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Effect of Orifice Geometry in Plasma-Arc Welding of Ti-6AI-4V

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Effect of Orifice Geometry in Plasma-Arc W e l d i n g of Ti-6AI-4V

Compared to conically divergent and conventionally cylindrical geometries, conically convergent orifices

produce greater voltage drop, higher plasma temperatures, deeper weld penetration and smaller

width-to-depth nugget ratios

BY C. B. SHAW, JR.

ABSTRACT. Comparisons have been made among the performances of three plasma-arc welding torch orifice geometries: conically convergent, con- ventional cylindrical, and conically divergent. The m i n i m u m orifice diam- eter and all welding parameters (in- cluding constant torch-to-workpiece distance rather than constant voltage) were identical for the three designs w h e n test butt welds were made in TP6AI-4V up to 12.7 mm (0.5 in.) thick.

Measurements were made during welding of the plasma voltage drop and plasma temperature distribution.

The resultant weldments were cross- sectioned and examined. Compared to the standard orifice, the convergent design produces a greater voltage drop, higher plasma temperature, deeper weld penetration, and smaller nugget w i d t h - t o - d e p t h ratio. The d i - vergent design has the opposite effect on all these points.

A complete plasmadynamic analysis has not been made, but analysis of simple models relates all these phe- nomena to "pneumatic constriction"

produced by flow through the conver- gent orifice.

I n t r o d u c t i o n

The plasma-arc welding (PAW) pro- cess is distinguished from the gas tungsten arc w e l d i n g (GTAW) process

by the presence of an orifice cup w h i c h surrounds the tungsten elec- trode, normally the cathode, c o m m o n to both processes. The melting w o r k - piece normally serves as anode in the arc discharge through f l o w i n g inert

gas, e.g., argon. A small pilot-arc cur- rent, limited by the series resistor w h i c h connects the orifice cup to the workpiece, flows between electrode and orifice cup to pre-ionize the gas flowing out through the orifice, but the major current flow between elec- trode and workpiece is constrained to pass through the plasma w h i c h streams through the orifice toward the workpiece. The tungsten electrode (a cylinder w i t h a broad conical tip), the orifice and the axis of symmetry of gas and current flow are all coaxial.

The welding performance of a PAW torch is w e l l - k n o w n on empirical grounds to be superior in several ways to that of a GTAW torch,1 a superiority w h i c h is popularly attributed to " c o n - striction" of the arc discharge by causing the plasma to flow through the orifice. In view of this central importance of the orifice to PAW performance, published research on the effects of orifice geometry and its optimization seem surprisingly scant.

Demyantsevich and Sosnin- in- cluded orifice geometry among the variables they explored in a search for a more "efficient" plasma torch, using concentration of heat flux and current flow to measure efficiency. They stud- ied both convergent and divergent orifices, the latter including a parabo- loid of revolution chosen to simulate erosion of the orifice cup through prolonged use. No evaluation of actual welds was presented; however, by their measure of efficiency, the per-

C B. SHAW, JR., is with the Rockwell International Science Center, Thousand Oaks, California.

formance of a divergent orifice is inferior, and that of a convergent orif- ice superior to that of a cylindrical orifice. Also, the use of tangential gas feed and an annular electrode is better yet.

Schultz' g r o u p ' reported an exten- sive metallographic evaluation of the effects of orifice geometry on w e l d nugget morphology and explored the range of welding parameters over which each design permitted satisfac- tory welding performance. Clear su- periority was shown for a design based on the concept of "striction pneumat- i q u e " (pneumatic constriction) which used a conically convergent orifice section. However, to avoid the prob- lem of double arcing (in which the main current f l o w goes from the elec- trode to a spot on the orifice cup and from there to the workpiece) which causes erratic welding and rapid ori- fice-cup deterioration, they f o l l o w e d the convergent orifice w i t h a conically divergent section at the outlet. A simi- lar design concept was used to achieve the greatest single-pass penetration depths yet attained by the PAW pro- cess in the welding of titanium and steel alloys.4

Schultz discussed qualitatively the effects of orifice geometry on gas f l o w , but neither measured nor analyzed the flow.3 The present work was, there- fore, undertaken to supplement metal- lographic observation of the effects of orifice geometry on welds in Ti-6AI-4V by measurement of plasma tempera- ture distributions produced by the various orifice shapes and by gasdy- namic modeling of the flows. The objective was to explore the physical mechanisms whereby a convergent

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orifice produces welds w h i c h are superior to those made by a cylindrical orifice using the same welding param- eters, while a divergent orifice (e.g., one eroded by prolonged use) yields inferior results. The modeling has not been as successful as hoped, but the reasons for this and limited results obtained are discussed below along w i t h the experimental data.

