) technique and characterisation of EXW joints

VTT- VAL5'-Z.2o ¥ FI9900105

Development of Explosion Welding(EXW) Technique and

Characterisation of EXW Joints

Report VALB220

Seppo Tahiinen Uila Ehrnsten

Paivi Karjaiainen-Roikcmeri Pekka MoHanest

Pentti Kauppinen Pertti Auerkari Klaus Rahka

Espoo, Finland 30 May, 19973 0-24

MANUFACTURING TECHNOLOGY

A Work reportB Public report XC Confidential report

Title of reportDevelopment of Explosion Welding (EXW) Technique and Characterisation of EXWJointsClient/sponsor of project and orderTEKES -Euratom Association/FFUSION

Report No.VALB220

ProjectFFUSION / FUSION MATERIALS / NET/ITER TASK 212

Project No.V7SU00516

Author(s)Seppo Tahtinen, Ulla Ehrnsten, Paivi Karjalainen-Roikonen, Pekka Moilanen, Pentti Kauppinen, Pertti Auerkari, Klaus Rahka

No. of pages/appendices 30 p. + app. 9 p.

KeywordsExplosion welding, copper alloy, stainless steel, fusion materialSummary

The integrity of the EXW copper alloy stainless steel compound plates were examined by ultrasonic examination. The metallurgical structure and mechanical performance of the copper stainless steel joints were characterised using tensile, fatigue, creep, shear and fracture resistance tests at ambient and elevated temperatures.

The results indicate that explosion welding can be applied in manufacturing fully bonded copper stainless steel compound plates with adequate bond strength. In tensile and fatigue testing the failure mode of the bond specimens was primarily a ductile failure of copper alloy both at ambient and at elevated temperature indicating that the bond strength was higher than the strength of copper alloy. However, the failure mode in creep fatigue and creep testing at elevated temperatures changed to interfacial failure propagating along the bond interface.

The observation that specific loading conditions at elevated temperatures can induce a bond failure at low stress levels indicates that creep behaviour of copper alloy may have a major role in failure mechanisms of copper alloy stainless steel compound structures under ITER operation conditions. Creep behaviour of copper alloy and copper alloy stainless steel compound specimens and structures at elevated temperatures requires more detailed studies and experimental work.Date

Y?

Espoo 1997

Rauno Rintamaa Seppo Tahtinen Senior Research Scientist

iV/MChecked

Distribution:Client, 3 copiesVTT Manufacturing Technology/VAL6, 12 copies

VTT Manufacturing Technology Phone: +358 9 4561Materials and Structural Integrity Telefax: +358 9 456 7002, +358 9 456 5875P.O. Box 1704 E-mail: [email protected] VTT, Finland WWW: http://www.vtt.fi/manu/

Cu/SS and Cu/Be Joining Techniques, Development and Testing

Development of Explosion Welding (EXW) Technique and Characterisation of EXWJoints

G 16 TT 12 (EC)

NET/ITER TASK T212,FINAL REPORT, May 1997

Association EURATOM-TEKES

S. Tahtinen, U. Ehrnsten, P. Karjalainen-Roikonen, P. Moilanen P. Kauppinen, P. Auerkari, K. Rahka

VTT Manufacturing Technology P.O.Box 1704

FIN-02044 VTT Finland

1

Final Report Task T212Development of Explosion Welding (EXW) Techniqueand Characterisation of EXW Joints

G 16 TT 12 (EC)EURATOM - TEKES

May 1997

TABLE OF CONTENTS

EXECUTIVE SUMMARY 3INTRODUCTION 4EXPERIMENTAL 5

Materials 5Explosion welding 6Ultrasonic examination 6Preparation of test specimens 6Mechanical testing 8

RESULTS 9Ultrasonic examination 9Metallography and hardness 9Tensile properties 13Fatigue and creep fatigue properties 15Creep rupture properties 18Shear strength properties 18Fracture resistance properties 20

FIRST WALL MOCK-UP MANUFACTURING 21B ending experiments 21

DISCUSSION 23Metallurgy 23Mechanical properties 24Comparison of EXW and HIP joints 26

CONCLUSION AND RECOMMENDATIONS 29REFERENCES 30APPENDIX 1 31

2



May 1997

EXECUTIVE SUMMARY

Development work on joining techniques of Cu-alloy to stainless steel in European Home Team have focused on solid HIP, powder HIP and explosion welding (EXW) techniques using two types of copper alloys, namely CuAI25 andCuCrZr.

In the present study the explosion welding was applied to manufacture CuCrZr copper alloy 316 LN IG stainless steel compound plates. The compound plate dimensions were 140 mm by 1050 mm by 37 mm. The thickness of copper alloy plate was 30 mm and that of stainless steel was 7 mm. Some of the compound plates were further machined and four stainless steel cooling tubes were tightened in copper alloy part of the compound plate by using explosives inside the tubes. Some of the compound plates were further bend to an angle of 90 degrees.

The integrity of the compound plates were examined by ultrasonic examination. The metallurgical structure and mechanical performance of the copper stainless steel joints were characterised using tensile, fatigue, creep, shear and fracture resistance tests at ambient and elevated temperatures.

The results indicate that explosion welding can be applied in manufacturing fully bonded copper stainless steel compound plates with adequate bond strength. In tensile and fatigue testing the failure mode of the bond specimens was primarily a ductile failure of copper alloy both at ambient and at elevated temperature indicating that the bond strength was higher than the strength of copper alloy. However, the failure mode in creep fatigue and creep testing at elevated temperatures changed to interfacial failure propagating along the bond interface.

The observation that specific loading conditions at elevated temperatures can induce a bond failure at low stress levels indicates that creep behaviour of copper alloy may have a major role in failure mechanisms of copper alloy stainless steel compound structures under ITER operation conditions. Creep behaviour of copper alloy and copper alloy stainless steel compound specimens and structures at elevated temperatures requires more detailed studies and experimental work.

