WELDING RESEARCH m SUPPLEMENT TO THE WELDINC JOURNAL, MARCH 1986 Sponsored by the American Welding Society and the Welding Research Council
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Development of a Weldability Test for Pulsed Laser Beam Welding
Relatively easy test can predict cracking sensitivity in laser beam welded alloys
BY L. A. WEETER, C. E. ALBRIGHT AND W. H. JONES
ABSTRACT. A weldability test designed a rating of a material's weldability is dency to hot crack (Refs. 1, 2). However, specifically for small component pulsed obtained. The results of the 796 data none of these previously developed tests laser beam welding was developed and points collected were consistent and consider the unique characteristics of tested on thirteen alloys and two dissimi reproducible. A crack susceptible alloy pulsed laser beam welding. lar alloy combinations. The weldability will begin to crack at a much smaller Pulsed laser beam welds are smaller test consists of drilling small holes of cavity depth than a crack insensitive alloy, and cool much more rapidly than other various diameters (0.343-0.572 mm/ due to the inability of the susceptible types of welds. As a result of the small 0.013-0.023 in.) and depths (0.127-0.508 alloy to cope with the increased degree sizes, low residual stresses are produced mm/0.005-0.02 in.) in the alloys or of restraint and solidification stresses of in pulsed laser beam weldments. These between the dissimilar alloys. Then, a the deeper cavities. low magnitude residual stresses reduce single YAG laser pulse is impinged directly The pulsed laser beam weldability test the effectiveness of the self-restraining on each of the drilled holes. The absence rated the alloys and alloy combinations weldability tests (Ref. 2) (such as the of metal in the hole causes the solidified from least crack sensitive to most crack circular patch and T-joint tests) in rating spot weld to have a concave contour. sensitive as follows: Hastelloy B2, Hastel the cracking sensitivity of alloys. The Varying the diameter and depth of the loy S, Hastelloy C4, stainless 304A, stain rapid cooling rates of pulsed laser beam drilled holes changes the depth of the less 304B, Inconel 625, Inconel 718, stain welds also reduce the accuracy and con cavity produced in the spot weld. By less 316A, stainless 316B, stainless 310, sistency of the augmented strain weld recording the depth of the cavity at Modified Inconel 600, stainless 330, Has ability tests (Ref. 2) (such as the Vares which the alloy exhibits incipient cracking, telloy X. traint and Murex tests). The application of Dissimilar alloy results indicated that strain at a reproducible instance in a Hastelloy B2 joined to modified Inconel weld's solidification cycle is critical for the 600 was a crack insensitive combination, accuracy of the augmented strain tests. L A. WEETER and W. H. JONES are with while stainless 304(A) to Inconel 718 was Pulsed laser beam welds cool so rapidly Monsanto Research Corporation, Miamisburg, crack sensitive. that the accurate, reproducible applica Ohio. Monsanto Research Corporation is tion of strain during solidification is not operated for the U. S. Department of Energy feasible. under contract No. DE-AC04-76-DP00053. Introduction C. E. ALBRIGHT is with The Department of In addition, pulsed laser beam welds Welding Engineering, Ohio State University, Many weldability tests have been are usually made on small components Columbus, Ohio. developed for evaluating an alloy's ten which are fabricated from small bars or
WELDING RESEARCH SUPPLEMENT 151-s pools is very similar to the solidification of weld craters. Depending on the circumstances, either increased segregation (Ref. 10) or lack of sufficient flow to fill the round crater (Ref. 10) can be responsible for crater cracking. For whatever reason, it is widely documented (Refs. 10, 11) that unless proper welding techniques are implemented, even a normally weldable alloy can exhibit crater cracking. Pulsed laser beam welds solidify very similarly to weld craters; thus, pulsed laser beam welds would be expected to be more crack sensitive than continuous wave IBHHHHI welds. Because pulsed laser welding can sig nificantly alter the cracking tendencies of alloys, a weldability test specifically designed for small component pulsed f laser beam welding is needed. The objec tive of this study was to develop a weldability test for pulsed laser beam Fig. 2 — Various weld pool solidification pat welding that would use a small sized weld terns: (A) elliptical (B) tear-dropped (C) round sample and that would be relatively easy B 100X (crater) to perform and evaluate. Fig. 1 — Cross-sections of a (A) continuous wave electron beam weld and a (B) pulsed laser beam weld on the same sample of less steels. They found that in two of the Experimental Procedure stainless 316. Note the extensive cracking in stainless steels, the most rapidly cooled the pulsed laser beam weld areas of the laser beam welds deviated Alloy Composition and Sample Preparation from the expected microstructures. Stain Table 1 lists the chemical compositions less type 308, which normally has 5-7% of the alloys used in this study. These ferrite in the final weld microstructure, sheet stock. It is often desirable to evalu alloys were chosen for testing because was completely austenitic. Type 312 ate each lot of stock for crack sensitivity most of them are commonly used in stainless steel, which normally has 35- in welding precision components, be industry and because