Mapping the Droplet Transfer Modes for an ER100S-1 G M A W Electrode Basic information about the relationship between arc signals and transfer modes advances automated welding BY P. R. H E A L D , R. B. M A D I G A N , T. A. SIEWERT A N D S. LIU ABSTRACT. Welds were made with a 1.2mm-diameter AWS ER100S-1 electrode using Ar-2% 0 2 shielding gas to map the effects of contact-tube-to-work distance (13,19 and 25 mm), current, voltage, and wire feed rate on metal transfer. The droplet transfer modes were identified for each map by both the sound of the arc and images from a laser back-lit highspeed video system. The modes were correlated to digital records of the voltage and current fluctuations. The maps contain detailed information on the spray transfer mode, including the boundaries of drop spray, streaming spray and rotating spray modes. The metal transfer mode boundaries shifted with an increase in contact-tube-to-work distance. Increasing the contact-tube-to-work distance from 13 to 19 mm resulted in a 1 5 mm/s increase in the wire feed rate for the globular-to-drop-spray transition. Introduction Intelligent welding systems are needed to offset the projected shortage of skilled welders, as well as to position the operator further from the uncomfortable environment near an arc. To replace a welder, these systems need to replicate the ability of a welder to manipulate the electrode along the joint, detect unsatisfactory conditions, and either offer a corrective action, or terminate welding. ExP R. HEALD is with Phillips 66 Co. — Refining Division, Borger, Tex. R. B. MADIGAN and T. A. SIEWERT are with the Materials Reliability Division — NIST, Boulder, Colo. S. LIU is with the Center for Welding and joining Research, Colorado School of Mines, Golden, Colo. 38-s I FEBRUARY 1 9 9 4 isting robotic actuators can duplicate the precision of human arm movements in manipulating the electrode along the joint, but substantial improvements are needed in sensor and control system technology. Monitoring of the electrical signals (through-the-arc sensing) is one sensing strategy for an intelligent welding control system. Electrical perturbations related to metal droplet transfer in gas metal arc welding (GMAW) can be used to sense changes in the welding conditions. The control loop would contain rules based on the expertise of a human welder and allow the controller to make the proper responses to the conditions detected by the sensors. A through-the-arc sensing strategy does not intrude into the arc region and eliminates sensor-workpiece interference and sensor blinding problems. Although the electrical signals were correlated to arc characteristics as early as 1955 (Ref. 1), recent advances in digital signal capture and processing tech- KEY WORDS GMAW Metal Transf. Modes ER100S-1 Electrode CTWD Spray Transfer Mapping Wire Feed Rate Voltage Current niques have greatly increased the capabilities of using arc signals to control the welding arc. Recent reports have described digital data collection and analysis systems, and have demonstrated clear correlation between arc signals and arc behavior (Refs. 2-8). These reports also indicated that it is possible to use through-the-arc sensing to detect various unsatisfactory conditions in the arc. The results of this study, metal transfer mode maps, will provide a basis for developing arc control strategies for the G M A W process. Knowledge of the interrelationships between arc signals and metal transfer modes allows the control system to determine an appropriate response to an unsatisfactory condition and decide if further correction is necessary. Metal Transfer Modes Within the G M A W arc, the molten metal from the electrode commonly transfers to the base metal in one of three distinct modes: short circuit, globular and spray (Refs. 9, 13). Each of these modes has a characteristic arc length, weld penetration and weld pool shape. Since the welding engineer specifies a transfer mode to obtain the desired weld characteristics, the control system must be able to detect when the mode changes and develop a response that will return the welding process to its preset behavior. Short circuit transfer is accompanied by a cyclic extinction and reestablishment of