The results of this study are basically in accordance with studies carried out by other EU Associations using HIP techniques to bond CuCrZr alloys and 316LN stainless steel. However, there is still a lack of data for feasible comparison and selection of bonding method. Metallurgy and mechanical properties of EXW and HIP bonds are different due to basic differences in bonding methods. However, after post weld heat treatments EXW bonds have similar behaviour with HIP bonds. This observation is belived to give additional flexibility for design of blanket module manufacturing by making it possible to use explosion welded compound plates in manufacturing processes with subsequent heat treatments and HIP cycles.

3



May 1997

INTRODUCTION

The blanket is the innermost part of the ITER machine and directly exposed to the plasma. Its functions are to remove the surface heat flux from the plasma and the volumetric heating caused by the interaction of the fusion neutrons with the blanket materials. In addition to heat removal the blanket is also used to reduce the activity and radiation damage in the vacuum vessel structure material and to protect the superconducting coils from excessive nuclear heating and radiation damage.

The present ITER design is based on a modular structure where shield blanket will be mounted on a continuous back plate as a supporting structure. Each module consists of a shield block and a first wall which is an integral part of blanket module. Shield block together with first wall is a water cooled stainless steel structure where copper alloy is used as a heat sink covered with protection material to cope with the plasma interactions. The material selection is based on preselected materials as 316 LN IG for structural material, CuA125 IG for heat sink and Be for plasma facing material. For shield blanket module it is of outmost importance to apply proper joining and manufacturing techniques for copper to stainless steel and copper to plasma facing material to ensure high heat conduction properties and adequate mechanical integrity of these multimetal structures under ITER operating conditions.

Development work on joining techniques of copper alloy to stainless steel in European Home Team have focused on further development of solid HIP, powder HIP and explosion welding techniques using two types of copper alloys, CuA125 and CuCrZr.

In the present study the explosion welding was applied to manufacture CuCrZr copper alloy 316 LN IG stainless steel compound plates where stainless steel cooling tubes were further tightened in copper alloy matrix by using explosives inside the tubes. The integrity of the explosion welded joints were studied by ultrasonic examination and mechanical performance of the joints e.g. tensile, fatigue, creep and shear strength and fracture resistance were determined.

This work was part of NET/ITER Task T212 performed under the agreement of Euratom- TEKES by VTT Manufacturing Technology. Explosion welding operations and CuCrZr alloy plates were provided by High Speed Tech Oy and Outokumpu Poricopper Oy. Part of the results have been published in following meetings and conference papers (10-12)

4


G 16 TT 12 (EC)EURATOM - TERES

May 1997

EXPERIMENTAL

Materials

Materials used in this study were AISI 316 LN IG austenitic stainless steel supplied by Joint Research Centre Ispra and precipitation hardened CuCrlZr copper alloy supplied by Outokumpu Poricopper Oy. For explosion welded compound plate used in bending experiments also commercial AISI 316 steel plates were used. The steel plates were used in hot rolled conditions with a plate thickness of 3.5 mm and the copper alloy was used in solution annealed and precipitation heat treated condition e.g. without intermediate cold rolling procedure. The chemical analysis and material conditions are summarised in Tables 1-3.

Table 1 The materials used in EXW experiments.

Material / supplier

Product form Final heat treatment HardnessHV10

ElectricalconductivityIASC%

AISI 316 LNIG

JRC Ispra

Original plate size 2000x250x30 mm3 hot rolled to1150x210x3.5 mm3

1100 °C / 30 min 155-159

CuCrlZr

Outokumpu Poricopper Oy

hot rolled at 920 °C 1050x140x30 mm3

solution annealed at960 °C / 120 min, wq, precipitation annealed460 °C/ 120 min

130-140 70-75

Table 2. The chemical analysis of CuCrlZr copper alloy.

Material Cu Cr Zr Total of other elementsCuCrlZr base 0.78 0.13 <0.031

Table 3. The specification of AISI 316 LN IG.

Material Ni Cr Mo C N Si Mn P S316 LNIG 12-12.5 17-18 2.3-2.7 <0.03 0.06-0.08 <0.5 1.6-2 <0.035 <0.025

5



May 1997

Explosion welding

Explosion welding was performed by High Speed Tech Oy. For explosion welding (EXW) experiments the copper plate was used as a base plate and stainless steel plate as a flyer plate. Two subsequent explosion welding operations were applied to obtain a total thickness of 7 mm of stainless steel on top of 30 mm thick copper plate. Final dimensions of explosion welded copper stainless steel compound plates were 1050x140x37 mm3.

Ultrasonic examination

A C-mode scanning acoustic microscope operating at frequencies up to 100 MHz was used to study the integrity of the plate-to-plate EXW joints. The ultrasonic acoustic examination was performed with the sample immersed in water and the ultrasonic transducer was scanned in a X-Y-pattern over the sample. The echo amplitudes reflected back to the transducer from the sample were recorded and their intensity was displaced in false colours in a C-scan image. The image shows the location of structural discontinuities in the sample on the plane of the sample surface.

By adjusting the time-gate, the image can be generated from different depths and interfaces of the sample. Depending on the location of the time-gate the discontinuities on the bonding interfaces are recognised from higher or lower amplitudes within the gate. This is caused by the change of acoustic impedance occurring in the case of discontinuity. In most of the C- scans presented here the gate has been placed to cover the interfaces and low amplitudes in the gate indicate good quality of bonding.

