Jul 1, 2013 - Posiva has welded series of plate welding experiments at Patria Aviation Facilities. At. Tampere University of Technology, Department of ...
Working Report 2013-14
Metallographic Studies of Electron Beam Welded Copper Plates: EBSD Studies of the Cross-Sections and Determination of EBSD Reference Curves by EB-Welded Tensile Test Samples Taru Karhula Tampere University of Technology
July 2013
Working Reports contain information on work in progress or pending completion.
The conclusions and viewpoints presented in the report are those of author(s) and do not necessarily coincide with those of Posiva.
ABSTRACT This work is part of Posiva’s spent nuclear fuel disposal canister sealing development. Posiva has welded series of plate welding experiments at Patria Aviation Facilities. At Tampere University of Technology, Department of Materials Science (TUT DMS) metallographic and electron microscopy studies of electron beam welded copper samples have been carried out. In this report a part of the welding test program is analyzed. The results of the crosssections of the test welds X436-X440 and X453-X458 are presented here together with the methods used. These two sets of welds were conducted to study the effects of welding speed, annealing temperature and the presence of cosmetic pass. The aim of this study was to estimate the residual stresses present in the EB-welds using electron backscatter diffraction (EBSD) technique. For this task various EBSD reference curves with tensile test samples were constructed: the recrystallized, substructured and deformed fractions of grains, the occurrence of 1.5º and 2.5º misorientations, the average of mean intra-grain misorientation AMIS, and the hardness could be related to the applied strain. It was found that the hardness was higher if there was a cosmetic pass on the weld. The welding speed and the annealing temperature did not seem to have a noticeable effect on the measured properties. The estimated residual stresses were mainly very low, in the range 27-34 MPa. In the values estimated based on the hardness reference curve, the maximum residual stress was 58.7 MPa (in X455A). Keywords: EBW, electron beam welding, copper, vertical plate on flat position, EBSD, electron backscatter diffraction, residual stress.
Elektronisuihkuhitsattujen kuparilevyjen metallografiset tutkimukset: Hitsien poikkileikkausten EBSD-tutkimukset ja EBSD:n referenssikäyrien määritys EB-hitsatuilla vetokoenäytteillä TIIVISTELMÄ Tämä työ on osa Posivan käytetyn ydinpolttoaineen kapselin sulkemiskehitystyötä. Posiva on hitsannut levykoesarjoja Patria Aviationin tehtaalla Nokian Linnavuoressa. Tampereen teknillisen yliopiston Materiaaliopin laitoksella (TTY MOL) on tutkittu elektronisuihkuhitsattuja kuparisia näytteitä metallografisin ja elektronimikroskopian menetelmin. Tämän raportin yhteydessä on analysoitu vain osa hitsauskoeohjelman hitseistä. Tässä raportissa esitellään hitsien X436-X440 ja X453-X458 poikkileikkausten tulokset sekä tutkimuksissa käytetyt menetelmät. Näiden kahden hitsauskoesarjan tutkittavina parametreina olivat hitsausnopeus, hehkutuslämpötila ja kosmeettinen hitsipalko. Tämän tutkimuksen perimmäinen tarkoitus oli arvioida jäännösjännityksen suuruutta elektronisuihkuhitsatussa levyssä käyttäen EBSD-tekniikkaa. Tätä varten laadittiin vetokoenäytteiden EBSD-datan avulla vertailukäyrät: rekristallisoituneiden, alirakenteisten ja deformoituneiden rakeiden osuudet, 1.5° ja 2.5° misorientaatioiden esiintymät, keskiarvo rakeiden sisäisestä keskimääräisestä misorientaatiosta sekä kovuus näyttivät olevan riippuvaisia näytteen venymästä. Kokeiden tuloksista huomattiin, että hitsissä, jossa on kosmeettinen hitsipalko varsinaisen liitoshitsin päällä, oli suurempi kovuus kuin hitsissä ilman kosmeettista palkoa. Hitsausnopeus ja hehkutuslämpötila eivät näyttäneet vaikuttavan tutkittuihin ominaisuuksiin. Hitsien arvioidut jäännösjännitykset olivat pääosin varsin alhaisia, luokkaa 27-34 MPa. Kovuustulosten mukaan laaditun vertailukäyrän perusteella suurin jäännösjännitys oli 58.7 MPa (hitsissä X455A). Avainsanat: EBW, elektronisuihkuhitsaus, kupari, pystylevy, kapselin hitsaus, EBSD, jäännösjännitys.