the arc, which results in very large fluctuations in the voltage and current. These fluctuations are easily detected and recognized (Refs. 2, 5-7). Globular metal transfer is characterized by a round globular drop of molten fer, the electromagnetic forces have become so large that the metal in the arc column experiences forces with nonaxial components (Refs. 9, 11, 12, 14). These nonaxial force components cause the molten column to have an initial velocity that is at an angle to the electrode axis. The liquid metal follows a helical course from the electrode to the base metal. Transistorized Welding Power Source Data Acquisition System GMA Welding Gun Focusing Lens Mirror V Focusing [_ Band-Pass Lens Filter Contact-Tube-to-Work Distance Frosted Glass Screen Composed of the electrode extension (distance from the contact tube to the arc) and the arc length, the contact-tube-towork distance (CTWD) affects the metal transfer mode by altering the amount of ohmic heating occurring in the electrode. The effect of CTWD on the circuit resistance requires that a new voltagecurrent map be developed for each CTWD. The three CTWDs (13, 19 and 25 mm) selected in this work bracket the range for most GMAW applications. The three maps were compared to determine the effectiveness of through-the-arc sensing over the entire range. Weld Pool High Speed Video System Laser Source Fig. 1 — Schematic diagram of the experimental setup including the data acquisition and laser imaging system. characterized as the absence of the large fluctuations seen with the globular or short circuit transfer (Refs. 6, 7). metal, of diameter greater than the diameter of the electrode, forming at the electrode tip (Refs. 9, 11). When the droplet has attained a sufficient size for gravity and the electromagnetic pinch force to overcome the surface tension, the droplet detaches and is transferred across the arc to the weld pool. Again, the transfer of large metal droplets results in large fluctuations in the voltage and current, which are easily detected and are distinctly different than those observed in short circuit transfer (Refs. 6, 7). Spray Transfer Subclassifications As a result of the higher current density and Lorentz force, spray transfer is typically characterized by the small droplets transferred across the arc. The droplet diameter is approximately that of the electrode or smaller (Ref. 9). Droplet transfer for spray mode is quite similar to the globular transfer. The distinction between them is based upon the size and, therefore, frequency of the droplets transferred. The detachment of small droplets has a small effect on the current and voltage, so small that spray transfer has been 40 V 38 - o 3 > • a* 32 ^ • • •• ••* • *»• • — m* • 34 c 2 5i 5 30 c o 0) S * - 38 - T T • - - : - • OJkf o°tf^ . 1 .&250 3 TT T " T T Rp gs? Vim W v V V a afflo LQvV W V T w - V Short Circuit D Globular Drop Sproy Streaming Spray R o t a t i n g Spray o CTWD = 13 m m i 300 WT • • * DD • O V . - V T o •• • • Xm ao«« • t * B Q» • • • • « • v * -D M&*&n w v 1 1 • CTWD = 19 m m D - The main focus of this study was to investigate the spray transfer mode and to establish the boundaries of the spray transfer mode in terms of voltage, current and wire feed speed values. The modes were determined by methods such as the level of spatter, the audible sound of the arc, a through-the-arc laser imaging system, direct visual inspection of the arc, and arc signal data recording. Since the first four methods are not suitable for automated control, they were used simply to identify the modes and to confirm the interpretation of the electrical signals over the range of the maps. In the cases where the transfer modes overlapped significantly, the audiovisual data were used to better define the mode boundaries. • • • • * • %- 28 26 , . - Short Circuit Drop Spray S t r e a m i n g Spray 36 Experimental P r o c e d u r e The spray transfer region can be further divided into three subclassifications: projected drop spray, streaming spray and rotating spray (Ref. 10). Drop spray transfer is characterized by roughly spherical droplets of molten metal and is the subclassification most often referred to as spray transfer by welding professionals. With further increases in wire feed rate and voltage, individual droplets become less distinct, and an almost continuous column of molten metal extends from the electrode to the base plate. H igh speed cinematography or video systems (at 10,000 frames per second or greater), however, show that despite the appearance of a continuous metal column, metal transfer occurs in the form of droplets, and therefore, the name streaming spray transfer. In rotating spray trans- 350 , . i 400 i . . 