Ultrasonic examination of the tube-to-plate interfaces were performed using the internal rotating inspection system (IRIS). Rotating mirror with ultrasonic transducer was inserted into the tubes and was manually guided through the tubes. The echo amplitudes reflected back to the transducer from the inner and outer circumference of the tube were detected on the oscilloscope screen and only the typical echo information was recorded by polaroid camera. The circumference of the tube was detected on the oscilloscope screen and if only inner surface was detected it indicated good quality of bonding.

Preparation of test specimens

Cross weld tensile specimens for mechanical testing were prepared by applying Electron Beam (EB) welding to add extra stainless steel material on specimen blanks. Tensile specimens were also taken parallel to the joint interface from both stainless steel and copper alloy. The cutting plan and specimen dimensions used in mechanical tests are shown in Figs.1-2.

6



May 1997

Propagation of explosive front

Figure 1. Cutting plan of test specimens used in mechanical testing.

Type a

80-120

TENSILEType b

Type c

CREEP

Typed

SHEAR 5x5x55

Type eFRACTURE 10x 10x55RESISTANCE 10% side grooved

sraace

Figure 2. Specimens used in mechanical testing.

7



May 1997

Mechanical testing

Mechanical properties of the composite copper-stainless steel specimens were determined both in as received and in post weld heat treated (PWHT) condition. PWHT was performed at 980°C for 180 minutes followed by air cooling and subsequent precipitation heat treatment at 460°C for 120 minutes.

Cross weld tensile tests were performed at room temperature and at 300°C using Instron 1185 testing machine. Most of the tensile tests of cross weld specimens were performed using a constant cross head displacement rate of 0.2 mm/min. Tensile tests of flat specimens taken parallel to joint interface were performed at room temperature with constant cross head displacement rate of 2 mm/min.

Low cycle fatigue test were performed at room temperature and at 300°C using 10 kN MTS universal testing machine. Fatigue tests were performed in strain control at a constant strain range of ± 0.4 % and ± 0.2 % with a cycle frequency of 60 seconds. Creep fatigue tests were conducted using the same experimental parameters but applying a 29 minutes hold at maximum tensile strain value. Axial strain to control the low cycle fatigue tests was measured using a strain gage extensiometer with quartz rod extension arms to contact the specimen gage length. Independent strain gauges were also used in some tests to measure strain in copper and stainless steel parts of the composite specimens during fatigue testing.

Constant load creep tests were performed at 300°C using constant load arrangement. During creep tests time to rupture were recorded.

The shear tests were performed using a servohydraulic 10 kN MTS universal testing machine, with the shear tooling shown in Fig. 3. Cross head displacement rate of 1 mm/min was used. The shear plane was aligned with the joint plane visually, and the resulting alignment was controlled after each test from the fracture surface. During the tests the loading force and test head displacement were recorded.

Fracture resistance curves of EXW joints were determined using beam specimens prefatigued to the a/W-ratio of 0.5 and side groowed 10% on both sides under three point bend testing at a constant cross head displacement rate of 3 x 10"4 mm/min. Load and displacement wererecorded during the test and crack growth was measured using direct current potential drop method (DC-PD).

8



May 1997

Sped man

Figure 3. Shear strength testing method.

RESULTS

Ultrasonic examination

C-scan images on plate-on-plate interfaces of EXW composite plates are shown in Appendix 1. Interfaces between both copper alloy to stainless steel and stainless steel to stainless steel are completely bonded and only some isolated discontinuities with an estimated diameter of 1-2 mm were detected on the bond interface. Specimens for mechanical testing were taken from the middle part of the EXW composite plate no. 1.

Metallography and hardness

The copper alloy stainless steel EXW joint interface had a typical wavy like appearance, Fig. 4. Both metals were heavily deformed near the joint interface and some small local areas were identified where stainless steel and copper alloy were mixed together. These areas composed of a mixture of copper and stainless steel and were typical interface wave vortices formed during explosion welding. No diffusion of alloying elements across the joint interface was observed.

Post weld heat treatment at 980°C for 180 minutes followed by air cooling and subsequent precipitation heat treatment at 460°C for 120 minutes resulted in recrystallization and grain growth of constituent metals. Also diffusion of alloying elements across the joint interface

9



May 1997

was observed. Extensive grain growth of copper alloy occurred within a distance of 3 mm from the joint interface.

In stainless steel side nickel was found to deplete and chromium together with molybdenum to enrich next to joint interface, Fig 5. Also copper was found to diffuse into stainless steel. The thickness of this nickel depleted zone was about 5 pm. This nickel depleted zone was also observed in optical and scanning electron micrographes and was identified as a ferrite phase by x-ray diffraction, Fig. 6.

In copper side nickel and iron diffused into the copper alloy. A narrow zone with different precipitation structure seemed to form within copper alloy next to the joint interface when compared to initial structure in EXW as received condition. Also zirconium precipitated at the original joint interface.

Hardness across EXW joint interface showed a general increase in hardness of both metals and a clear hardness gradient in stainless steel due to work hardening, Fig. 7. Hardness of the copper alloy was much less affected by EXW welding when compared with hardness of stainless steel. Heat treatment recovered the hardness gradient in stainless steel and resulted in slight hardness reduction in both stainless steel and copper alloy measured far from the joint interface. Ferrite phase formed at the EXW joint interface after post weld heat treatment showed higher microhardness values than austenite or copper alloy.

b)

Figure 4. Optical micrographs showing the explosion welded copper alloy stainless steel interface a) EXW as received and b) EXW after PWHT.

10



May 1997

c)Figure 5. SEM micrographs showing the joint interface structure and chemical composition measured by EDS across the joint interface a) EXW as received, b)EXW after PWHT and c) EDS analysis across the joint interface.

11



May 1997

ANGLE 20 ('

Figure 6. X-ray diffraction curve measured on the EXW joint interface.