Experiments

Blank orifice cups were obtained from the torch manufacturer, and three types of orifices were machined in them:

1. A right circular conically conver- gent orifice, the cone having 15 deg included angle.

2. A straight right circular cylindrical orifice.

3. A right circular conically diver- gent orifice, the cone again having 15 deg included angle.

These w i l l be referred to as —15 , 0, and + 1 5 deg tapers, respectively. ( A d - ditional orifices w i t h + 3 0 and + 4 5 deg tapers were also produced but quickly dropped from the test pro- gram; these produce welds so broad that excessive sag and even cutting result before full penetration welds can be obtained.)

The m i n i m u m diameter of each ori- fice was 3.45 m m (0.136 in.), the diam- eter recommended by the manufactur- er for the intended current range. The edges at the entrance and exit of each orifice were polished smooth w i t h abrasives. This was done on the assumption that double arcing is engendered by electric field concen- tration, w h i c h is k n o w n to be signifi- cant at a sharp edge or the point of a burr on a conductor.5 No double arcing was encountered, even w i t h the

—15 deg taper.

Single-pass butt welds were made on Ti-6AI-4V bar stock, each strip being 38.1 by 305 mm (1.5 in. by 12 in.), w i t h thicknesses from 3.2 to 12.7 mm (0.125 to 0.5 in.). The edges to be joined were filed clean and smooth immediately before clamping the specimens to the water-cooled grooved copper back-up bar. The bars were tack-welded together w i t h a gap of 0.36 mm (0.014 in.).

All welds were made in an inert- atmosphere chamber filled w i t h argon to prevent oxidation and nitridation of the hot t i t a n i u m . No shielding gas flow was therefore necessary, but the ori- fice f l o w rate was the normal value for full penetration welding in the key- hold mode.1 Travel speed was selected in the range 50 to 400 mm7minute according to specimen thickness in order to keep the current in the range 40 to 120 amperes (A). Constant stand- off distance (from orifice to w o r k - piece) was used rather than automatic voltage control, the distance being 3.2 mm (0.125 in.) at low current and 6.3 mm (0.25 in.) for currents of 80 amperes and above.

For every combination of workpiece thickness, travel speed, and current employed, the f o l l o w i n g results were uniformly obtained. Relative to the 0 deg taper orifice, the —15 deg taper (convergent orifice) produced a plas- ma such that the potential difference between workpiece and electrode was 3 to 6% (1 to 2 volt, i.e., V) greater-that is, the plasma column had 3 to 6%

higher overall electrical resistance and the electrical energy delivered per unit length of w e l d was correspondingly greater. The + 1 5 deg taper (divergent orifice) produced a plasma for w h i c h the potential difference was reduced 6

-1.5 -1.0 -0.5 0.0 0.5 1.5 R(MM)

Fig. 1—Plasma temperature vs. radius for three orifice geometries, temperature measured 0.97 mm below torch at 70 A; o — 7 5 , D-0, Ji- + 75 deg.

to 10% (2 to 3 V), indicative of corre- spondingly reduced overall electrical resistance and reduced electrical ener- gy delivered per unit length of w e l d . Measurement of the plasma temper- ature distributions6 showed that the plasma produced by the —15 deg orif- ice was 8 to 12% (2.5 to 3.8 X 101 K) hotter than that from the 0 deg orifice, w h i l e the plasma from the + 1 5 deg orifice was 20 to 30% (6.3 to 9.5 X 103

K) cooler than the reference. The full w i d t h at half maximum of the temper- ature distributions did not depend sig- nificantly on taper, but the radius at which the intensity of the measured spectral lines dropped to the smallest useful value was less for the —15 deg taper and greater for the + 1 5 deg taper than for 0 deg.

