to the weld pool, the electromagnetic pinch force, gravity, shielding gas drag force and surface tension are the major forces that act on the electrode tip. Using high-speed cinematographic techniques, Muller, Greene, and Rothschild (Ref. 2) found that large spherical liquid-metal droplets in a GMA arc decreased in size with increasing current. As the electrode feed rate was continuously increased, however, a sudden decrease in droplet size occurred at what was termed the transition current. In addition, they determined that with inert gas shielding, the droplet composition remained constant during the metal transfer. Lesnewich (Refs. 3-5) investigated the physics of arc welding using SMA and CMA welding. Particularly, he studied the effects of welding process parameters such as current, voltage, electrode polar-
KEY W O R D S Droplet Transfer to G M A W Metal Transfer Droplet Rate Droplet-Transfer Mode Short Circuiting Globular Transfer Statistical Analysis Current Variations Voltage Variations Arc Behavior
ity, electrode extension and diameter on the electrode melting rate and metal transfer. In carbon steel welding of 1.2mm (0.045-in.) diameter electrodes with constant arc length and 99% Ar-1% 0 2 shielding gas, a transition in metal transfer occurred at approximately 250 A, and below this current, the droplet rate decreased abruptly from more than 200 to 10 s~1. Figure 1 illustrates this transition in droplet-transfer rate with welding current. The transition current increased with increasing electrode diameter and decreased with increasing electrode extension. From his GMAW studies, Ludwig (Refs. 6, 7) confirmed that the electromagnetic force is the major force among the chemical and physical forces that affect the formation and subsequent transfer of metal droplets. Over a welding current range of 200 to 450 A, no abrupt change in metal transfer was observed. The frequency of droplet transfer decreased continuously with decreasing welding current, as shown in Fig. 1. Pintard (Ref. 8) also observed no abrupt variation in the transfer rate at current levels below 220 A. However, there was a transfer mode change at 190 A. Above this current level, the electrode tip shape changed from hemispherical to conical, and the metal droplets, which detached from the tip of the electrode became smaller than the electrode diameter. Needham, Cooksey and Milner (Ref. 9) investigated the droplet-transfer rate for aluminum G M A W and confirmed the presence of a transition current. Disconti-
5. LIU is with the Center for Welding and joining Research, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colo. T. A. SIEWERT is with the Fracture and Deformation Division, National Institute of Standards and Technology, Boulder, Colo. Based on a paper presented at the 68th Annual AWS Meeting, held March 22-27, 1987, in Chicago, III.
nuity in the d r o p l e t rate w a s seen at the transition current. In a m o r e recent investigation o f aluminum-magnesium alloys w e l d i n g , W o o d s (Ref. 10) s h o w e d the i m p o r t a n c e of the e l e c t r o d e chemical c o m p o s i t i o n in d e t e r m i n i n g the metal transfer m o d e a n d spatter levels. He indicated that the transfer characteristics of all alloy filler metal electrodes w o u l d also d e p e n d u p o n the c o n c e n t r a t i o n o f the high-vapor-pressure alloying elements i n c o r p o r a t e d in t h e electrodes. V a p o r i z a t i o n of elements such as m a g n e sium and zinc c o u l d cause arc instability and in-flight explosion of t h e metal d r o p lets, resulting in unstable metal transfer. O n e of the most u p - t o - d a t e a n d c o m prehensive r e v i e w s of metal transfer m o d e s d u r i n g arc w e l d i n g w a s w r i t t e n b y Lancaster (Ref. 11). A c c o r d i n g t o t h e IIW n o m e n c l a t u r e r e f e r e n c e d in his b o o k (Ref. 12), metal transfer can b e classified into t h r e e main g r o u p s : free-flight transfer, bridging transfer and slag-protected transfer. In free-flight transfer, the elect r o d e does n o t c o n t a c t t h e m o l t e n metal p o o l . M e t a l d r o p l e t s detach f r o m t h e tip of the e l e c t r o d e and m o v e across t h e arc c o l u m n . W h e n the e l e c t r o d e contacts the w e l d p o o l , bridging transfer occurs. For w e l d i n g processes that use large a m o u n t s o f fluxes, metal transfer m a y i n v o l v e layers o f slag, k n o w n as slagp r o t e c t e d transfer. The c o m p l e t e IIW classification of metal transfer m o d e s w i t h examples is r e p r o d u c e d in Table 1 a n d schematically illustrated in Fig. 2. In most of the w e l d i n g literature, the metal transfer m o d e is described o n l y b y t h e t e r m s of short-circuiting, globular and spray. At l o w - c u r r e n t and l o w - v o l t a g e levels, short-circuiting transfer (intermittent-bridging transfer) occurs. A t a slightly higher v o l t a g e level, globular (free-flight) transfer results. A n d a b o v e t h e transition c u r r e n t , fine metal droplets are p r o p e l l e d across the arc t o w a r d s the w e l d p o o l and spray (free-flight) transfer b e c o m e s the predominant mechanism.
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e c o n o m i c a l shielding gas (Refs. 13-15). In C 0 2 w e l d i n g , Dilthey (Ref. 16) r e p o r t e d t h r e e m o d e s o f metal transfer: globular, subarc a n d short-circuiting. During globular transfer, large metal d r o p s are f o r m e d . S o m e of these d r o p l e t s are repelled f r o m the e l e c t r o d e tip; they m o v e u p w a r d or sideways in an irregular p a t t e r n , a n d deposit outside t h e m o l t e n p o o l in t h e f o r m of spatter. T h e subarc m o d e is characterized b y a higher current density; t h e arc has a v e r y short length ( l o w voltage) a n d occurs b e l o w the w o r k p i e c e surface. Small d r o p l e t s are d e t a c h e d a n d transferred across the arc. A c c o r d i n g t o Rothschild (Ref. 17), subarc
c a n n o t b e classified as a t r u e spray transfer since the droplets are transferred laterally into t h e w e l d p o o l . In the short-circuiting m o d e , the elect r o d e f e e d rate is slightly higher than the melting rate in t h e arcing m o d e , so the tip o f the e l e c t r o d e eventually dips into the m o l t e n p o o l a n d extinguishes the arc (Ref. 18). This short circuit reduces the arc voltage t o z e r o a n d causes t h e w e l d i n g current t o rise v e r y rapidly. During this p e r i o d o f increased c u r r e n t f l o w , t h e e l e c t r o d e heats according t o Joule's law a n d begins t o melt. The e l e c t r o m a g n e t i c pinch f o r c e also increases strongly, pushing the m o l t e n metal in the e l e c t r o d e
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Short-Circuiting Transfer in CO2 Welding In t h e early 1950's, c a r b o n d i o x i d e (CO2) w a s i n t r o d u c e d in Europe as an alternate shielding gas in arc w e l d i n g . Aside f r o m t h e high w e l d i n g s p e e d and d e p o s i t i o n rate, and g o o d w e l d p e n e t r a t i o n achieved in C O 2 w e l d i n g , t h e main reason for utilizing this reactive gas is its l o w cost. H o w e v e r , t h e i m p r o p e r use of C O 2 causes a large a m o u n t of spatter, w h i c h m a y clog the gas p o r t s of a w e l d ing g u n and a l l o w n i t r o g e n t o enter the w e l d p o o l and e m b r i t t l e it. F u r t h e r m o r e , p o s t w e l d cleaning and grinding o p e r a tions are o f t e n r e q u i r e d t o r e m o v e the spatter. T h e r e f o r e , several researchers have tried t o d e v e l o p a b e t t e r understanding of the metal transfer characteristics of this process f o r b e t t e r use o f this
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