they provide a 40% ferrite, was completely ferritic. Vitek cause lot-to-lot metallurgical variations in wide range of hot cracking tendencies. hypothesized that the large undercool alloys can cause variations in cracking Arc welding weldability studies and the ings generated in laser beam welding tendency. Many weldability tests use alloy manufacturer's welding information produced massive transformations (Refs. large welding samples; thus, they are not rate the weldability of the alloys (Refs. 4, 5), which modified the microstruc easily adaptable to the smaller dimen 12-14) in descending order from most to tures. sions encountered in pulsed laser beam least weldable as follows: welding. Molian and Wood (Ref. 6) demonstrat Most Weldable The motivation for developing a weld ed that laser processing can produce Hastelloy S ability test for pulsed laser beam welding massive transformations in Fe-Cr-Ni Hastelloy B2 was derived through experience at the alloys. The Fe-6%Cr-2%Ni alloy they stud Hastelloy C4 Monsanto Research Corporation (Mound ied exhibited a structure of both massive Stainless 304A* Facility). Researchers observed that in ferrite and martensite. Stainless 316A* some cases pulsed laser beam welds tend The effect of massive transformations Inconel 718 to crack more prevalently than other on the crack sensitivity of alloys has not Inconel 625 types of welds. Cross-sections of a con been investigated. However, if, as Vitek Stainless 304B* tinuous wave electron beam and a demonstrated, massive transformations Stainless 316B* pulsed laser beam weld are shown in Fig. produce a completely austenitic or ferritic Stainless 310 1. These welds were made adjacent to matrix, massive transformations could Modified Inconel 600 each other on the same sample of stain alter the hot cracking tendencies in stain Hastelloy X less steel, type 316. Each weld was cross- less steel. Stainless 330 Least Weldable sectioned at four locations and examined. Another possible explanation for the The modified Inconel 600 has an exces No cracks were found in any of the increased cracking in pulsed laser beam sive amount of copper (0.41% compared electron beam weld cross-sections, but welds is an alteration of the weld pool to 0.25% maximum for Inconel 600). every cross-section of the pulsed laser shape. Continuous wave welds assume Because of the excess copper, modified beam weld exhibited cracking. At this either an elliptical or a tear-drop shaped Inconel 600 was expected to be crack time, the reason for the increased crack pool (Ref. 8)-Figs. 2A, 2B. In both of sensitive. ing in pulsed laser beam welds is not these shapes, growth of the solidification The two stainless 304 alloys and the completely understood. However, it is boundary occurs only from the trailing two stainless 316 alloys were chosen thought that an alteration of the final edge of the weld pool into the liquid. because of the differences in their pre- weld microstructure and/or of the weld Pulsed laser beam welds have a nearly pool shape caused the cracking. round shape, and, when solidification is Vitek, ef al. (Ref. 3), studied the effects complete between pulses, they solidify * These are different alloys with compositions of laser beam welding in modifying the from all sides of the pool —Fig. 2C. The within the specifications for stainless 304 and microstructures of three austenitic stain- solidification of these round individual stainless 316.
52-s | MARCH 1986 Table 1—Chemical Compositions of the Experimental Alloys
Alloys C Mn Si Cr Ni Cu Mo Fe Co Ti Al Cb + Ta SN W Q. Stainless 304A .065 .89 .41 .015 .028 18.33 8.20 .44 .45 Bal .15 .052 - - - - - — o -I Stainless 304B .023 1.7 .41 .001 .007 18.3 11.4 , - .04 Bal .04 .037 - - - - - — LU Stainless 310 .050 1.65 .48 .009 .030 25.10 19.90 .22 .26 Bal - .08 - - - - - — > Stainless 316A .050 1.55 .51 .0015 .010 17.8 10.9 2.55 Bal .01 LU — - — - - - - - O Stainless 316B .050 1.46 .44 .021 .017 16.25 13.11 .11 2.23 Bal .04 .065 — - - - - - Mod. Inconel 600 .06 .26 .26 .001 — 15.98 74.00 .41 - 9.03 — — ------Inconel 625 .044 .15 .14 .001 .009 21.15 Bal - - 4.42 .19 - .28 .30 3.51 - - - o Inconel 718 .04 .10 .13 .002 .005 18.42 52.73 .04 3.09 18.39 .12 - .98 .60 5.30 .0032 - -
WELDINC RESEARCH SUPPLEMENT j 53-s locating cracks. The SEM was also imple CAVITY DEPTH mented to photograph cracks in selected samples. The cavity depth at which the alloy exhibited incipient cracking was used to rate the alloys' weldabilities. A total of 796 data points were collected. The data points shown in Figs. 7, 8 and 9 demon strate that the results were reproduc ible. Fourteen welds on each alloy were then cross-sectioned and examined. This was performed to determine if the laser weldability test was bringing the majority Fig. 4 — Schematic showing the cavity depth measurement of hot cracks to the surfaces of the welds or if some alloys were cracking in their heat affected zones without causing sur face cracks. Other reasons for cross- ALLOY 1 sectioning were to observe the shape of the cavity and the growth direction of the grains in the weld zone. The cross-sections of the austenitic stainless steel welds (304A, 304B, 316A ALLOY 2 and 316B) were also inspected for any ferrite in the weld microstructure. All of the stainless steel welds were cross- sectioned, polished, and electrochemical- DISSIMILAR ALLOY SAMPLE ly etched with potassium cyanide. The welds were then optically inspected at Fig. 5 — Schematic of a dissimilar alloy sample 1000X for the presence of ferrite.