450 • • i 500 . . i . . . . i . . . . i 250 24 Mean Weldinq Current [A) Fig. 2 — Metal transfer mode map as a function of mean welding current and voltage for 13-mm CTWD. Mean Welding Current (A) Fig. 3 — Metal transfer mode map as a function of mean welding current and voltage for 19-mm CTWD. W E L D I N G RESEARCH SUPPLEMENT I 39-s 40 40 CTWD = 25 mm 13 mn - CTWD 38 38 36 34 5- 36 a> o 34 • • • > 32 £* ";c -Vvv f ¥ 30 • • fl W V 28 - 0 D W V a> * V • o • 26 • Short Circuit Globular Drop Spray Streaming Spray Rotating Spray 250 300 350 400 450 hiiiiM 30 AZ 26 8 S * vv 24 500 B ,100 O n c e t h e b o u n d a r i e s of t h e s p r a y transfer m o d e w i t h respect t o g l o b u l a r and short c i r c u i t transfer w e r e establ i s h e d , an investigation o f the l o c a t i o n s of the transfer m o d e b o u n d a r i e s (subclassifications) w i t h i n the spray m o d e was c o n d u c t e d . To establish the b o u n d a r i e s of the spray transfer r e g i o n , an e x p e r i m e n t a l m a t r i x c o v e r i n g the v o l t a g e range o f 2 4 to 4 0 V a n d c u r r e n t range f r o m 1 75 to 5 0 0 A w a s e x p l o r e d . T h e w i r e feed rates v a r i e d f r o m 3.3 to 1 3.2 in./s (84 to 3 3 5 m m / s ) . For each e x p e r i m e n t a l w e l d , the w e l d i n g current and voltage were m a i n t a i n e d constant u n t i l c o m p l e t i o n . A t least o n e w e l d w a s p r o d u c e d for e a c h w e l d i n g current-voltage c o m b i n a t i o n . O t h e r v a r i a b l e s , s u c h as e l e c t r o d e c o m p o s i t i o n (ER100S-1), e l e c t r o d e d i ameter (1.2 m m ) , s h i e l d i n g gas c o m p o s i t i o n a n d f l o w rate ( A r - 2 % 0 2 , 0.3 L / m i n , respectively), and C T W D were m a i n t a i n e d c o n s t a n t (at e a c h o f t h r e e C T W D s ) . Base metal t h i c k n e s s (1/2 i n . ; 12.5 m m ) a n d travel speed (10 i n . / m i n ; 4.2 mm/s) of the a u t o m a t i c carriage as w e l l as w e l d o r i e n t a t i o n (flat) w e r e also h e l d c o n s t a n t t h r o u g h o u t t h e investigation. Bead-on-plate welds were made. Equipment Description A c o m m e r c i a l constant p o t e n t i a l D C arc w e l d i n g p o w e r source s u p p l i e d the current and voltage for the w e l d i n g process. H o w e v e r , the p o w e r o u t p u t f r o m the p o w e r source w a s r e g u l a t e d a n d filtered by a 600-A transistorized DC w e l d i n g c u r r e n t regulator. T h e c u r r e n t r e g u l a t o r r e m o v e d a r t i f a c t s s u c h as p o w e r l i n e ripples f r o m the o u t p u t of the p o w e r source, p e r m i t t i n g the d e t e c t i o n of the natural arc response. A m a n u a l G M A w e l d i n g gun was held stationary w h i l e 40-s I FEBRUARY 1994 150 V V V V V V V V v V Short Circuit O Drop Spray # Streaming Spray 8 81 Mean Welding Current (A) Fig. 4 — Metal transfer mode map as a function of mean welding current and voltage for 25-mm CTWD. v O V 24 200 32 200 250 300 350 Wire Feed Rote ( m m / s ) Fig. 5 — Metal transfer mode map as a function of wire feed rate and mean welding voltage for 13-mm CTWD. the plate w a s m o v e d u n d e r the g u n d u r ing w e l d i n g . This a r r a n g e m e n t p e r m i t t e d a stationary v i e w of t h e e l e c t r o d e t i p d u r ing w e l d i n g using the through-the-arc laser i m a g i n g system. d e t e r m i n e the metal transfer m o d e a n d d e f i n e the m o d e b o u n d a r i e s . A s c h e m a t i c d i a g r a m of the t h r o u g h the-arc i m a g i n g system is presented in Fig. 1 . T h e system c o m p o n e n t s consisted of a 2 0 - m W h e l i u m - n e o n laser; a h i g h speed (up to 1 0 0 0 frames/s) v i d e o c a m era