Typical results are shown in Fig. 1.

The temperature in thousands of degrees Kelvin is plotted against dis- tance in millimeters from the torch axis of symmetry, as measured 0.97 mm (0.038 in.) below the torch orifice while welding 5.3 mm (0.21 in.) thick Ti-6AI-4V at 70 A and 8 ipm. The standoff distance was 3.2 mm (0.125 in.). The measured voltages were 34, 32 and 28 volts for the - 1 5 , 0, and + 1 5 deg orifice tapers, respectively.

For every set of w e l d i n g conditions the —15 deg taper gave deeper pene- tration and a smaller average ratio of w i d t h to depth than the 0 deg taper.

The + 1 5 deg taper gave shallower penetration and a much greater ratio of w i d t h to depth. Quantitative c o m - parison is limited by the frequent pro- duction of pipe defects in the welds.

In order to make the most direct comparison of the effects of orifice geometry, it was decided to accept defects as they came, rather than to adjust orifice gas flow rate according to the weld penetration achieved by each orifice design. The pipes, w h i c h are continuous voids parallel to the line of w e l d i n g and extend for many millimeters, are not infrequently pro- duced in PAW w h e n the orifice gas flow rate is adjusted for full penetra- tion keyhole-mode w e l d i n g but the current is insufficient to achieve full penetration. Their presence cannot be detected until the weld is radio- graphed or sectioned.

Once thermal expansion closes the initial gap between plates, the vigo- rous f l o w of argon into the w e l d pool can produce a bubble of appreciable size w i t h i n the circulating w e l d metal which flows above the gas to a cooler region where it solidifies. The pipe marks the location in w h i c h argon (together w i t h surface tension and inertia) was supporting the molten metal.

Figures 2-4 show polished and etched sections of welds made at 90 A and 2 ipm in 12.7 mm (0.5 in.) Ti-

1 2 2 - s l APRIL 1980

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Fig. 2—Weld made in TI-6AI-4V12.7 mm (4 in. thick with convergent orifice, current 90 A, travel speed 0.85 mm/s, standoff 6.3 mm (2 in.)

Fig. 3—Weld made with straight orifice-same parameters as Fig. 2

Fig. 4—Weld made with divergent orifice—same parameters as Fig. 2 and 3

6AI-4V, using the - 1 5 , 0, and + 1 5 ° ta- pers, respectively. The standoff dis- tance was 6.3 mm (0.25 in.). A pipe is clearly visible in the 0 deg specimen.

The fusion zone is delimited by the boundary between large columnar sol- idification grains and the smaller equi- axed grains of the inner heat-affected zone, w h i c h underwent transition to the /3 phase.

Figure 2 shows that the —15 deg taper achieved full penetration, pro- ducing a nugget 8.4 mm (0.33 in.) wide at the top (0.66 times penetration depth), and 2.9 mm (0.11 in.) at the b o t t o m , for an average w i d t h of only 5.6 mm (0.22 in.), which is 0.44 times the penetration depth of 12.7 mm.

There is a slight drop-through of 0.2 mm (0.008 in.), and undercutting to a depth of 0.4 mm (0.016 in.) along either edge of the w e l d .

As shown in Fig. 3, the 0 deg taper produced penetration to 11.4 mm (0.45 in.), 10% less than full penetration. The nugget w i d t h at the top is 9.00 mm (0.35 in.), or 0.79 times the penetration depth. There is no undercutting. The pipe is 2.1 mm w i d e and 1.4 mm high (0.08 X 0.06 in.), w i t h an area of approximately 2.3 mm- (0.004 sq in.);

this accounts for approximately 'A of the cross-sectional area of the raised top-bead of 7.7 mm- (0.012 sq in.), the rest being attributable to shrinkage.

Figure 4 shows that the + 1 5 deg taper produced a weld nugget 13 mm (0.51 in.) wide at the top surface, 1.4 times the maximum penetration depth of only 9.5 mm (0.37 in.). These last measurements are less precise than the preceding ones, since the w i d e flat top surface of the weld was roughly rip- pled, and a section taken elsewhere in

the weld w o u l d show slightly different values for depth and w i d t h .