PREFACE This report is one part of the residual stress evaluation of the EB-welded nuclear waste canister made of copper. Target of the residual stress evaluation is to assess level of the residuals stresses of the weld and also effect of the welding parameters and stress relief on residual stresses. Evaluation can be divided on following parts: -
Evaluation deformations and residual stresses on EB-welds using numerical modeling (finite element method, FEM) Residual stress measurements of the EB-welds using different measurement methods Destructive testing of the weld to assess residual stresses
Effect of the residual stresses on microstructure using destructive testing is reported in this report. Also effect of the stress relief annealing and welding parameters on microstructure is evaluated using destructive testing.
1
TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ PREFACE LIST OF SYMBOLS AND ABBREVIATIONS ................................................................. 3 1
INTRODUCTION .................................................................................................... 5
2
DETERMINATION OF THE EBSD REFERENCE CURVE .................................... 7
3
4
5
2.1
Preparations of the tensile samples ................................................................ 7
2.2
Tensile tests .................................................................................................... 8
2.3
EBSD studies of the tensile test samples ..................................................... 10
2.4
Summary of the EBSD results for the tensile tests ....................................... 12
2.5
Hardness measurements of the tensile test samples ................................... 14
2.6
Verification of the reference curves with the confirmation test ...................... 15
2.7
The obtained reference curves ..................................................................... 18
STUDIES OF THE EB-WELDED PLATES ........................................................... 21 3.1
Identification of samples ............................................................................... 21
3.2
Welding parameters ...................................................................................... 22
3.3
Sample preparation for macroscopic examination ........................................ 23
3.4
EBSD studies of the cross-sections .............................................................. 23
3.5
Summary of the EBSD results of the cross-sections .................................... 23
3.6
Hardness measurements of the cross-sections ............................................ 24
DISCUSSION........................................................................................................ 27 4.1
The effect of welding speed .......................................................................... 27
4.2
The effect of annealing temperature ............................................................. 30
4.3
The effect of cosmetic pass and annealing ................................................... 33
4.4
The effect of cosmetic pass and welding speed ........................................... 34
4.5
Estimated residual stresses in the cross-sections ........................................ 36
CONCLUSIONS.................................................................................................... 39
REFERENCES ............................................................................................................. 41 APPENDICES............................................................................................................... 43
APPENDIX U on DVD
2
3
LIST OF SYMBOLS AND ABBREVIATIONS A
breadth of cross-sectional sample
[mm]
AA
working distance
[mm]
AF
focal length
[mm]
ACX
oscillation input in x-direction for EB-machine
ACY
oscillation input in y-direction for EB-machine
AMIS
average of the mean intra-grain misorientation
av(G)
average of grades
av(s)
average of penetrations
[mm]
α
angle of groove
[º]
β
angle of slot
[º]
B
measure from the weld centerline to lid side of cross-sectional sample [mm]
BSE
backscattered electron
C
distance from inner surface of tube to inner surface of lid
[mm]
D
distance from inner surface of tube to outer surface of tube
[mm]
Delta
the highest minus the lowest average for each factor
E
distance from weld centerline to inner surface of tube
EBSD
electron backscatter diffraction
EBSP
electron backscatter diffraction pattern
εeng,pl
plastic engineering strain
[mm/mm]
εeng,tot
total engineering strain
[mm/mm]
εtr,pl
plastic true strain
[mm/mm]
F
distance from outer surface of lid to inner surface of lid
[mm]
[º]
[mm]
FESEM Field Emission SEM h
height of an imperfection
[mm]
h1
height of a spike, h1 = S – s
[mm]
hmax
maximum height of welding imperfections in certain welding distance [mm]
HAZ
heat affected zone
4 IB
beam current
[mA]
Ifoc
focus current i.e. lens current
[mA]
L
width of slot
[mm]
L osc
longitudinal oscillation measure
[mm]
MAD
mean angular deviation
s
penetration depth without spiking