Dissimilar Alloy Test schematic of a typical sample is shown in drilled hole, the degree of concavity Fig. 3. The holes were 0.35, 0.36, 0.41, could be altered. The concept of drilling small holes was 0.46, 0.51, 0.53 or 0.57 mm (0.013, A coaxial light, binocular microscope also investigated for rating the weldability 0.014, 0.016, 0.018, 0.020, 0.021 or 0.022 and a dial indicator were then used to of two dissimilar alloy combinations. in.) in diameter with 0.13, 0.25, 0.38 or measure the depth of the cavity —Fig. 4. Stainless steel 304A to Inconel 718 was 0.51 mm (0.005, 0.010, 0.015 or 0.020 in.) The dial indicator was mounted so that it chosen because they represent two rela depths. All tolerances were ± 0.03 mm could measure the amount of movement tively weldable alloys which should (± 0.001 in.). Holes with a diameter larg of the microscope stage. The microscope produce an unweldable combination. er than 0.57 mm (0.022 in.) were not was set at 200X magnification and was Hastelloy B2 to Modified Inconel 600 was used because the focused laser beam focused on the deepest point of the also selected because it was hypothe would pass into the hole without hitting cavity. The dial indicator was zeroed. The sized that the weldable Hastelloy B2 and the top surface of the sample. Initial tests microscope was then focused on the the crack sensitive Modified Inconel 600 used a series of holes with the complete surface of the sample. The dial indicator would form a weldable combination. set of diameters and a constant 0.38 ± reading was then recorded as a measure In this experiment, two 6.35 X 0.03 mm (0.015 ± 0.001 in.) depth. Later ment of the depth of the cavity. An 6.35X25.4 mm (0.25X0.25X1.0 tests varied the hole diameter and depth Indi-Trace measuring device was used to in.) bars were tack welded together using to focus over the critical ranges for each verify the accuracy of the microscopic a Korad Model KWD YAG laser. Control alloy. For example, wide, deep holes technique. The Indi-Trace's results indi samples of similar alloy welds, e.g., 304A were drilled for crack insensitive alloys, cated that the microscopic method of to 304A, 718 to 718, etc., were also tack and narrow, shallow holes were drilled measuring cavity depth was accurate welded to compare to the 304A-718 and for crack sensitive alloys. within 0.06 mm (0.002 in.). B2-600 samples. A small drill press was After the holes were drilled, the sam While measuring the cavity depth, the then used to drill holes along the interface ples were ultrasonically cleaned in tri- 200X binocular microscope was also between the tack welded samples —Fig. chlorethylene, then freon, then alcohol to employed to inspect the spot welds for 5. Finally, just as in the similar alioy study, remove any residual machining oil. cracks. The accessibility and simple oper a single laser pulse was impinged on each A Korad Model KWD YAG laser was ation of the binocular microscope made it drilled hole, and then the resulting cavity employed to impinge a single laser pulse the choice for inspection rather than a depths were measured. The data for this directly on the hole. The Korad laser was more complex scanning electron micro study is shown in Fig. 15. equipped with a coaxial viewing system scope (SEM). An SEM was used in initial tests to verify that the binocular micro for alignment of the laser pulse on the Continuous Wave Experiment center of the drilled hole. Error in scope was accurately identifying cracks. It alignment is estimated at ± 0.03 mm was found that the binocular microscope After the data for a single laser pulse (± 0.001 in.). The parameters developed could be used to detect cracks over 0.06 on each drilled hole were collected, an in the laser study were used to make the mm (0.002 in.) long in the crater of the experiment was performed to determine welds (see Table 2). The absence of welds. It was felt that with this degree of if the test could be adapted to continu metal in the hole caused the solidified accuracy, coupled with a consistent stan ous wave laser beam welding. Only two spot weld to have a concave contour. By dard for identifying a crack, the binocular alloys, Inconel 625 and stainless steel 310, varying the diameter and/or depth of the microscope was a competent tool for were studied in this experiment.