350 -- EXW as received

300 -- ■S— EXW after PWHT316LNIG

200 --

150

d 200 --100interface CuCrlZr

-0.1 -0.05 0 0.05 0.1Distance, mm

-25 -20 -15 -10 -5 0 5 10 15 20 25DISTANCE FROM JOINT INTERFACE, (mm)

Figure 7. Hardness profdes measured across the EXW joint interface.

12



May 1997

Tensile properties

Tensile test results of base materials before and after EXW are summarised in Tables 4 - 6 and Fig 8. Hardening due to explosion welding was clearly seen in tensile properties of stainless steel where yield and tensile strength almost doubled from 320 MPa and 620 MPa to about 970 MPa and 1051 MPa, respectively. Elongation decreased from original value of 71% to about 13 % after explosion welding. Stainless steel plate next to copper joint interface showed somewhat higher tensile strength and lower elongation values compared to steel plate on top of that due to additional hardening induced by second explosion welding operation. Yield and tensile strength of copper alloy increased from 248 MPa and 371 MPa to about 420 MPa and 433 MPa, respectively. Elongation of copper alloy next to joint interface decreased from original value of 25 % to 13 % due to explosion welding operations.

PWHT reduced yield and tensile strength of both stainless steel and copper alloy. Yield strength of stainless steel decreased 322 MPa and tensile strength to 641 MPa. Yield strength of copper alloy decreased to 190 MPa and tensile strength to 314 MPa next to EXW joint interface.

Cross weld tensile test results are summarised in Table 7. Cross weld tensile testing invariably showed failure outside the joint interface, in the copper alloy, indicating that the actual interface strength was better than the softer base material.

The tensile strength of as received EXW cross weld specimens was reduced from 412 MPa to 311 MPa, when testing temperature increased from room temperature to 300 °C. This shift in testing temperature also reduced the observed reduction in area from Z = 50 to 55% to the range of Z = 35 - 45%. PWHT at 980°C/180 min + 460°C/120 min reduced the tensile strength about 20% to about 328 MPa at room temperature and to about 262 MPa at 300°C. The reduction in area remained approximately on the same level as for as received material, but elongation increased, apparently due to recovery of the high dislocation density of the copper alloy.

Table 4. Tensile properties of base materials 316 LNIG and CuCrlZr before explosion welding at room temperature.

Material Orientation Rp0.2

MPaRm

MPaA,% Z%

316 LN IG IL 324 619 71CuCrlZr TL 248 371 25 65

13



May 1997

Table 5. Tensile test results of base materials 316 LNIG and CuCrlZr after EXW in as received condition at room temperature.

Material Orientation Distance from joint interface,

mm

Rp0.2

MPaRm

MPaAs%

Z%

n

316 LN IG TL 2.5 970 1051 13 53 0.113CuCrlZr IL 2.5 420 433 13 80 0.051

Table 6. Tensile test results of base materials 316 LN IG and CuCrlZr after EXW and subsequent PWHT at 980°C for 3 hours followed by 460°C for 2 hours at room temperature.

Material Orientation Distance from joint interface,

mm

R0.2Mpa

RmMpa

A5%

Z%

n

316 LN IG TL 3.5 322 641 66 74 0.281CuCrlZr TL 3.5 194 312 30.5 68 0.231

Table 7.Ccross weld tensile test results of explosion welded (EXW) 316 LN IG stainless steel to copper CuCrlZr alloy composite specimens. T = temperature; dA = displacement rate in testing. Note that due to cross weld specimens, yield strength and elongation values are nominal only. In all cases the failure occurred on the side of the copper alloy.

T°C

dAmm/min

Rpo.iMPa

Rp0.2MPa

RpOSMPa

Rpi.oMPa

RmMPa

A5%

Z%

EXW as received

RT 1 353 375 403 409 411 13 53RT 1 325 353 392 408 413 12 55300 0.2 237 256 280 305 311 11 47300 0.01 290 309 312 312 312 9.5 42300 0.005 277 291 311 311 311 6 33

PWHT 980°C / 180 min + 460°C / 120 min

RT 0.2 188 198 216 237 334 21 57RT 0.2 183 195 211 229 321 16.5 46300 0.2 164 174 193 208 271 14.5 33300 0.2 163 173 182 195 252 16 37

14



May 1997

316 LNIG EXW as received

316 LNIG EXW after PWHT

CuCrlZr EXW as received800 - CuCrlZr EXW after PWHT

600 -

200 t

ENGINEERING STRAIN, %

Figure 8. Engineering stress versus engineering strain curves of 316 LN IG and CuCrlZr - alloy after explosion -welding in as received EXW and in PWHT conditions. Tensile specimens were taken parallel and next to EXW joint interface.

Fatigue and creep fatigue properties

The strength mismatch between stainless steel and much softer copper alloy was clearly seen in stress strain loops of continuous cycling of cross weld copper stainless specimens. During the first tension, 1/4 of fatigue cycle, copper yields at lower stress level than stainless steel and takes about 80% of the total strain when a total strain amplitude of ±0.4% was used in fatigue test. During continuous cycling with a total strain amplitude ±0.4%, over the gauge length with the joint in the middle, the copper part of the cross weld specimen experienced a strain amplitude of ±0.6% and the stainless steel part only about ±0.2% , Fig. 9.

Cyclic tests were performed with strain control in order to simulate the effect of rapidly changing temperature. Tests with or without hold times at maximum tensile strain at room temperature and 300°C were run. The stress amplitude versus number of cycles showed continuous softening for the as received material and initial hardening for the heat treated material. Cyclic life at room temperature was slightly decreased by heat treatment, Fig. 10.