w i t h a 9 0 m m , F/3.5 lens; a m i r r o r ; t w o f o c u s i n g lenses; a n a r r o w b a n d pass laser f i l t e r ; a n d a frosted glass p r o j e c t i o n screen. A d e s c r i p t i o n of the basic features of the laser i m a g i n g system has been p r o v i d e d e l s e w h e r e (Refs. 5 - 7 , 1 5). T h e syst e m used in this study w a s m o d i f i e d f o l l o w i n g p r e v i o u s studies (Refs. 5 - 7 ) b y t h e a d d i t i o n o f a higher speed v i d e o c a m e r a and better laser o p t i c s . Transfer Mode Maps Method of Data Collection A c o m p u t e r e q u i p p e d w i t h an a n a l o g to-digital (A/D) conversion board was used to s a m p l e the c u r r e n t a n d v o l t a g e values d u r i n g w e l d i n g . T h e actual c u r rent a n d v o l t a g e data w e r e c o l l e c t e d w i t h a p r o g r a m w r i t t e n in a h i g h - l e v e l p r o g r a m m i n g language (Ref. 1 5). A s a m p l i n g rate of 5 0 0 H z w a s selected for this w o r k . T h e c u r r e n t a n d v o l t a g e d a t a , averaged w i t h respect t o t i m e for e a c h w e l d , w e r e t h e n used for the c h a r a c t e r i z a t i o n of t h e different transfer m o d e s . T h e actual currents a n d voltages w e r e c o r r e l a t e d w i t h the v i d e o i m a g e of the t i p of the e l e c t r o d e as the metal transfer w a s t a k i n g p l a c e . A n e l e c t r i c a l pulse w a s g e n e r a t e d a n d i m posed o n the arc signal to i n d i c a t e the start of data c o l l e c t i o n . This pulse w a s also s y n c h r o n i z e d w i t h the v i d e o i m a g e o f metal transfer. Together w i t h t h e o t h e r a u d i o v i s u a l data c o l l e c t e d , the v i d e o r e c o r d i n g a n d arc signals w e r e used t o Results a n d D i s c u s s i o n A l t h o u g h the w e l d i n g current and voltage data are specific t o the o u t p u t of the c u r r e n t regulator, t h e w e l d i n g g u n a n d the e l e c t r o d e , o t h e r factors that i n f l u e n c e the transfer m o d e (besides current, v o l t a g e a n d C T W D , variables in this e x p e r i m e n t a l p r o g r a m ) are the e l e c t r o d e t y p e a n d s h i e l d i n g gas c o m p o s i t i o n . T h e r e f o r e , these data also p r o v i d e useful metal transfer m o d e m a p s of the o p e r a t i n g range for the ER1 0 0 S - 1 / A r - 2 % 0 2 c o m b i n a t i o n . Figure 2 is a m a p of t h e m e t a l t r a n s f e r m o d e as a f u n c t i o n o f w e l d i n g c u r r e n t and v o l t a g e , for a C T W D of 13 m m (0.5 i n . ) . For t h e 1 3 - m m C T W D , no g l o b u l a r or rotating spray transfer w a s o b s e r v e d . For 19 m m (0.75 in.) and 25 m m (1 in.) C T W D s , all transfer m o d e s w e r e present, as illustrated in Figs. 3 a n d 4 . W h i l e Figs. 2 - 4 s h o w t h e presence of the different transfer m o d e s , substantial o v e r l a p p i n g o f t h e r e g i o n s c a n be o b served, w h i c h indicates that alternate plots m a y be n e e d e d to better d e f i n e the metal transfer m o d e b o u n d a r i e s . In fact, c o n t r a r y t o the general p r a c t i c e , c u r r e n t s h o u l d not be c o n s i d e r e d as an i n d e p e n d e n t v a r i a b l e in G M A W . W i r e feed rate is the true i n d e p e n d e n t v a r i a b l e . T h e w i r e feed rate d e p e n d s o n l y o n the w i r e feeder c o n t r o l setting s i n c e t h e p o w e r for the w i r e feeder is separate f r o m the p o w e r supplied for w e l d i n g . Therefore, n e w metal transfer m o d e m a p s w e r e p l o t t e d as a f u n c t i o n of w e l d i n g v o l t a g e a n d w i r e feed rate as s h o w n in Figs. 5 - 7. A s e x - 40 40 CTWD = 19 mm J • n Y » 36 o«a T • • • • • « • • • T • • TT • • 34 38 T 36 T ! I < V CTWD = 25 mm ' <J < T 38 <J 30 28 3 V V V 32 V a n a rarv v v v 26 V Short Circuit D Globular V v O Drop Spray # Streaming Spray T Rotating Spray v v v v v v DQ • 28 26 V Short Circuit D Globular O Drop Spray • Streaming Spray T Rotating Spray 24 100 150 200 250 300 100 350 150 Wire Feed Rate ( m m / s ) Fig. 6 — Metal transfer mode map as a function of wire feed rate and mean welding