Analysis

The plasmadynamic equations and boundary conditions governing flow in a channel w i t h conductive walls, such as the PAW orifice, are well understood even when an interior boundary exists (such as the tungsten electrode if it protrudes into the ori- fice), and numerical methods for their solution have been thoroughly ex- plored and critiqued.' The correspond- ing equations and boundary condi- tions for the standoff region (between torch and workpiece) are more diffi- cult to analyze because of the increased difficulty in determining the electric field and treating computa- tionally a boundary condition at infin- ity; they have, however, been mas- tered, at least in an approximation w h i c h neglects turbulence."

What cannot be treated correctly at the present time is the abrupt transi- tion from channel f l o w to free f l o w which takes place at the outlet of the orifice. An attempt was made to match numerical solutions for flow in the t w o regions by only requiring that the total rates of transport of mass, charge, m o m e n t u m , and energy by the t w o flow fields should match at the outlet, instead of requiring that they coincide point by point.

The results obtained were not physi- cally significant; for example, the c o m - puted density field oscillated in such a

way as to include negative values. It is believed that this failure stems from neglect of turbulence (which destroys axial symmetry of the flow) generated by the abrupt transition at the orifice.

In reality, it is k n o w n experimentally that any atmospheric-pressure arc dis- charge w h i c h is not stabilized by pres- ence of a wall or by some artifice is highly turbulent.* (Landes et al.' refer to a " T I G * similar arc," rather than to a TIG arc, because an artifice was used to suppress turbulence.) Rather than attempt analysis of a three dimension- al turbulent plasmadynamic flow, it was decided to see what could be inferred about the effect of orifice geometry on PAW by modeling sim- pler f l o w problems.

Inspection of the plasmadynamic equations for channel flow7 shows that the trends of variation in the flow velocity field u, pressure p, sound speed a, temperature T, and density p w i t h changes in channel cross-section- al area A should be in the same direc- tion as in the much simpler case of flow of an un-ionized gas. A lower bound on the effect of orifice geome- try on the f l o w was therefore f o u n d by neglecting ohmic heating, dissipative processes such as viscosity, heat con- duction, ionization and electromag- netic forces, and calculating just the effect of the three orifices on adiabatic

*The American Welding Society recom- mends use of the term "GTAW" rather than the older term "TIG," which stands for tungsten inert gas, to denote the process.

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and isentropic flow of a perfect gas.

Conservation of mass means that the mass m' of gas flowing through any cross-section A per unit time is inde- pendent of axial position in the orifice.

In particular,

pAu = pAu

~RT (1)

at the exit,of the 0 deg taper orifice, the right-hand expression f o l l o w i n g from the perfect gas law. (Note that R here is not the gas constant but that quantity divided by the molecular weight of argon.)

Equation (1) permitted u to be cal- culated from k n o w n quantities since m' can be found from the measured orifice gas flow rate in cfh or cubic meters per second, the static pressure p can be taken as approximately one atmosphere, and T was measured. The remaining one-dimensional gasdy- namic equations" (which ignore any variation in flow variables transverse to the axis of the orifice) as applied to flow through a channel of variable area10 then permitted calculation of the reservoir or stagnation tempera- ture and all flow variables at the entrance to the 0 deg orifice. It was verified that the flow was in the far subsonic region, u less than 0.01 a.

Assuming that gas conditions were approximately the same at the en- trance to all three orifices, it follows that the convergent orifice w o u l d cause the temperature to be higher at the orifice exit than w o u l d the straight orifice, whereas the divergent orifice w o u l d cause the exit temperature to be lower. This is in qualitative agree- ment w i t h the experimental results.

Given identical initial conditions, the convergent orifice w o u l d also pro- duce higher dynamic pressure, Vi pu-, at the outlet than the straight orifice, while the divergent orifice w o u l d pro- duce a lower dynamic pressure. This trend in dynamic pressure means that the stream from the convergent orifice w o u l d support a greater head of mol- ten metal and thus enhance penetra- t i o n , while the stream from the diver- gent orifice w o u l d support a lesser head, to the detriment of penetration.