Elevated temperature continuous cycling at 300°C was equally damaging for as received and heat treated materials. Heat treated material was much softer both at room and elevated temperature cycling. When prolonged exposure was used introducing a hold at peak strain, cycle life was drastically reduced and the failure occurred at the interface. Cycling with tension hold was more damaging for as received than for heat treated material, Fig. 11 and 12. Cyclic fatigue properties of copper stainless steel cross weld composite specimens are summarised in Table 8

15



May 1997

STRAIN, %

Figure 9 Typical stress strain behaviour of copper alloy stainless steel cross weld composite specimen in EXW after PWHT condition during continuous cycling. Strains of copper alloy and stainless steel were measured by independent strain gauges. Total strain amplitude is ±0.4%

□ EXW as received

OEXW + 980 C/180 min + 460C / 120 C400 -

300 -

200 -

10000

NUMBER OF CYCLES

Figure 10. Stress amplitude versus number of cycles of copper stainless steel cross weld composite specimens during continuous cycling with total strain amplitude of ±0.4 % at room temperature.

16



May 1997

D EXW as received

OEXW + 980C/ 180 min + 460 C/ 120 min

300 -

e = +- 0.2 %e = +- 0.4 %

10000

NUMBER OF CYCLES

Figure 11. Stress amplitude versus number of cycles of copper stainless steel cross weld composite specimens during continuous cycling with total strain amplitude of '±0.4% at 300 °C. Post weld heat treated specimens were tested also with total strain amplitude of ±0.2%

□ EXW as received

O EXW + 980 C /180 min + 460 C /120

10000

NUMBER OF CYCLES

Figure 12. Stress amplitude versus number of cycles of copper stainless steel cross weld composite specimens during cycling with tensile hold of 29 minutes at maximum strain at200 °C.

17



May 1997

Table 8. Summary of the cyclic fatigue and creep fatigue tests for copper stainless steel composite specimens.

Specimen Temperature

°C

cyclic mode Total strain amplitude

±%

Cycles to failure

Plastic strain range

%

Stressamplitude

MPa

EXW as received

V22 20 continuous 0.4 657 0.22 294.5V23 20 continuous 0.4 437 0.33 301.6V24 300 continuous 0.4 225 0.42 242.3V27 300 continuous 0.4 205 0.52 264.0V25 300 tensile hold 0.4 39 0.48 276.8V26 300 tensile hold 0.4 73 0.45 248.1

EXW + 980°C/180 min + 460°C/120 min

H34 20 continuous 0.4 285 0.46 241.8H35 20 continuous 0.4 721 0.43 235.4H31 300 continuous 0.2 1223 0.15 159.6H32 300 continuous 0.4 221 0.51 202.5H33 300 tensile hold 0.4 125 0.56 186.8

Creep rupture properties

The creep test results at low stresses show failure at the joint interface and time to failure that is clearly less than literature data (2-3) for the precipitation hardened CuCrlZr or stainless steel base materials Fig. 13. The as received EXW joints appear to have a higher creep strength than the corresponding PWHT joints.

Shear strength properties

Shear strength of as received EXW joint of copper stainless steel was about 360 MPa and that of the copper alloy 270 MPa. Heat treatment reduced the shear strength of EXW joint to 280 MPa and that of the copper alloy to 247 MPa. Shear strength results are summarised in Table 9.

18



May 1997

RA9612D

100 : ISO-data average for 316 SS at 550°C literature data for CuCrtZr at 300°C EXW as received at 300°C EXW after PWHT at 300°C

50 -

1000 10000

TIME TO RUPTURE (h)

100000

Figure 13. Creep strength of EXW copper alloy stainless steel composite specimens at 300°C in comparison with average creep strength of base material (refs 2-3).

Table 9. The results from shear testing of EXW joints at room temperature.

Shear strength, MPaSpecimen EXW joint CuCrlZr base Remarks

EXW as received5x5 mm 346.7 263.4 orientation S-T

371.7 265.9369.6 262.3332.3 273.7379.8 283.6

average 26P.&EXW+ 980°C/180 min + 460°C/120 min

5x5 mm 277.6 249.0 orientation S-T257.8 248.0288.3 248.9289.4 245.0283.9 245.0

average 27P.4 247.2

19



May 1997

Fracture resistance properties

The fracture resistance of the EXW joint after PWHT was clearly higher than that of the EXW joint in as received condition, Fig 14. The initiation values, J0.2/BL, for ductile crack growth were higher for EXW joints in the PWHT condition, 148 kJ/m2, than in the as received condition, 52 kJ/m2. Higher fracture resistance of PWHT joints compared to that of EXW joints in as received condition is also shown in the higher tearing resistance values, dJ/da, for PWHT joints, Table 10. In PWHT condition the fracture took place parallel and close to the joint interface in copper but in as received condition fracture seemed to deviate from interface and propagate into copper. The fracture mode in both conditions was fully ductile.

Table 10. Fracture resistance properties of copper stainless steel EXW joints.

J0.2/BL(kJ/m2)

dJ/da(kJ/m2/mm)

M Note

56 84 2.3 EXW as received53 80 2.3 EXW as received46 52 2.3 EXW as received130 133 1.7 after PWHT167 170 1.7 after PWHT

M = Strength mismatch, Ro.2(steel)/Ro.2(copper)

200 - EXW after PWHT

100 --

EXW as received

0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8CRACK EXTENSION, mm

Figure 14. Fracture resistance curves of EXW copper alloy to stainless steel joints. Characteristic fracture resistance values are summarised in Table 10.

20



May 1997

FIRST WALL MOCK-UP MANUFACTURING

Cooling tube insertion

Total of seven explosion welded copper alloy stainless steel composite plates No. 2-8 with dimensions of 1050x140x37 mm3 were additionally produced. In these compound plates commercial AISI 316 steel plates were used instate of 316 LN IG steel plates. All the compound plates were examined using C-scan ultrasonic inspection. Results of ultrasonic examinations are shown in Appendix 1.