54-s|MARCH 1986 A miniature drill press was used to drill surface of the welds —Fig. 12. However, Table 3—Continuous Laser Beam Weld two lines of holes on 25.4 mm (1 in.) long none of the alloys exhibited subsurface 3 samples of the two alloys. A Coherent cracking at cavity depths significantly dif Parameters' ' Model 525 CO2 laser was then used to ferent from the shallowest cavity depth produce a continuous wave weld over to exhibit surface cracking — Fig. 13. Thus, Power: 300 W each line of holes on the two alloys. The examining only the surface of the weld Travel Speed: 21.1 mm/s parameters, listed in Table 3, were cho under the conditions of this study was an Lens: 2.5 in. CaAs sen because they would produce welds accurate method of evaluating weldabili Focus: Sharp focus with a 0.89 ± 0.03 mm (0.035 ± 0.001 ty- (a)Coherent Model 525 Laser in.) width and 0.64 ± 0.03 mm (0.025 ± The cross-sections also illustrate that 0.001 in.) penetration. A photograph of the cavity contours change as the cavities the completed Inconel 625 welds is become deeper. The diameters of the Continuous Laser Beam Welding Results shown in Fig. 6. welds remain almost constant. Only the The effect of cavity depth on the The binocular microscope technique cavity depths increase. For a small cavity cracking tendency of Inconel 625 and was then used to measure the cavity depth, the weld is almost flat (Fig. 12A), stainless steel 310, when continuously depth at the locations of the drilled holes while for a large cavity depth, the weld laser beam welded, is shown in Fig 16. and to detect cracks. contour becomes very concave —Fig. The pulsed laser data is also shown in Fig. 12C This change in contour is one reason 16, for comparison. No cracks were the deep cavities exhibit a greater ten Results detected in any of the Inconel 625 contin dency for cracking than shallow cavities. uous welds, even at cavity depths greater Pulsed Laser Beam Welding Results Another interesting result of this study than depths that exhibited cracking in is that no ferrite was found in any of the pulsed welding. Cracks were detected in The effect of cavity depth on the hot austenitic stainless steel welds. Even stain the stainless 310 welds, but the incipient cracking tendency of the three alloy fam less steel 304A and stainless steel 316A, cracking depth was much larger for the ilies is shown in Figs. 7, 8 and 9. The which the Delong diagram predicted to continuous welds (0.23 mm/0.009 in.) number of samples made at each cavity have 7% and 8% ferrite, respectively, than for the pulsed welds (0.08 mm/ depth is included in these figures to were completely austenitic —Fig. 14. The 0.003 in.). demonstrate the consistency of the weld cooling rates and undercooling in the ability test. From Figs. 7 through 9, it is liquid in the pulsed laser beam welds clear that increasing the cavity depth must have completely impeded the for Discussion increases the tendency for cracking in all mation of the ferrite. The absence of Test Description of the alloys. Furthermore, it is clear that delta ferrite in these welds is important the shallowest cavity depth to exhibit for explaining their weldabilities and will The rapid solidification rates and cracking can be used to rate weldability. be discussed in the next section. changes in pool solidification shape of Crack susceptible alloys, Hastelloy X and pulsed laser beam welding can alter the stainless 330, begin to crack at much cracking sensitivity of some alloys. In Dissimilar Alloy Weld Results shallower cavity depths than crack insen addition, the low residual stress levels and sitive alloys, Hastelloy B2 and Hastelloy S. The results of the dissimilar alloy welds fast solidification rates of laser beam The other alloys begin to crack at cavity are shown in Fig. 15. These results closely welding could make the previously depths between these two extremes and resembled the similar alloy study except developed weldability tests unadaptable demonstrate a wide range of weldabili that the incipient cracking depth was to pulsed laser beam welding. Therefore, ties. The cavity depth that exhibits incipi slightly lower for Inconel 718 and stainless a weldability test, designed specifically ent cracking can be used to express the 304A. This implies that the joint interface for pulsed laser beam welding, was weldability for each of the alloys, as does influence cracking, as expected. developed. illustrated in Table 4. To use the pulsed laser beam weldabil It is shown in Figs. 7, 8 and 9 that the ity test, the following procedure is sug cavity depth definitely influences crack gested: ing. All of the alloys, except Hastelloy X 1. Determine a set of laser parameters and stainless 330, which cracked at all that will produce welds with the same depths, and Hastelloy B2, which did not diameter and depth on all of the alloys to crack at any depth, exhibited the same cracking sequence: 1. No cracks at small cavity depths. Table 4—Pulsed Laser Weldability Rating 2. A critical cavity depth range is reached where the alloy sometimes Alloy Weeter Index'3' cracks and sometimes does not crack. 3. Above a certain depth, the alloy Hastelloy B2 0.48 always cracks. Hastelloy S 0.38 The appearance of both uncracked Hastelloy C4 0.34 and cracked welds in stainless steel 316B Stainless 304A 0.28 is shown in Fig. 10. Stainless 304B 0.23 Inconel 625 0.18 Figure 11 is an SEM photograph of a Inconel 718 0.15 crack in stainless steel 316B. The surface Stainless 316A 0.10 morphology of the crack is that of inter Stainless 316B 0.10 dendritic separation. Cracks in the other Stainless 310 0.08 alloys exhibited similar surface morpholo Modified Inconel 600 0.03 gies. The cause of the cracks will be Stainless 330 0.01 discussed in the next section. Hastelloy X 0.01
The cross-sections of the welds reveal Fig. 6 — Photograph of a continuous laser beam ta}The Weeter Index is the cavity depth, in millimeters, that that not all of the hot cracks came to the weld on an Inconel 625 sample exhibited incipient cracking.