The empirical fact that the convergent orifice produced a plasma of higher resistance, higher voltage, and there- fore higher energy per unit length of w e l d , while the divergent orifice had the opposite effect, also contributes to the observed effect on penetra- tion—one cannot judge the relative importance of the t w o effects.

Although the above trends coincide qualitatively w i t h the experimental results, they are at odds quantitatively.

The temperature differences c o m - puted from the one-dimensional gas- dynamic equation for adiabatic and

isentropic flow of an ideal gas were less than one degree Kelvin instead of the thousands of degrees observed.

Likewise, the computed dynamic pres- sures for the —15 deg, 0 deg, and + 1 5 deg orifices were 7, 5, and 2 Nm"'-, almost t w o orders of magnitude too small to account for the effect on penetration via hydrostatic support of molten metal. These quantitative dis- crepancies must be attributed to the gross oversimplification of-neglecting ohmic heating and heat c o n d u c t i o n .

It is somewhat surprising that the hotter plasma was f o u n d to have the higher overall resistance. If one had a uniform plasma of area A and length L, the overall resistance R w o u l d be

R = rjL/A (2)

where q is the electrical resistivity. For a fully ionized plasma, q is proportion- al to T-:1"-." For a weakly ionized plas- ma the dependence is less simply expressed, but q decreases w i t h increasing T.1- (A plasma is " f u l l y "

ionized, i.e., dominated by Coulomb collisions rather than by collisions between charged and neutral particles, when the degree of ionization is more than about 1%.'3 For pure argon at one atmosphere pressure, this requires a temperature between 10 and 20 t h o u - sand degrees Kelvin.) It must be, there- fore, that the effective area of the plasma is changed more than the resistivity by use of the convergent or divergent orifices. This, of course, is in accord w i t h the concept of pneumatic constriction.1 Indeed, the breadth of the region emitting the ionized-argon spectral radiation used for tempera- ture measurement6 was roughly 10%

less for the convergent orifice and 10%

more for the divergent orifice than for the straight cylindrical orifice. The dif- ferences in diameter of the plasma column imply a 20% change in area A.

The plasma temperature differences were about 10%, w h i c h implies a 15%

change in resistivity in the " f u l l y " ion- ized region, so that according to Eq.

(2) the resistance of the plasma pro- duced by the convergent orifice should have been higher by some 5%, and that from the divergent orifice lower by some 5%, compared to the cylindrical orifice. This is consistent w i t h the observed changes.

The reduction in area by pneumatic constriction also tends to account for the observed differences in plasma temperature, even though the mini- m u m orifice cross-section was the same for all three orifices. Neglecting conduction and compression or ex- pansion, the rate of change of temper- ature T' for a unit mass of gas is:

where q is the rate of generation of thermal energy by ohmic heating, j is the current density and I the total current. But it has already been noted that:

n = kT-3'2 (4)

w i t h k a constant, and if t is a charac- teristic time for heating the gas, rough- ly the same for all three orifices since the mass flow rate and m i n i m u m ori- fice area are the same, T' is related to the final temperature T by:

T = T/t. (5) Substitution of equations (4) and (5)

into (3) gives:

kT-3/2|2

pCpA2

\ p cp/

(6)

(7) The observed change of some 20% in A w o u l d then account for a change in the opposite direction of some 16% in T, certainly close enough to the observed results, considering the crudeness of this analysis.

Quantitative understanding of the effects of pneumatic constriction re- quires the plasmadynamic analysis of flow through and out of the orifice, including the effect of the abrupt tran- sition from channel flow to free f l o w , w h i c h has not yet been accomplished.

However, it can be noted in closing that if the flow were collisionless (which of course it is not) the velocity vectors of all particles emerging from the orifice w o u l d be confined to a cone whose axis is the torch axis of symmetry and whose outer limit is the extension of the conical surface of the orifice. For the 15 deg convergent ori- fice, the radius of this flow field w o u l d vanish at a distance z„ from the torch, where

z„ = 3.45 m m / 2 tan

7.5° = 13.1 mm (8)

q

pc„ PC„ pc„A2

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since the diameter of the orifice outlet is 3.45 mm and the included angle of the cone is 15°. (This happens to be very close to the center of the work- piece w h i c h is at 12.7 mm for the standoff distance of 6.3 mm (0.25 in.) and workpiece thickness of 12.7 m m (0.5 in.) used to produce the weld shown in Fig. 2.)