voltage for 19-mm CTWD. pected, these three figures show better defined transfer mode regions, with fewer cases of overlapping. The subboundaries between drop spray, streaming spray and rotating spray transfer can also be distinguished easily. The effect of CTWD also became more noticeable. For example, an increase in the CTWD from 13 to 19 mm caused the globular-to-drop spray transition to shift from a wire feed rate of 105 to 1 20 mm/s (248 to 283 in./min) at a constant voltage of 29.5 V. With a further increase of C T W D to 25 mm the same transition shifted to 1 30 mm/s (307 in./min). The shifts in the different spray transfer mode sub-boundaries as a result of the CTWD changes are summarized in Figs. 8 and 9. Figure 8 shows a shift of the drop spray transfer region as a result of increasing the CTWD from 13 to 25 mm. The shift was more noticeable with the CTWD changing from 1 3 to 19 mm than with a CTWD changing from 19 to 25 200 250 300 350 Wire Feed Rate ( m m / s ) Fig. 7 — Metal transfer mode map as a function of wire feed rate and mean welding voltage for 25-mm CTWD. mm. The arrow indicates the direction of such a shift. Figure 9 illustrates the locations of the streaming spray metal transfer mode for the three different CTWDs. Both figures indicate that to accommodate changes in a welding arc because of an increasing CTWD while maintaining a preselected metal transfer mode, higher wire feed rates and higher mean voltages are required. As the CTWD increases for a given voltage, two effects are observed. With more of the electrode extending beyond the contact tube, the length of electrode that is carrying the current increases (assuming the current enters the electrode at the extreme of the contact tube and continues to the electrode tip). Because of the inherent resistance of the electrode which increases with length and temperature, both the resistance of the electrode and the total resistance of the circuit increase. This higher resistance causes more ohmic heating in the electrode (Ref. 16), which preheats the electrode and increases the melting rate. However, the increased resistance also reduces the current in the circuit, which may result in a power loss. Overall, the power loss effect is more significant than the ohmic heating effect and an increase in current (wire feed rate) and/or voltage is required to maintain heat balance and to maintain the metal transfer mode. Consequently, the transfer mode regions tend to shift to higher wire feed rate (current) and/or voltage with an increase in CTWD. At a wire feed rate of 120 mm/s and 13-mm CTWD, the globular-to-drop spray transition occurred at approximately 26 V, whereas with a 19-mm CTWD, the same transition occurred just under 29 V — Figs. 5, 6. Increasing further the CTWD to 25 m m , the transition occurred at nearly 29.5 V — F i g . 7. Another example of the voltage increase necessary to maintain a similar transfer mode, as a result of the CTWD in- 40 Streaming Spray 38 36 34 32 30 28 O CTWD = 13 m m • CTWD = 19 m m V CTWD = 25 m m 26 100 150 200 250 300 35 Wire Feed Rate ( m m / s ) Fig. 8 — Shift in drop spray transfer due to CTWD changes. 100 150 200 250 300 350 Wire Feed Rate ( m m / s ) Fig. 9 — Shift in streaming spray transfer due to CTWD changes. W E L D I N G RESEARCH SUPPLEMENT I 41-s 200 50 CTWD = 19 m m Globular T r a n s f e r CTWD = 13 m m S h o r t - C i r c u i t Transfer 40 •50 E "> 30 100 20 50 10 0 300 350 400 500 450 ' ' 50 100 150 Welding Current (A) Fig. 10 — Typical current histogram for short 200 250 300 350 Welding Current (A) circuit crease, is the transition from short circuit to streaming spray transfer mode. At a constant wire feed rate of 1 70 mm/s, the transition occurred at 28.5, 29.0 and 29.5 Vfor 13-, 19- and 25-mm CTWDs, respectively. A notable shift in transfer mode also occurred at 270 mm/s and 31 V, streaming spray for 1 3-mm CTWD and short circuit transfer for 19- and-25 mm CTWDs. The increase of CTWD from 1 3 to 19 mm reduces the power available to melt the electrode. Consequently, the melting rate decreases and becomes less than the wire feed rate, which allows short circuit transfer to occur. In summary, with a gradual increase in CTWD for a given voltage, the arc length becomes