Three of the copper stainless steel compound plates were further fabricated to insert a total number of four stainless steel cooling tubes into the compound plates. The surfaces of the compound plates were first machined and then four holes were drilled through the copper alloy along the longitudinal direction of the compound plates. Stainless steel tubes were inserted into the holes and were mechanically tightened using explosives inside the stainless steel tubes. Two of the compound plates with embedded cooling tubes are shown in Fig. 15.

Typical tube to plate images produced by IRIS method are shown in Appendix 1. Tube to plate interfaces can be identified from the IRIS images indicating that mechanical tightening due to explosives inside the tubes does not create a perfect interface between stainless steel tube and copper alloy plate.

Bending experiments

Preliminary experiments were performed for getting experience on bending behaviour of the copper alloy stainless steel compound plates. The experiments using a bending radius of 50 mm showed that there was no indications of debonding or lamination of the EXW joint interface. However cracking of copper alloy at the outer surface of the bend area was observed depending on actual condition of copper alloy stainless steel compound plate. No cracks were observed in solution annealed compound plate whereas cracking occurred if the compound plate was used in EXW as received condition. The cooling tubes were deformed at the bending area. Deformation of the cooling tubes was less savage for solution annealed compound plate. Examples of bend compound plates are shown in Fig. 16.

21



May 1997

Figure 15. Copper stainless steel compound plate after explosion welding operation and after machining and mechanical tightening of stainless steel cooling tubes. Explosion welding was performed by High Speed Tech Oy.

Figure 16. Copper alloy stainless steel compounds plates after bending operation showing the deformation of cooling tubes upper element is in EXW as received condition and lower element is EXW after solution annealing at 960° C for 30 minutes followed by water quenching.

22



May 1997

DISCUSSION

Metallurgy

Explosion welding is a solid state welding operation although local melting can occur at the interface area. Cast microstructures are possible to form in vortex areas where also cooling cavities can be observed. However, due to rapid cooling rate no diffusion of alloying elements across the joint interface was observed after explosion welding.

EXW joint interface showed a typical wavy like appearance with front and rear vortices formed in association with each wave as expected for joining materials with similar density. The vortices at the interface contained a mixture of both component materials. Rear vortices were mainly composed of stainless steel from flyer plate and front vortices were mainly composed of copper alloy from the base material. These observation were in accordance with the models proposed for explosion wave formation mechanism (1,4).

Overall hardness increase with an increase in yield and tensile strength and a reduction in ductility was observed although visible grain distortion was limited to a distance of 0.1 mm from the EXW joint interface. The presence of high residual concentration of point defects and dislocations is expected to contribute to an observed increase in hardness and reduction in ductility.

Recrystallization of copper alloy due to PWHT indicated that explosion welding induced a deformation or defect gradient along the thickness of base plate copper alloy. Relatively small grains were present in area of large grain distortion whereas extensive grain growth was observed within a distance of 3 mm from the EXW joint interface. This kind of recrystallization behaviour is in accordance with the general correlation between degree of deformation and annealing temperature. Obviously a critical strain level or critical amount of defects or specific defect structure were generated due to explosion welding within a distance of 3 mm from the EXW joint interface although no visible grain distortion was observed.

PWHT induced diffusion of alloying elements across the EXW joint interface. Diffusion of alloying elements is expected to be further enhanced by plastic deformation and specific defect structure of the constituent materials next to EXW joint interface. Main diffusing elements from stainless steel to copper were nickel and iron and elements diffusing into opposite direction were copper and possibly chromium. Also some zirconium precipitates were found at the joint interface. Depletion of nickel from stainless steel is believed to enhance the austenite to ferrite phase transformation. The composition of austenite and ferrite phases are in accordance with phase stability diagram of ternary Fe-Cr-Ni - system.

23



May 1997

Mechanical properties

Explosion welding altered the mechanical properties of the component metals. Increase in hardness and tensile strength and corresponding reduction in ductility with little gross deformation and no visible grain distortion can be explained by dislocation and defect substructure generated by explosion welding. In particular the loss of work hardening ability is expected to result when the dislocation substructure rearrange to a more stable substructure during tensile deformation without generating new dislocations. PWHT simulating the HIP joining cycle of copper to stainless steel recovered the dislocation substructure and the tensile properties of component metals.

Due to strength mismatch and different strain hardening properties of copper alloy and stainless steel the tensile tests using cross weld specimens reveals information mainly on the tensile properties of softer material, in this case of the copper alloy. This is clearly seen when comparing the tensile results summarised in Tables 4-6.

To estimate the fatigue life of copper stainless steel composite specimens the Coffin exponent c in the equation A£pc N = C was obtained from the 300°C continuous cycling data, Fig. 17. Coffin exponent c = 0.6 is a reasonable value but without statistical reliability for the time being. The effect of hold time during cycling is to reduce the number of cycles to failure. The as received material was more adversely affected by hold time than the heat treated material. Post weld heat treatment appeared to prolong the cyclic life. This may be due to changes in dislocation and defect structure, reduction of residual stresses and local mismatch at joint area. Further test data are needed at lower strain amplitudes and hold times at elevated temperatures.

Tensile tests and continuous fatigue cycling tests with cross weld specimens at room temperature and 300°C resulted in ductile failure of copper alloy. However, at elevated temperature with tensile hold at peak strain during cyclic fatigue test or in a constant load creep test, the resulting failure mode changed from ductile failure of copper alloy at high stresses (short testing times) to interface failure at low stresses (longer testing times), Fig. 18. Experience and FEM analysis of creep loaded dissimilar and other welds show that the stress redistribution between different materials sections of cross-weld specimens at sufficiently long times of exposure will introduce a stress peak close to the interface (5, 9). This is essentially why the subsequent creep failure will occur close to the interface. At shorter term testing there is insufficient time for the stress redistribution to evolve and then the failure is more likely in the weaker base metal.