There w o u l d be a " l i n e focus" of plasma from that point on along the axis, and—perhaps more realistical- ly—the diameter of the plasma stream w o u l d already have been reduced 10%

at a distance 1.31 mm from the torch.

Conclusion

The concept of pneumatic constric-

1 2 4 - s l APRIL 1980

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t i o n q u a l i t a t i v e l y ties t o g e t h e r several o b s e r v e d facts. A c o n i c a l l y c o n v e r g e n t P A W t o r c h o r i f i c e p r o d u c e s a p l a s m a w h i c h :

1 . Has r e d u c e d d i a m e t e r a n d cross- s e c t i o n a l area.

2. Has h i g h e r t e m p e r a t u r e in t h e s t a n d o f f r e g i o n .

3. Has h i g h e r o v e r a l l r e s i s t a n c e . 4. For f i x e d s t a n d o f f d i s t a n c e , g e n - erates m o r e e n e r g y p e r u n i t l e n g t h o f w e l d .

5. A c h i e v e s g r e a t e r p e n e t r a t i o n . 6. P r o d u c e s a d e s i r a b l y n a r r o w e r w e l d w i t h s m a l l e r r a t i o o f t h e w i d t h s at t o p a n d b o t t o m t h a n a c o n v e n t i o n a l s t r a i g h t c y l i n d r i c a l o r i f i c e .

N o t r o u b l e w i t h d o u b l e - a r c i n g is o b s e r v e d if t h e o r i f i c e o u t l e t is a b r a - sively p o l i s h e d s m o o t h a n d f r e e o f b u r r s ; t h u s , it is n o t n e c e s s a r y t o w e a k - e n t h e e f f e c t o f t h e c o n v e r g e n t o r i f i c e s e c t i o n b y a d d i n g a f i n a l d i v e r g e n t s e c t i o n . T h e c o n i c a l l y d i v e r g e n t o r i - f i c e , w h i c h s i m u l a t e s t h e e f f e c t o f o r i - f i c e e r o s i o n , has t h e o p p o s i t e , d e l e t e r i - o u s , e f f e c t in all d e t a i l s . A p l a s m a d y - n a m i c analysis o f f l o w t h r o u g h a n d o u t o f t h e o r i f i c e is s t i l l n e e d e d f o r q u a n t i - t a t i v e c o m p a r i s o n w i t h e x p e r i m e n t a l m e a s u r e m e n t s .

Acknowledgments

T h e a u t h o r is d e e p l y g r a t e f u l t o B.I.

D a v i s f o r d e d i c a t e d s u p p o r t a n d v a l u - a b l e o r i g i n a l c o n t r i b u t i o n s t o all aspects o f t h e e x p e r i m e n t a l w o r k . D . W . T r o v e r c o n t r i b u t e d b o t h t o l a b o - r a t o r y w o r k a n d t o d a t a r e d u c t i o n . W . E . L a w r e n c e e v a l u a t e d w e l d q u a l i t y

a n d o f f e r e d v a l u a b l e s u g g e s t i o n s a n d e n c o u r a g e m e n t . R.A. S p u r l i n g s k i l l f u l l y p r e p a r e d m e t a l l o g r a p h i c s p e c i m e n s w h i c h J.C. C h e s n u t t c a r e f u l l y e v a l u - a t e d a n d i n t e r p r e t e d .

U s e f u l d i s c u s s i o n s w e r e h e l d w i t h S.S. G l i c k s t e i n , T . W . Eagar, N . D . M a l - m u t h , D.R. B r u b a k e r a n d W . C . P e r k i n s , a n d v a l u a b l e s u g g e s t i o n s as w e l l as e n c o u r a g e m e n t a n d s u p p o r t c a m e f r o m W . F . H a l l , R.E. D e W a m e s , B.A.