progressively shorter and eventually the arc extinguishes and short circuit transfer occurs. During the short circuit portion of the cycle (high current), sufficient ohmic heating occurs to melt the wire and an arc is re-established. At shorter CTWDs for a given voltage, transfer. Fig. 11 — Typical current streaming spray transfer remains the transfer mode at higher electrode feed rates, permitting a greater deposition rate with a stable transfer of metal. A longer CTWD is desirable for manual welding because it allows a better view of the weld area and, therefore, the operator can react to the changing conditions of the weld. Development of a control system for mechanized w e l d i n g , able to react quickly to changing conditions, w o u l d permit use of a shorter C T W D and, therefore, a greater metal deposition rate for a given voltage. histogram for globular transfer. Figs. 10, 11 and 12, respectively. The short circuit transfer mode, which has the largest current standard deviation, is characterized in Fig. 10 by the w i d e range in current recorded. Due to the limitations of the data acquisition method, current values lower than 500 A had their actual values recorded while current readings greater than 500 A simply assumed the value of 500 A. The broad peak centered near 390 A in Fig. 10 represents the current readings during the arc period. The higher currents occur during the short circuit period and are represented as a straight line at 500 A. Short circuit transfer fluctuates over this current range, between the peak for arcing and the peak for short circuit. Current Standard Deviation As mentioned in a previous section, metal transfer generally causes welding current and voltage fluctuations. The source of the standard deviation in current can be found by examining the current histogram for each transfer mode: short circuiting, globular and spray in Globular transfer is also characterized by two distinct current ranges, closer together than for shortcircuiting transfer. The two currents in Fig. 11 are characteristic of globule growth and the period immediately after globule detachment. 40 200 • CTWD | E "> 100 - 36 34 1 g 32 <u * - 32 30 o 1> JrW 400 450 s 530 histogram 42-s I FEBRUARY 1 9 9 4 for spray •f '• • • • • • • • • •/ • • • • • • • • • • 1 • • •^ V • !• • V V * V 28 26 Welding Current (A) current • • - >O C Fig. 12 — Typical 13 m m 2.5 A 5- 150 350 = 38 CTWD = 19 mm Sproy Tronsfer - "<\A V V l-tf V 20A V o • S> 5 A - • - • ^ f 10 A_ 9 V V Short Circui Drop Spray Streoming Spray - 24 100 transfer. 150 200 250 300 350 Wire Feed Rate ( m m / s ) Fig. 13 — Current standard deviation isopleths plotted on metal CTWD. fer mode map for 13-mm trans- 38 - 20 A ' • 30 A / / 36 _40 A / / / 34 32 Y T • /• 28 - !{• D f /• -p ip* • • » • SU 1 • i I i 10A m t ' YY T • • Y • • V, V Sr^v. 2.5 A T • Y X X . 10 A. T %J*^ . V 20A. JL30A- 'VJL- 20 A -2.40A . 60A V — 80 A . v / v V Short Circu it P Glob j l a r i CTWD = 2 5 m m 5A ,2.5A T T Y TT.- ^ » o 2b 5A Y • • .« • • • •/ • #-- ^ v 5 / T<JSTH-? \nrjj, CTWD = 19 mm 5A / T • T Drop Spray Streaming Spray Rototing Spray V • Short Circuit Globular O Drop Spray • Streaming Spray Y Rotating Spray 24 100 150 w're 200 250 300 150 350 Feed Rate ( m m / s ) Fig. 14 — Current standard deviation isopleths plotted on metal transfer mode map for 19-mm CTWD. T h e high c u r r e n t peak o c c u r s as t h e g l o b u l e is g r o w i n g , the arc length is short, a n d t h e v o l t a g e d r o p across t h e arc is s m a l l . I m m e d i a t e l y after the g l o b u l e detaches, the arc length b e c o m e s longer, resulting in a higher v o l t a g e and a l o w e r c u r r e n t . The current standard deviation for the g l o b u l a r transfer is smaller t h a n that of the short c i r c u i t transfer. Spray transfer is e v i d e n c e d by a narrow current histogram, with the two p e a k s b e c o m i n g e s s e n t i a l l y o n e , as s h o w n in Fig. 1 2 . T h e c u r r e n t range over w h i c h the arc varies has b e c o m e q u i t e s m a l l , o n t h e order of ±2 5 A . Based o n the discussion presented in the p r e v i o u s paragraphs, the standard d e v i a t i o n s of t h e w e l d i n g c u r r e n t data can be