[mm]
sintact
clean penetration i.e. minimum intact fusion zone thickness
[mm]
smin
penetration minimum in certain welding distance
[mm]
smax
penetration maximum in certain welding distance
[mm]
Δs
variation in penetration depth in certain welding distance
[mm]
S
penetration depth to the tip of a spike
[mm]
Smax
maximum penetration depth (to the tip of a spike) across the welding distance [mm]
SE Fit
standard error of fits
SEM
scanning electron microscope
standard deviation
stress
[MPa]
T osc
transversal oscillation measure
[mm]
UB
accelerating voltage
[kV]
v
welding speed
[mm/s]
x
axis in the direction of welding
y
axis in the direction of the side of welding
z
axis in the direction of the weld penetration
w
width of weld or width of an imperfection
[mm]
5
1
INTRODUCTION
This report contains the methods used at Tampere University of Technology, Department of Materials Science (TUT DMS), in metallographic and electron microscopy studies of electron beam welded copper samples welded by Posiva at Patria Aviation Facilities. The aim of the work was to evaluate the residual stresses in electron beam welded plate samples using electron backscatter diffraction (EBSD). To obtain true data for comparing the results with, a reference curve with tensile tests of known strains was first constructed. Also one tensile test was carried out without revealing the actual strain to the writer until all the tests were carried out: the purpose of this was to act as a confirmation test for the reference curve. Additionally a reference curve based on hardness of the tensile samples was generated. The samples, where the residual stresses were to be estimated, were divided into 2 sets of welds: the 1st set (welds X436-X440) for studying the effect of welding speed and cosmetic pass and the 2nd set (welds X453-X458) for studying the effect of annealing temperature and cosmetic pass. The results were partly examined by the means of statistical design of experiment (DOE).
6
7
2
DETERMINATION OF THE EBSD REFERENCE CURVE
The reference curve for EBSD measurements with different plastic strain conditions was constructed using electron beam welded samples. The reason for selection of welded samples over base material samples was the correspondence to the cross-section samples later to be evaluated. The values obtained from the EBSD studies might be somewhat affected by e.g. the large difference in grain size. 2.1
Preparations of the tensile samples
The tensile test samples were taken from the plate welding experiments X438, X453 and X458. The tensile test specimens were cut by wire electric discharge machining WEDM. The drawings of the samples are presented in Appendix A. The samples were lightly etched with technical grade nitric acid HNO3. The macrostructure of the tensile samples is shown in the images in Appendix B. The welding was carried out in a medium vacuum (10−2 - 10−3mbar) using a vertical electron beam. The welding speed v was 2.5 mm/s, the beam current IB was 300 mA and the accelerating voltage UB was 150 kV. The oscillation pattern was E32 using the relative set values of ACX 4.2 and ACY 3.8. The lens current IL was 2395 mA. The slope up started in x=30 mm and the slope down started in x=320 mm. The welded material was phosphorus micro-alloyed oxygen-free copper Cu−OFP with P= 30−70 ppm, O < 5 ppm, H < 0.6 ppm and S < 8 ppm. The tensile test samples were annealed at 600°C for 2 hours and they were left to cool down with the furnace. The annealing was carried out in order to bring the tensile test samples to unstrained condition. To estimate the effect of annealing on the grain size, a grain size study with a smaller specimen was carried out at TUT DMS. This small EBwelded sample was annealed at TUT DMS at 600 °C for 1 hour and cooled down with the furnace. The grain size was determined using the intercept procedure according to the standard SFS-EN ISO 2624:1995 before and after annealing from the same sample. The grain size was measured with calculating the intersecting grains on 14-15 defined lines in 2 locations. The images of the macro- and microstructure of this sample before and after annealing are shown in Appendix C. The average grain size before annealing was 297.7 µm (with 56.72 µm standard deviation) and after annealing 281.6 µm (with 50.62 µm standard deviation). According to this, no change in grain size due to annealing can be seen. A non-destructive testing (NDT) with penetrant testing was carried out for each tensile test sample. The purpose was to eliminate such samples in which there might be welding imperfections affecting the result. In the samples X453L1 (tensile test 09), X453L2 (tensile test sample used for preliminary testing of the setup), X458A1 (tensile test 06), X458A3 (tensile test 13), X458L3 (tensile test 10) a very small reddish area could be seen, but these were considered either due to an insufficient cleaning or too small to have a measurable effect on the tensile test results. As an example two of these indications are shown in Figure 1.