WELDING RESEARCH SUPPLEMENT 155-s 0.019 0.4826 2 • • Cracks were detected in weld or welds 0.018 — • No cracks were detected in weld or welds — 0.4572 The number beside the • or • denotes the number of Q samples with that cavity depth. • 0.017 0.4318 • _ • 0.016 0.4064 •
0.015 0.3810 4«
_ • _ 0.014 0.3556 • 3* • 0.013 • 0.3302
2* 2«# 0.012 0.3048
Vi CD 2 1/T | 0.011 * • / * 0.2794 *-r JZ 0) 4-> / 4» •• / S 0.010 0.2540 § > 2« 3« ># _C "> • * A, • 2» • »3 _ CL o 0.009 0.2286 CU Q 2* • 2« t • > 4-> ll 3« / »4 _ > 0.008 0.2032 CD CJ • 2» / • }3
0.007 _ 2« 4» / *3 _ 0.1778 • 2* # 2* t _ 3» • 2« g »2 0.006 0.1524 2* 2»./ • • ••/ 0.005 2 • 2 •/ • 2 0.1270 2» 2»»2 3.03 _ 2» ft+2 *#* 0.004 0.1016 •-**»* * • 4 3* »^ *>3 •« • *6 lU 0.003 _ 3« # *5 M 0.0762 • / 3 »3 2 6 0.002 - * / J3 * 0.0508
0.001 0.0254 3« *5 • 0 000 II II I 1 0.000 Stainless Stainless Stainless Stainless Stainless Stainless 330 310 316A 316B 304B 304A
Material
Fig. 7 —Cavity depth measurements for the stainless steel samples
56-s | MARCH 1986 0.019 0.4826
• Cracks were detected in weld or welds 0.018 • No cracks were detected in weld or welds. 0.4572 The number beside the • or • denotes the number of samples with that cavity depth. 0.017 0.4318
0.016 - 0.4064
0.015 0.3810 • 0.014 — • 0.3556
• 0.013 • 0.3302 • • 0.012 • 0.3048
• 0.011 — 2* • _ 0.2794 is •w CD • 2a» a) CU E CJ •• c 0.010 — 2* 3* 0.2540 I JZ • a. 2* • 3 a 0.009 • • • 3 0.2286 S > • 2* '> • • • CO • *> CJ 0.008 •"• • 3* • 2 • • 0.2032 o 2* 2*c) • 2* >* »2 0.007 — 2» 2» S~ <) 2 0.1778 2« • ^r T ft^r I 3 3« 3«#* I 0.006 - 4 # 3 2 - ill * ^ »3 0.1524 4» / 3 2» / »2 »3 0.005 4« / t>6 2 _ 0.1270 2 0.004 — 2* / • 4 / • 0.1016 3-M / 43 4«t i • 0.003 • i *3 0.0762 2«A # • ••2 • • 3«t)2 0.002 — • 2 0.0508 3» Fig. 8 —Cavity depth measurements for the Inconel samples WELDING RESEARCH SUPPLEMENT | 57-s 0.4826 • / • Cracks were detected in weld or welds • / • 0.018 • No cracks were detected in weld • _ 0.4572 or welds • / " 0.017 thThee numbenumberr obesidf samplee thse wit• ohr •tha denotet s ** A r — 0.4318 cavity depth. g t 0.016 • 0.4064 • / 0.015 0.3810 0.014 - 0.3556 •/' • 0.013 f• • 0.3302 — 1 I I 0.012 0.3048 * t/i CD 0.011 / »3 »2 0.2794 Si CU • — o I • = c I ? 5 0.010 .C • 0.2540 1 •a-* Q. a CL (U f • DCD Q > 0.009 s • • 0.2286 *->> > > CO 1 • O 1 CroJ 0.008 — 0.2032 0.007 - 0.1778 : 5 ! f3 0.006 • - 0.1524 • * • »3 0.005 — • / J4 J2 • — 0.1270 2. • M J2 • 0.004 — 3« / «4 • 2 0.1016 3» / »3 V 0.003 3* / »2 • 2 0.0762 2» / «2 0.002 • 4 ~~ 0.0508 " 2J« »2 0.001 ~" 4«| »2 0.0254 2." 0.000 1 1 1 1 1 0.000 Hastelloy X Hastelloy C4 Hastelloy S Hastelloy B2 Material Fig. 9 —Cavity depth measurements for the Hastelloy samples 58-s j MARCH 1986 Fig. 10—SEM photographs of the spot welds on a stainless 316B sample: (A) Cavity depth0.127 mm; no cracks are seen. (B) Cavity depth = 0. 178 mm; a small crack is evident. (C) Cavity depth = 0.229 mm; a large crack is observable K Fig. 11 — SEM photograph sho wing the surface morphology of a typical crack in stainless 3I6B Fig. 12 — Photomicrographs of cross-sections of welds on Inconel 600 (I00X): (A) Cavity be tested. In this study, laser power and depth = 0.08 mm. (B) Cavity depth = 0. 16 mm. (C) Cavity depth = 0.20 mm. (D) Cavity laser beam focused spot size were varied depth = 0.25 mm to produce spot welds with a 0.89 ± 0.03 mm (0.035 ± 0.001 in.) diameter and 0.58 ± 0.03 mm (0.023 ± 0.001 in.) consistent method. more than two relative positions' differ depth on all of the alloys. 6. Identify cracks with a consistent ence in the above rating systems are 2. Use two alloys with known crack method. stainless steel 304B and stainless steel sensitivities as standards. One alloy 7. Use the cavity depth where incipi 316A. Considering their respective pre should be relatively crack insensitive and ent cracking is observed to rate the dicted levels of delta ferrite, 0% and 8%, the other alloy should be relatively crack weldabilities of the alloys. Alloys that stainless steel 304B should be crack sensi sensitive. These alloys are necessary to begin to crack at shallow cavity depths tive and stainless steel 316A should be compare with the unknown alloys and to are more crack sensitive than alloys that crack insensitive (Ref. 7). However, the ensure that the test is working properly. begin to crack at deep cavity depths. converse was true. One possible expla 3. Drill a wide matrix of holes. If the nation of these variations in weldability is diameter and depth of the spot welds are the effect of laser beam welding's fast Similar Alloy Weldability Rating 0.89 ± 0.03 mm and 0.58 ± 0.03 mm solidification rate on segregation and the respectively, the matrix of holes in Table The pulsed laser weldability test rated formation of delta ferrite. 