p l o t t e d o n t o t h e transfer m o d e maps to e v a l u a t e t h e stability of each transfer m o d e . Figures 1 3 - 1 5 are plots of w e l d ing c u r r e n t standard d e v i a t i o n isopleths for t h e three C T W D s . These plots s h o w that t h e large c u r r e n t standard d e v i a t i o n s (20 to 8 0 A) w e r e o b s e r v e d for short circ u i t transfer. G l o b u l a r t r a n s f e r has a s o m e w h a t smaller c u r r e n t standard d e v i a t i o n (10 to 4 0 A ) . T h e three spray m o d e s ( d r o p , s t r e a m i n g , a n d rotating) all have s i m i l a r m a g n i t u d e s for the c u r r e n t stand a r d d e v i a t i o n (< 10 A), w h i c h are the smallest values of all five transfer m o d e s . T h e results i n d i c a t e that standard dev i a t i o n of the c u r r e n t readings is a c o n v e n i e n t m e a s u r e o f the transfer m o d e . W e l d s w i t h a c u r r e n t standard d e v i a t i o n greater than 2 0 A ( w i t h w i r e feed rates greater t h a n 2 0 0 m m / s ; 4 7 2 i n . / m i n ) exh i b i t short c i r c u i t metal transfer. T h o s e w i t h c u r r e n t s t a n d a r d d e v i a t i o n s less t h a n 10 A e x h i b i t spray transfer. The c h a n g e f r o m a standard d e v i a t i o n o f 2 0 A t o 1 0 A signals t h e t r a n s i t i o n f r o m short c i r c u i t to spray. The g l o b u l a r - t o - d r o p spray t r a n s i t i o n is also c h a r a c t e r i z e d by a r e d u c t i o n in t h e c u r r e n t standard d e v i a t i o n f r o m 2 0 A to 1 0 A . In the case of 200 250 300 350 Wire Feed Rate ( m m / s ) Fig. 15 — Current standard deviation isopleths plotted on metal transfer mode map for 25-mm CTWD. w e l d s m a d e w i t h w i r e feed rate l o w e r t h a n 150 m m / s (354 i n . / m i n ) , a c u r r e n t standard d e v i a t i o n less t h a n 10 A denotes spray metal transfer characteristics, a n d those w e l d s w i t h c u r r e n t standard d e v i a t i o n s greater t h a n 2 0 A e x h i b i t g l o b u l a r transfer. A l t h o u g h the standard d e v i a t i o n o f c u r r e n t itself c a n n o t d i s t i n g u i s h g l o b ular f r o m short c i r c u i t transfer, the stand a r d d e v i a t i o n c a n be used to signal a d e v i a t i o n f r o m the spray transfer m o d e . As s u c h , metal transfer m o d e maps a n d c u r rent standard d e v i a t i o n data c a n be used in the f o r m u l a t i o n of c o n t r o l strategies for GMAW. Conclusions 1) V o l t a g e - c u r r e n t o r v o l t a g e - e l e c t r o d e feed rate maps are a p p r o p r i a t e for r e p o r t i n g t h e response of an ER100S-1 e l e c t r o d e to G M A W . 2) T h e standard d e v i a t i o n o f the c u r rent is a robust i n d i c a t i o n of metal transfer m o d e for an ER100S-1 e l e c t r o d e . T h e highest standard d e v i a t i o n is i n d i c a t i v e of short c i r c u i t transfer; m o d e r a t e stand a r d d e v i a t i o n , o f g l o b u l a r transfer; a n d the l o w e s t standard d e v i a t i o n , of spray transfer. 3) S p r a y m e t a l transfer p r o d u c e s a c u r r e n t standard d e v i a t i o n of less t h a n 1 0 A w i t h the transitions to o t h e r m o d e s i n d i c a t e d by the c u r r e n t standard d e v i a t i o n increasing into the range of 10 to 2 0 A . S i n c e t h e s t a n d a r d d e v i a t i o n is a s m o o t h l y v a r y i n g f u n c t i o n , changes in it c a n be used in c o n t r o l strategy. 4) T h e t h r e e s u b c l a s s i f i c a t i o n s o f spray transfer exist in c l e a r l y d e f i n e d regions o n transfer m o d e m a p s . T h e maps c a n be used to m o n i t o r transfer m o d e s a n d are u s e f u l in f o r m u l a t i n g c o n t r o l strategies for G M A W . 5) A t a g i v e n v o l t a g e , as the C T W D i n creases, the r e q u i r e d w i r e feed rate for t r a n s i t i o n f r o m g l o b u l a r t o d r o p spray transfer also increases. Acknowledgments Financial a n d t e c h n i c a l s u p p o r t f r o m the U.S. N a v y P r o g r a m m a b l e A u t o m a t e d Welding System (PAWS) program, Charles N u l l , NAVSEA 5 1 4 2 , Program Manager, and R. A . M o r r i s , N a v a l Surface W a r f a r e Center, T e c h n i c a l M a n a g e r , is acknowledged. References 1. Lesnewich, A. 1955. Electrode activation for inert-gas-shielded metal arc welding. Welding journal 34 (12): 1167-1178. 