8
Figure 1. Samples X453L1 (upper) and X453L2 (lower) after penetrant testing. Small indications are circled. 2.2
Tensile tests
The tensile tests were carried out using the hydraulic testing machine MTS 810 TestStar with 100 kN load cell and 25 mm gauge length extensometer. The straining speed was set to 10-3 s-1, except in the tests 11 and 12 the speed was 5 mm/min. The chosen end levels of strain are shown in Table 1 together with the corresponding weld identification. Due to the limitations of the extensometer the tests 11 and 12 were performed until a predetermined length was traveled. The resulting plastic strain was then measured from the ruling made to the samples before testing. The test number 13 was a blind test; the writer did not know the end level until all the EBSD and hardness studies of the tensile samples had been carried out. This was made as a sort of a confirmation test for the reference curve. The extensometer measures the total strain introduced in the sample. Total strain is the sum of elastic and plastic strains, see Equation 1. The elastic strain can be determined using Hooke’s law, Equation 2. In order to have samples with the target plastic strains shown in Table 1, the needed total strain had to be determined and thus, an extra test was conducted to measure the elastic modulus E of the material. The test X458L2 was carried out with 10 intermediate reductions of the load to 1 kN around each target strain. The true stress vs. true strain curve and the figures showing each reduction with a linear fit to obtain the elastic modulus are presented in Appendix D. As a summary of these measurements the elastic moduli as a function of strain is shown in Figure 2. The average of the elastic moduli Eaver=100.9 GPa was used in the determination of the total strain.
9
(Equation 1) (Equation 2)
Table 1. The set end levels of strain in tensile tests. Tensile test 00 01 02 03 04 05 06 07 08 09 10 11 12 13
Eng. strain, total [%] 0 1.04 1.38 1.83 2.41 3.20 4.25 5.64 7.49 9.95 13.2 ~20 ~25 Xeng
Eng. strain, plastic [%] 0 1.00 1.33 1.77 2.35 3.13 4.16 5.53 7.36 9.79 13.0 19.4 23.9 Xeng,pl
True strain, plastic [mm/mm] 0 0.00995 0.01321 0.01755 0.02323 0.03082 0.04076 0.05383 0.07102 0.0934 0.1224 0.1773 0.2143 Xtr,pl
Sample X453A2 X438A3 X458L1 X438A1 X458A2 X453L3 X458A1 X453A3 X453A1 X453L1 X458L3 X453A4 X458A4 X458A3
10
Figure 2. The elastic moduli as a function of strain determined from the tensile test X458L2 with intermediate reductions of load to 1 kN.
The true stress vs. true strain curves for the tensile test samples 01-13 are shown in Appendix D. 2.3
EBSD studies of the tensile test samples
Electron backscatter diffraction (EBSD) measurements were performed using Zeiss ULTRAplus UHR FESEM microscope with the HKL Premium-F Channel EBSD ultrafast Nordlys F400 detector provided by Oxford Instruments, see Figure 3. EBSD was first discovered nearly a hundred years ago, but a true utilization has been made in the last two decades mainly because of the development of fully automated EBSD systems [1]. EBSD technique provides crystallographic information (orientation maps, orientation relationships between two phases, grain boundary disorientations, etc.). Briefly, the principle of this technique is to exploit information given by backscattered electrons from the specimen. Diffracted electrons are collected on a phosphorus screen where they form Kikuchi pseudo-bands. From the Kikuchi patterns, the corresponding orientations of the crystal lattice are calculated automatically using the HKL Channel 5 software. [2]
11
Figure 3. Zeiss ULTRAplus UHR FESEM microscope with the HKL Premium-F Channel EBSD ultrafast Nordlys F400 detector. From the tensile test samples an EBSD sample was prepared from the center of the parallel length. The studied surface was the cross-sectional surface. EBSD analysis requires specific conditions for the preparation of the surface specimens. To obtain a good quality EBSD pattern, residual deformation or stress due to mechanical polishing must be removed [3]. As a consequence, the sample preparation presents many stages. First the samples were mechanically wet ground with silicon carbide papers from P320 to P600. Polishing was carried out with 9 μm and 3 μm diamond suspensions. Final polishing was performed with colloidal silica particles. Between the stages the samples were carefully washed with water and cleaning agent. Finally the samples were cleaned in an ultrasonic bath for 10 min in distilled water with cleaning agent and for 10 min in ethanol. The samples were stored in desiccator. Just before EBSD studies the samples were cleaned in plasma cleaner. EBSD was carried out as matrix of three maps covering the whole fusion area and some base material outside the weld. The scanned area of one map was 4.4 mm wide and 3 mm high with the step of 5 µm, which give 533886 measurement points for each of the three maps. During EBSP analysis the detection was made using band centers with 6 to 7 bands. The EBSP imaging was set to 26-28 ms with 4x4 binning and gain 7.