5 should provide enough data to ade the weldabilities of the tested alloys as Increasing the solidification rate de quately rate most alloys. similar to arc welding weldability studies, creases the amount of segregation. At 4. Impinge a single laser pulse directly as seen in Table 6. These results indicate extremely high solidification rates, com on each drilled hole. that the pulsed laser beam weldability pletely homogeneous alloys are obtained 5. Measure the depth of the cavity test is in general agreement with arc (Ref. 4). Laser beam welding's fast solidifi that forms in the spot weld due to the welding weldability tests. cation rate could reduce the segregation absence of metal in the hole with a The only major discrepancies with of detrimental low melting point constitu- WELDING RESEARCH SUPPLEMENT 159-s 019 0.48 018 Incipient surface cracking depth 0.46 017 Incipient subsurface cracking depth 0.43 016 0.41 015 0.38 014 0.36 013 0.33 012 0.30 011 0.28 010 0.25 009 0.23 008 0.20 007 0.18 006 0.15 Q05 0.13 004 0.10 003 0.08 002 0.05 001 _ 0.03 330 X 600 310 316A 3168 718 625 304B 304A C4 S B2 Material Fig. 13--Surface vs. subsurface cracking data showing no significant change in cracking tendencies for the alloys B * • .v sa* • N - • V > :v VTVV - • v .* Fig. 14 — Cross-sections of spot welds on the 0.019 • Cracks were detected —0.4 8 stainless steel samples showing no ferrite in any of the welds (1000X). (A) Stainless 304B. 0.018 • No cracks were detected — 0.46 (B) Stainless 316A 0.017 —0.4 3 0.016 * 0.41 point constituents, high solidification rates 0.015 — 0.38 might not "trap" all of the impurities in 0.014 • 0.36 the matrix. Some low melting point con stituents could segregate to interdendritic 0.013 • 0.33 spaces. Without any delta ferrite to 0.012 _ 0.30 impede the spreading of the liquid, a 0.011 • — 0.28 harmful liquid film forms on the forming 0.010 0.25 grain boundaries. If a stress is applied 0.009 • _ 0.23 while the grain boundaries are still liquid, • hot cracks form. 0.008 • — 0.20 The concentrations of sulfur, phospho • 0.18 0.007 — rus and silicon, elements known to cause 0.006 • _ 0.15 hot cracking in stainless steels, are low in 0.005 • _ 0.13 both stainless steel 304B and stainless 0.004 • _ 0.10 steel 316A (Table 1). The high solidifica • 0.003 • — 0.08 tion rate encountered in laser beam welding may increase the solubility of 0.05 0.002 — these elements enough that they are 0.001 _ 0.03 completely absorbed in the matrix. _L Because stainless steel 304B has no other 304A-718 71! 304A 600 B2-600 B2 low melting constituents, stainless steel Material 304B becomes crack insensitive when Fig- 15 —Cavity depth measurements for the dissimilar alloy samples laser beam welded. Stainless steel 316A, however, has a high level of molybde num. Cieslack (Ref. 12) has shown that ents and thus decrease hot cracking sen duplex stainless steels. With the elimina molybdenum forms a low melting chi sitivity. tion of delta ferrite, laser beam welds are phase in stainless steels. This extra Although high solidification rates can more crack sensitive than slow cooling amount of low melting point liquid is a decrease hot cracking sensitivity by arc welds. plausible reason why stainless steel 316A reducing segregation, high solidification The complex interaction of reduced cracks when laser beam welded. rates can also increase hot cracking sensi segregation and lack of beneficial delta tivity by impeding the formation of delta ferrite may account for the variation in Dissimilar Alloy Weldability Rating ferrite in stainless steel welds. Alloys that weldabilities of stainless steel 304B and solidify as primary austenite without the stainless steel 316A. For alloys with low A major advantage of the pulsed laser production of any delta ferrite are well harmful impurity levels, high solidification beam weldability test is the ability of the known to be crack sensitive (Ref. 7). rates could reduce the segregation of the test to evaluate dissimilar alloy welds. The Vitek (Ref. 3) and this research have impurities to such a minute amount that experimental data indicates that drilling demonstrated that laser beam welding's the absence of delta ferrite is not a factor holes at a square butt joint interface has fast solidification rate can impede the in hot cracking sensitivity. Conversely, for the same effect on cracking tendency as formation of delta ferrite in normally alloys with a large amount of low melting drilling holes on the surface of an alloy: 60-s | MARCH 1986 0.4826 o • Cracks were detected in weld or welds Table 5- Suggested Drilled Hole Matrix (in • 4 No