M a c D o n a l d a n d J.C. W i l l i a m s .

T h i s w o r k w a s s u p p o r t e d in part b y R o c k w e l l I n t e r n a t i o n a l u n d e r its I n d e - p e n d e n t Research a n d D e v e l o p m e n t P r o g r a m , in p a r t b y t h e A i r F o r c e M a t e - rials L a b o r a t o r y / L T M u n d e r C o n t r a c t F33615-74-C-5036, a n d in p a r t b y t h e U.S. N a v y O f f i c e o f N a v a l R e s e a r c h , P r o j e c t 4 7 1 , u n d e r C o n t r a c t N 0 0 0 1 4 - 7 5 - C - 0 7 8 9 .

References

1. AWS Arc W e l d i n g and Arc Cutting C o m m i t t e e , Recommended Practices for Plasma-Arc W e l d i n g , AWS publication C5.1-73, American W e l d i n g Society, M i a m i , Florida, 1973.

2. Demyantsevich, V. P., and Sosnin, N.

A., "Some Ways of Increasing the Efficiency of the Plasma Arc," Welding Production, 1974 (4) 26-29 (1974), Russian original Svar.

Proiz. 1974 (4) 15-17 (1974).

3. Demars, P., Schultz, J.-P., and Cuny, E.,

"Les Nouvelles Possibilites d u Procede Plas- ma en Soudage," Soudages et Techniques Gonnexees, 5/6 May-June, 211-230 (1973).

4. Brubaker, D. R., Padian, W „ and Sid- beck, P., " M a n u f a c t u r i n g M e t h o d s for Plas- ma Arc W e l d i n g , " Final Report on Contract F33615-74-C-5036, Report AFML-TR-76-152, Air Force Materials Lab., September 1976 ( A F M L / L T M , Fred R. Miller, Program M a n -

ager, AF Materials Lab., Wright-Patterson AFB, O h i o 45433).

5. Shaw, Jr., C. B., "Diagnostic Studies of the G T A W Arc. Part 1 - O b s e r v a t i o n a l Studies," Welding lournal, 54 (2), Feb. 1975, Research Suppl., pp. 33-s to 44-s; "Part 2 - M a t h e m a t i c a l M o d e l , " Ibid., 54 (3), March 1975, Research Suppl., pp. 81-s to 86-s.

6. Shaw, Jr., C. B., " D e t e r m i n a t i o n of Temperature Distributions in W e l d i n g Plas- mas," submitted for publication in IEEE Trans. Plasma Sci. (Science Center Preprint SC-PP-79-74, C B . Shaw, |r., Rockwell Inter- national Science Center—A35, P.O. Box 1085, Thousand Oaks, California 91360) 1979.

7. Brushlinsky, K. V., " N u m e r i c a l Simula- t i o n of T w o - D i m e n s i o n a l Plasma Flow in Channels," Computer Meth. in Appl. Mech.

and Eng., 6 293-308 (1975).

8. Landes, K., Seeger, G., and Tiller, W.,

"Evaluation of Mass Flow Fields and Elec- tric Current Density Fields in a Free Burning TIG Similar Arc," International Institute of W e l d i n g D o c u m e n t 212-462-79 (Prof. D r -

Ing. Gerhard Seeger, Fachbereich Elektro- technik, Hochschule der Bundeswehr M u n c h e n , Werner-Heisenberg-Weg 39, 8014 Neufiberg, Federal Republic of Germa- ny) 1979.

9. Liepmann, H. W., and Roshko, A., Ele- ments of Gas Dynamics, W i l e y , New York, 1957, Ch. 2.

10. Liepmann and Roshko, op. cit.

Ch. 5.

11. Spitzer, Jr., Lyman, Physics of Fully Ionized Gases, Interscience, New York, 1956, Ch. 5.

12. D r a w i n , H. W., "Evaluation of Electri- cal Conductivity, Heat Conductivity and Viscosity of Plasmas," Ch. 15 of W . Lochte- Holtgreven, ed., Plasma Diagnostics, W i l e y Interscience, N e w York, 1968.