The shear strength of the post weld heat treated EXW joint was reduced compared to the shear strength of the as received EXW joint. This shear strength reduction due to post weld heat treatment may be caused by the softening of the base materials.

The results of the fracture resistance tests were in a good accordance with the results of the tensile tests. The higher elongation, the lower yield strength and the higher strain hardening coefficient of the copper after PWHT compared to the corresponding values in as received

24



May 1997

EXW as received EXW + 980 C /180 min + 460 0/120 min

O20C, continuous cycling #200 continuous cycling

10 -r- □3000, continuous cycling ■ 3000, continuous cycling

; A300C, cycling with tensile hold A 3000, cycling with tensile hold

100 1000 NUMBER OF REVERSALS TO FAILURE, 2N(f)

10000

Figure 17. Plastic strain amplitude versus critical number of reversals to failure of copper stainless steel composite specimens.

Test type TENSILE CYCLIC FATIGUE CREEP FATIGUE CREEP

Fracturepath

Cu

SS

Cu

n

Cu Interface Interface

WvyvH

zww\

Temperature 20°C,300°C 20°C, 300°C 300°C 300°C

Strain rate 3*10"5 -6.6*10"4 1/s 1.3-2.7*1 CF 1/s 2.7 X10"4 1/s tensile hold 29 min

5*1 0"7 -3*1 (J8 1/s

ST962D

Figure 18. Typical fracture behaviour of explosion welded cross weld tensile test specimens. Comparison of EXW and HIP joint properties

25



May 1997

condition indicates higher ductility. This means higher fracture resistance for the joints in PWHT condition as more energy is consumed for plastic work. Strength mismatch is expected to affect the fracture propagation e.g. when strength mismatch was low fracture propagated parallel and close to the joint interface and when strength mismatch was large fracture seemed to deviate from joint interface into the softer material.

Comparison of EXW and HIP joints

Preliminary comparison of EXW and HIP joints of CuCrZr alloy and 316 LN IG stainless steel indicate that there is major differences in metallurgy of the joints due to differences in joining methods.

• EXW is basically a low temperature and HIP is a high temperature joining method.

• there is practically no diffusion of alloying elements across EXW joint interface where as the formation of HIP joint is based on diffusion.

• mechanical properties of component metals are changed due to dislocation and defect structure generated by EXW operation whereas no other than temperature induced changes are expected due to HIP cycle.

• post weld heat treatment of EXW joint recovers mechanical properties of component metals and induces diffusion of alloying elements across the EXW joint interface resulting in basically similar joint metallurgy with HIP joints e.g. ferrite phase is formed at the joint interface between CuCrZr alloy and 316 LN IG stainless steel

In the present study EXW joints were further HIP treated in order to ascertain that the mechanical properties of EXW joints are not degraded if the EXW copper stainless steel compound plates will be used in manufacturing process where additional HIP cycles are required.

Shear strength and hardness of explosion welded joints after PWHT and after additional HIP cycle were compared and only a minor differences were observed, Figs 19 and 20. Highest shear strength value of EXW joint was observed in as received condition and all the subsequent heat treatments decreased the shear strength of the EXW joint. Somewhat higher shear strength values observed after HIP treatments compared with PWHT treatment can be explained by different final heat treatments, e.g. water quenching after solution annealing favours optimum precipitation behaviour of copper alloy. Similar behaviour is also observed in measured hardness values. General increase in hardness was observed for copper alloy after explosion welding and hardness values after PWHT, air cooling after solution anneal, were lower than after HIP cycles.

These results are in accordance with earlier findings where about 80 % of tensile properties of CuCrZr alloy were regained after additional heat treatment when compared with properties

26



May 1997

of prime aged alloy. Reference for prime age condition is solution annealing followed by water quenching and subsequent precipitation annealing (8).

There is only limited amount data available for direct comparison of EXW and HIP joint properties of CuCrZr and 316 LN IG stainless steel. HIP joints produced and tested by CEA (6) show about 15% higher tensile strength values when compared with the tensile strength of the EXW joints in PWHT condition, Fig 21. This difference in tensile strength can be explained by differences in post weld heat treatments. The fracture behaviour of the joints was also different. For EXW joints the fracture behaviour in tensile tests was always ductile fracture of copper alloy whereas HIP joints produced by CEA showed interface fracture at elevated temperature. An interlayer of FeNi42 alloy was used in HIP joints studied by CEA. Joints produced by single step powder HIP cycle were studied by Studsvik and these joints show lowest tensile strength values (7). The highest tensile strength is observed for EXW joints in as received condition.

400

co 375CL

350X

o 325

£ 300 t—w 275 tr3 250 Xco 225

200c E E c E E c E E]o E *o E ;o E E E E5 a is a § e 8 t2- 8X X X 3 X Z3LU

oLLI 3o

LU O 3o m O =3o

EXW-as received

EXWPWHT

Figure 19. Comparison of shear strength of EXW joint and copper alloy after different heat treatments e.g. EXW as received (no heat treatment), EXW PWHT (980°C for 180 min + air cooling + 460°C for 120 min), EXW HIP 1000°C HT (HIP at 1000°C / 90 MPa for 180 min followed by 960°C. for 60 min + water quench + 460°C for 120 min) EXW HIP 900°C HT (HIP at 900 °C / 90 MPa for 180 min followed by 960°C. for 60 min + water quench + 460°C for 120 min). Copper samples were taken at 5 mm and 20 mm from the EXW joint interface.