cracks were detected in weld or wc Ids min)l'l 0018 - - 0.4572 Diameter Depth Diameter Depth 0.017 - - 0.4318 0.343 0.381 0.343 0.508 0.016 - - 0.4064 0.368 0.381 0.368 0.508 0.407 0.381 0.407 0.508 0.457 0.381 0.457 0.508 - - 0.3810 0.508 0.381 0.508 0.508 • 0.533 0.381 0.533 0.508 • 0.3556 0.572 0.381 0.572 0.508 • (a)Drill four holes of each depth and diameter. • • 0.3302 • • — 0 0.3048 the drilled hole concept for continuous • wave welds, larger diameter and/or - O • - 0.2794 • deeper drilled holes are necessary. o • • • Although additional work is required to 0.010 - - 0.2540 • o • fully develop the test for the continuous laser beam welding process, the initial 0.009 — o • —0.228 6 results indicate that the technique is a • promising method of rating weldability. 0.008 • 0.2032 •J Cracking in Pulsed Laser Welds 0.007 t 0.1778 • The surface morphologies of the j cracks generated along the grain bound • aries of the spot welds indicate that the 0.005 i interdendritic separation resulted in the j cracks. Interdendritic separation can be • - - 0.1016 caused by: 1. The separation of a liquid film on - - 0.0762 the grain boundaries, i.e., a true hot crack. 0.002 0.0508 2. Insufficient liquid metal present to 1 • i allow the growing dendrites to interlock, • 0.0254 i.e., a shrinkage crack. - • - • Either of these mechanisms can cause the i r i 1 1 0.000 cracking observed in this study. Pulsed Continuous Pulsed Continuous However, the good agreement be Stainless 310 Stainless 310 Inconel 625 Inconel 625 tween the expected and experimental Material results implies that the pulsed laser beam Fig. 16 —Cavity depth measurements for the continuous wave samples weldability test is an accurate method of evaluating an alloy's sensitivity to true hot cracking. In addition, the surface and subsurface cracking results (Fig. 13) show 1. No cracks at small cavity depths. drilled holes can also be applied to con that the test is detecting the majority of 2. A critical cavity depth range is tinuous wave welding. The stainless steel the cracks. reached where the alloy sometimes 310 welds were similar to the pulsed laser cracks and sometimes does not crack. beam welds, except for the incipient 3. Above a certain depth, the alloy cracking cavity depth. Both the pulsed always cracks. and continuous welds did not crack at This implies that the pulsed laser beam shallow cavity depths, began to crack at a weldability test can evaluate dissimilar certain depth, and continued to crack alloy welds. after that depth. The only difference was However, all of the cracks in the dis that the continuous welds began to crack similar alloy welds propagated along the at a 0.229 mm (0.009 in.) depth, while the joint interface between the alloys —Fig. pulsed welds began to crack at a 0.089 17. This implies that the interface contrib mm (0.003 in.) depth. The variation in utes to the formation of cracks in the welding mode may have decreased welds. Thus, the dissimilar alloy welds either the solidification stresses or the must be compared to similar alloy sam restraint of the weld, or both, to cause ples with a joint interface for proper this increase in incipient cavity depth. evaluation. The Inconel 625 continuous welds did not crack at any of the cavity depths produced. If the Inconel 625 is similar to Continuous Wave Welds stainless 310, deeper cavities than those Fig. 17-Photograph of a crack along the Joint The continuous wave laser experiment made in this experiment are needed to interface between Inconel 718 and stainless shows that the concept of welding over cause the Inconel 625 to crack. To use 304A WELDING RESEARCH SUPPLEMENT 161-s small sized samples and is relatively easy Table 6—Comparison of Laser Beam and Arc Weldability Rating to perform and evaluate. Microsegregation, fluid flow, heat Laser Beam Weldability Rating Arc Weldability Rating flow, restraint, welding mode, solidifica Hastelloy B2 Hastelloy S tion stresses, ductility, and variations in Hastelloy S Hastelloy B2 material properties and composition can Hastelloy C4 Hastelloy C4 all affect hot cracking. Until a hot cracking Stainless 304A Stainless 304A model that considers all of these variables Stainless 304B Stainless 316A is developed, practical techniques, such Inconel 625 Inconel 718 Inconel 718 Inconel 625 as the laser beam weldability test, are Stainless 316A Stainless 304B necessary to evaluate the weldabilities of Stainless 316B Stainless 316B alloys. Stainless 310 Stainless 310 Modified Inconel 600 Hastelloy X Hastelloy X Stainless 330 References Stainless 330 ""Modified Inconel 600 1. Welding Handbook. 