13. Hoyaux, Max F., Arc Physics, Spring- er-Verlag, New York, 1968, Ch. 3.

WRC Bulletin 254 November 1979

(1) A Critical Evaluation of Plastic Behavior Data and a Unified Definition of Plastic Loads for Pressure Components

by J. C. Gerdeen

(2) Interpretive Report on Limit Analysis and Plastic Behavior of Piping Products

by E. C. Rodabaugh

(3) Interpretive Report on Limit Analysis of Flat Circular Plates

by W. J. O'Donnell

T h e s e t h r e e r e p o r t s s u m m a r i z e a f o u r - y e a r e f f o r t by t h e PVRC T a s k G r o u p o n " C h a r a c t e r i z a t i o n of t h e Plastic B e h a v i o r of S t r u c t u r e s " t o m e e t t h e n e e d f o r u n i f i e d a n d s t a n d a r d i z e d m e t h o d s f o r l i m i t a n a l y s i s o n p l a s t i c c o l l a p s e d e t e r m i n a t i o n s .

P u b l i c a t i o n o f t h i s r e p o r t w a s s p o n s o r e d b y t h e P r e s s u r e Vessel R e s e a r c h C o m m i t t e e of t h e W e l d i n g Research C o u n c i l .

T h e p r i c e o f W R C B u l l e t i n 2 5 4 is $ 1 3 . 5 0 p e r c o p y . Please i n c l u d e $ 3 . 0 0 f o r p o s t a g e a n d h a n d l i n g . O r d e r s s h o u l d be s e n t w i t h p a y m e n t t o t h e W e l d i n g R e s e a r c h C o u n c i l , 3 4 5 East 4 7 t h St., R o o m 8 0 1 , New Y o r k , NY 1 0 0 1 7 .

(6)

WRC Bulletin 256 January 1980

Review of Data Relevant to the Design of Tubular Joints for Use in Fixed Offshore Platforms

by E. C. Rodabaugh

The program leading to this report was funded by 13 organizations over a two-year period. The objective was to establish a n d / o r validate design methods for tubular joints used in fixed offshore platforms. The report is divided into four self contained chapters (1) Static Strength (2) Stresses (3) Fatigue (4) Displacements, wherein detailed cross comparisons of various types of test data and design theories are reviewed.

Publication of this report was sponsored by the Subcommittee on Welded Tubular Structures of the Structural Steel Committee of the Welding Research Council.

The price of WRC Bulletin 256 is $13.00 per copy, plus $3.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, 345 East 47th St., Room 8 0 1 , New York, NY 10017.

WRC Bulletin 257 February 1980

Analysis of the Ultrasonic Examinations of PVRC Weld Specimens 155, 202 and 203 by Standard and Two-Point Coincidence Methods

by R. A. Buchanan, prepared for publication by 0. F. Hedden

This report describes two methods of analysis of ultrasonic examination data obtained in a 13-team round-robin examination of three intentionally flawed weldments. The objective of the examinations is to determine the accuracy of independently detecting, locating and sizing the weld flaws, using a fixed procedure.

Computer programs to facility comparison of flaw locations with the ultrasonic data, for each of the specimens and both of the methods, are appended to the report.

Publication of this report was sponsored by the Subcommittee on Nondestructive Examination of Materials for Pressure Components of Pressure Vessel Research Committee of the Welding Research Council.

The price of WRC Bulletin 257 is $11.00 per copy, plus $3.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, 345 East 47th St.. Room 8 0 1 , New York, NY 10017.

PVRC Monograph

Development of General Formulas for Bolted Flanges

Published by the Welding Research Council

Through the courtesy of G&W Taylor-Bonney Division, the 1949 Taylor Forge book, Development of General Formulas for Bolted Flanges, by E. O. Waters, D. B. Rossheim, D. B. Wesstrom and F. S. G. Williams has been reprinted.

The historical importance of this book is enhanced by the fact that the material is current and still forms the basis of the rules in the ASME Codes for the design of raised faced flanges. Additionally, the book continues to appear as a reference in technical papers published in the United States and abroad.

The price of this book is $18.00. Please send payment with orders to the Welding Research Council, 345 East 47th St., Room 8 0 1 , New York, NY 10017.

126-s I APRIL 1980

References

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