27



May 1997

-A— EXW as received

350 -- EXW after PWHT

EXW after HIP/1000oC and HTEXW after HIP/900oC and HT

250 -

200 "

150 -

DISTANCE FROM JOINT INTERFACE, mm

Figure 20. Comparison of hardness values measured across the EXW joint interface after different heat treatments.

cl 400

X 350<3 300cc 250

150LLI 100

TEMPERATURE, °C

—EXW Rm VTT

—A—HIP PWHT Rm CEA

-O—EXW PWHT Rm VTT

—6—HIP (powder) Rm STUD

Figure 21. Comparison of tensile strength of EXW and HIP joints of CuCrZr and 216 stainless steel. Joining condition are following, HIP by CEA (920°C /120 MPa / 60 min followed by PWHT 990°C / 60 min / water quench / 480°C / 240 min, note an interlayer of FeNi42 alloy). HIP by Studsvik (1035°C /140 MPa/240 min). EXW by VTT (EXW as received or followed by PWHT 980°C / 180 min / air cooling / 460°C / 120 min).

28



May 1997

CONCLUSION AND RECOMMENDATIONS

Explosion welding can be applied to produce fully bonded copper alloy stainless steel panels with good bond properties.

Metallurgy and mechanical properties of EXW and HIP bonds are different due to basic differences in bonding methods. However, after post weld heat treatment EXW bonds have similar metallurgy and mechanical properties e.g. shear strength with HIP bonds.

Failure mode of the EXW bond specimens in tensile and fatigue tests was primarily a ductile failure of copper alloy. However, failure mode in creep fatigue and creep tests at elevated temperature changed to interfacial failure along the bond interface.

Creep behaviour of copper alloy and copper alloy stainless steel compound specimens and structures at elevated temperatures requires more detailed studies and experimental work.

Presently, there is still a lack of data for feasible comparison and selection of bonding method between EXW and HIP for copper alloys to stainless bonding. However, as both bonding methods are known to produce good quality bonds the selection can be based on manufacturing requirements e.g., dimensions and geometry of components.

Flexibility in first wall mock-up manufacturing is offered by the observation that the properties of EXW bonds are not destroyed by subsequent heat treatments or HIP cycles.

29



May 1997

REFERENCES

1. BLAZYNSKY, T. Z. (1983). Explosive Welding, Forming and Compaction, Applied Science Publishers, London.

2. BAUKLOH, A., DREFAHL, K„ HEUBNER, U. & RUHLE, M. (1976). Zeitstanduntersuchungen an niedrig- und unlegierten kupferwerkstoffen. Metall., 30, p. 19-28.

3. BUTTERWORTH, C.J. & FORTY, C.B.A. (1992). A survey of the properties of copper alloys for the use as fusion reactor materials. J. of Nuclear Materials, 189, p. 237-276.

4. CROSSLAND (1982). Explosive Welding of Metals and Its Application, Clarendon Press, Oxford.

5. DE WITTE, M. & COUSSEMENT, C. (1992). Creep behavior of2.25CrlMo base material and weldments. VGB-Conf. “Residual Service Life 1992”, Mannheim, July 6 - 7, Vol. 2, p. 38.1-38.28.

6. GENTZBITTEL, J. M. (1996). T212 progress report, EU/JCT Joint meeting on Progress on ITER R&D Tasks T8, T212 and T213, February 8-9, Grenoble, France

7. LIND, A. and TEGMAN, R. (1995). Fabrication, examination of microstructure and mechanical testing of a Cu(Cr,Zr)-powder alloy joined to 316 LNIG and 316LN stainless steels by a one-step hot isostatic pressing (HIP) technique. Studsvik report, STUDSVIK/M-95/90-IVF 95/18.

8. SINGH, B. N. (1996). Results of Screening Experiments and Low Cycle fatigue testing of Copper and Copper Alloys. EU/JCT Joint meeting on Progress on ITER R&D Tasks T8, T212 and T213, February 8-9, Grenoble, France

9. TOWNLEY, C. H. A., GOODALL, I. W. & LEWIS, D. J. (1972). Dari#, criteriayhr weWingpressureparts, Proceedings of an International Conference on Welding Research related to Power Plant, September 17 - 21, Southampton. CEGB, Southampton, UK, p. 44 - 50.

10. TAHTINEN, S. (1996). T212progress report, EU/JCT Joint meeting on Progress on ITER R&D Tasks T8, T212 and T213, February 8-9, Grenoble, France

11. TAHTINEN, S., KAUPPINEN, P„ RAHKA, K. and AUERKARI, P. (1996). Performance of copper- stainless steel EXW welds, Proceedings of the 2nd International Symposium on Mis-Matching of Welds, April 24-26, Reinstorf-Luneburg, Germany. To be published.

12. TAHTINEN, S„ MOILANEN, P„ KARJALAINEN-ROIKONEN, P. and ERNSTEN, U. (1996). Characterisation of copper-stainless steel EXW joints, Proceedings of the 19 th Symposium on Fusion technology, September 16-20, Lisbon Portugal. To be published.

30



May 1997

APPENDIX 1 1/9

C-SAM image on copper stainless steel joint interface, plate No. 1.

31



May 1997

APPENDIX 1 2/9


32



May 1997

APPENDIX 1 3/9


33


G 16 TT 12 (EC) EURATQM - TEKES

May 1997


34



May 1997


35



May 1997

APPENDIX 1 6/9


36


G 16 TT 12 (EC) ETJRATOM - TEKES

May 1997

APPENDIX 1 7/9


37



May 1997

APPENDIX 1 8/9

C-S AM image on copper stainless steel joint interface, plate No. 8

38

APPENDIX 1 9/9

Final Report Task T212 G 16 TT 12 (EC)Development of Explosion Welding (EXW) Technique EURATOM - TEKESand Characterisation of EXW Joints May 1997

Typical oscilloscope images of IRIS ultrasonic examination on stainless steel tube copper interface a) explosion tightened tube, note: the outer surface of the tube is visible and b) HIP bonded tube, note: no reflections from the interface.

39

Documents

) technique and characterisation of EXW joints