1981. Volume l. Seventh Edition, American Welding Society, ta) Modified Inconel 600's weldability was unknown for arc welding. Miami, Fla. 2. Davidson, |. A., Konkol, P. |., and Sovak, Effects of Cavity Depth on Cracking tion. If the base metal surrounding the I. F. 1983. Assessing fracture toughness and cracking susceptibility of steel weldments —a weld has enough restraint to resist defor These researchers decided to use the review. Final Report, Department of Transpor mation under the stresses caused by the cavity depth that exhibits incipient crack tation, December, Washington D.C. contraction of the weld metal, the weld ing as a means of rating weldability, 3. Vitek, ). M., Dasgupta, A., and David, metal must accommodate the stresses. because measuring the cavity depth was S. A. 1983. Microstructural modification of Thus, the weld metal can either plastically consistent and simple. By measuring the austenitic stainless steels by rapid solidification. deform or crack. By varying the restraint Metal. Trans. 14A(9): 1833-1841. cavity depth and identifying cracks, a of the weld sample, the cracking suscep 4. Mehrabian, R. 1982. Rapid solidification. consistent cracking index was obtained. tibility of the weld sample can be Int. Metal. Reviews 27(4): 185-208. Other weldability tests use the total num altered. 5. Plichta, M. R. 1984. The nucleation kinet ber of cracks, the total crack length, or ics, crystallography, and mechanism of the The self-restraining weldability tests the longest crack length to rate weldabil massive transformation. Metal. Trans. (Lehigh, Tekken, Circular Patch and ity. As the cavity depth increased, the 15A(3):427-435. total number of cracks did change from Houldecraft) use different methods of 6. Molian, P. A., and Wood, W. E. 1983. zero to approximately five, but this is not varying the restraint of the weld sample Transformation behavior of laser processed a large enough difference to rank to evaluate the weldability of materials. In Fe-5"„ Cr, Fe-5",> Ni, and Fe-6% Cr-2"„ Ni materials. The cracks were also longer this study, the cavity depth altered the alloys. /. Metals Sci. 18:2555-2562. and wider as the cavity depth increased. restraint of the weld. 7. Hull, F. C. 1967. Effect of delta-ferrite on Thus, crack length measurements might The cavity depth is also important the hot cracking of stainless steels. Welding lournal 46(9):399-s to 409-s. be another index for rating weldability. because it affects the contour of the 8. Davies, C. )., and Garland, I. C. 1975. However, crack length measurements on weld. The cross-section of the weld in Solidification structures and properties of flat surfaces often vary widely, de Fig. 12 demonstrates that the contour of fusion welds. Int. Metal. Reviews 20:83-106. the cavity becomes more concave as the pending on the person making the mea 9. Savage, W. F. 1980. 1980 Houdremont cavities become deeper. As the cavity surements. In a deep sloping cavity, these lecture: solidification, segregation, and weld inconsistencies in measurements would becomes deeper, the surface tension imperfections. Welding in the World 18:2- be compounded. Because using the incip driven solidification stresses increase until 17. ient cracking depth was easier and more cracks are formed (Ref. 10). 10. Dudas, |. H. 1966. Preventing weld consistent, these other cracking indices cracks in high strength aluminum alloys. Weld were not used. ing lournal 45(6):241-s to 249-s. Conclusions 11. Baeslack 111, W. A. 1980. Observations Another reason for using the cavity of solidification cracking in Ti alloy weldments. depth as a means of rating weldability is From the above discussion, several Metallography 13:277-281. that the cavity depth influences the conclusions can be drawn: 12. Cieslak, M. |. 1984. Chi-phase formation restraint, which is a factor in hot cracking. 1. The laser beam weldability test can during solidification and cooling of CF-8M Restraint is the resistance to expansion evaluate the effect of either pulsed or weld metal. Welding Journal 63(4):133-s to 140-s. and contraction. In a laser beam weld, continuous wave laser beam welding on 13. Lundin, C. D., and Lingenfelter, A. C. relatively little heat is lost by thermal an alloy's cracking sensitivity. The Varestraint Test. WRC Bulletin 280. diffusion into the adjacent base metal 2. The laser beam weldability test can 14. Sponaugle, C. |. Fabrications Manager, while the pool is molten. Thus, thermal evaluate dissimilar alloy combinations Technical and Product Services Croup, Cabot expansion is minimized. The weld metal after pulsed laser beam welding. Corporation, Kokomo, Ind. Private correspon does, however, contract upon solidifica 3. The laser beam weldability test uses dence. 62-s I MARCH 1986