2. Lucas, W. 1 983. Trends in MIG-welding process development. Exploiting MIG WeldIhe Welding Institute, ing Developments, Cambridge, England, p. 1. 3. Kohn, C , and Siewert, T. A. 1987. The effect of power supply response characteristics on droplet transfer of C M A welds. Advances in Welding Science and Technology, ASM International, Materials Park, Ohio, pp. 299-302. 4. Johnson, J. A., Carlson, N. M., and Smartt, H. B. 1990. Detection of metal-transfer mode in GMAW. Recent Trends in Welding Science and Technology, ASM International, Materials Park, Ohio, pp. 3 7 7 - 3 8 1 . 5. Liu, S., and Siewert, T. A. 1989. Metal transfer in gas metal arc welding: Droplet rate. Welding Journal 68 (2): 51 -s to 58-s. 6. Liu, S., Siewert, T A., and Lan, H.-G. 1 990. Metal transfer in gas metal arc welding. Recent Trends in Welding Science and Technology, ASM International, Materials Park, Ohio, p . 475-479. 7. Adam, G., and Siewert, T. A. 1990. Sensing of G M A W droplet transfer modes using an ER100S-1 electrode. Welding Journal 69 (3): 103-s to 108-s. 8. Mahin, K. W., Shadbolt, T , Fuchs, E. A., Williams, D., and Franco-Ferreira, E. 1 9 9 1 . Characterization of the GMA welding process. Welding Technologies for Irradiated Materials, Progress Report March 1990 to March 1991, Nat'l. Tech. Inf. Ser., Springfield, Va., pp. 70 WELDING RESEARCH SUPPLEMENT I 43-s 9. Lancaster, J.F., ed. 1984. The Physics of Welding, First Edition, Chapter 7, Metal transfer and mass flow in the weld pool. International Institute of Welding (IIW), Pergamon Press. 10. American Welding Society. 1979. Recommended Practices for Gas Metal Arc Welding. AWS C5.6-79, American Welding Society, Miami, Fla. 11. Lyttle, K. A. 1982. Reliable G M A W means understanding wire quality, Equipment and Process Variables. Welding journal 61 (3): 43-48. 12. Lesnewich, A. 1958. Control of melting rate and metal transfer, Part II — Control of metal transfer. Welding journal 37 (8) 418s to 425-s. 13. American Welding Society. 1 9 9 1 . Welding Handbook, Volume 2: Welding Processes, American Welding Society, Miami, Fla. 14. Allemand.C. D., Schoeder, R., Ries, D. E., and Eagar, T. W. 1985. A method of filming metal transfer in welding arcs. Welding Journal 6 4 ( 1 ) : p. 4 5 - 4 7 . 15. Heald, P. R„ Madigan, R. B., Siewert, T. A., and Liu, S. 1991. Droplet Transfer Modes for a MIL 100S-1 G M A W Electrode. NIST Interagency Report 3976, National Institute for Standards and Technology, Gaithersburg, M d . 16. Lesnewich, A. 1958. Control of melting rate and metal transfer, part 1 — control of electrode melting rate. Welding journal 37(8): 343-sto353-s. RECOMMENDATIONS PROPOSED BY THE PVRC COMMITTEE ON REVIEW OF ASME NUCLEAR CODES AND STANDARDS Approved by the PVRC Steering Committee The ASME Board on Nuclear Codes and Standards (BNCS) determined in 1986 that an overall technical review of existing ASME nuclear codes and standards was needed. The decision to initiate this study was reinforced by many factors, but most importantly to capture a pool of knowledge and "lessons learned" from an existing generation of technical experts with codes and standards background. Project responsibility was placed with the Pressure Vessel Research Council, and activity was initiated in January 1988. Direction was vested in a Steering Committee which had overview of six subcommittees. Recommendations were provided by nuclear utilities and industry and these recommendations were combined with independent considerations/recommendations of the PVRC Subcommittees and Steering Committees. Publication of this document was sponsored by the Steering Committee on the Review of ASME Nuclear Codes and Standards of the Pressure Vessel Research Council. The price of WRC Bulletin 370 (February 1992) is $30.00 per copy, plus $5.00 for U.S. and Canada, or $10.00 for overseas, postage and handling. Orders should be sent with payment to the Welding Research Council, Inc. • 345 E. 47th St. • Room 1301 • New York, NY 10017 • (212) 7057956. 44-s I FEBRUARY 1994