12 Although 3 maps were scanned for nearly every sample, only the center map was used in the analysis. This map contained only grains in the center of the fusion zone. The data of all the maps can be found in the attached DVD (Appendix U). It was found out that the center map gave better correlation between the EBSD data and the strain than when using the stitched map of the 3 adjacent maps. In the HKL software the minimum angle to define a grain boundary was 1°. The EBSD data were noise reduced using a ‘‘wildspike’’ correction and a five-neighbour zero solution extrapolation. The EBSD maps showing the band contrast together with small angle grain boundaries (blue), high angle grain boundaries (green) and twin boundaries (red) are shown in Appendix E. The Inverse Pole Figure (IPF) maps with orientation X0 are shown in Appendix F and the maps with Euler Angle coloring are shown in Appendix G. The maps showing recrystallized, substructured and deformed fractions are presented in Appendix H. The local misorientation maps are shown in Appendix I. The maps showing the average of the mean intra-grain misorientation AMIS are presented in Appendix J. For the deformation maps the HKL Tango software measures the internal average misorientation angle within each grain. If the average angle in a grain exceeds the userdefined minimum angle to define a subgrain, (θc), the grain is classified as being "deformed". Some grains consist of subgrains whose internal misorientation is under θc but the misorientation from subgrain to subgrain is above θc. In that case the grain is classed as "substructured". All the remaining grains are classified as “recrystallized”. [4] In the deformation maps the minimum misorientation (°) to separate subgrains was set to θc=1° and to separate grains to 7.5°. The grains with minimum deformation (recrystallized) are marked by blue, the grains containing low angle boundaries (substructured) by yellow and the deformed grains by red. 2.4
Summary of the EBSD results for the tensile tests
The results of the EBSD data are summarized in Table 2 and in Table 3. The mean angular deviation (MAD) is given in degrees specifying the averaged angular misfit between detected and simulated Kikuchi bands [3]. In all cases the MAD was low indicating a high degree of fit between theoretical and measured EBSPs.
13
Table 2. The results of EBSD studies for the tensile samples. Weld Sample ID 00 01 02 03 04 05 06 07 08 09 10 11 12 13
X453A2 X438A3 X458L1 X438A1 X458A2 X453L3 X458A1 X453A3 X453A1 X453L1 X458L3 X453A4 X458A4 X458A3
Eng. strain, plastic 0 % 1 % 1.33 % 1.77 % 2.35 % 3.13 % 4.16 % 5.53 % 7.36 % 9.79 % 13.02 % 19.40 % 23.90 % X %
Eng. True strain, strain, EBSD total plastic indexing (exten‐ [mm/mm] someter) 0.00 % 1.04 % 1.38 % 1.83 % 2.41 % 3.20 % 4.25 % 5.64 % 7.49 % 9.95 % 13.21 %
0 0.00995 0.01321 0.01755 0.02323 0.03082 0.04076 0.05383 0.07102 0.0934 0.12239 0.17731 0.2143
97.3 % 98.4 % 96.4 % 97.6 % 95.1 % 97.8 % 98.1 % 95.5 % 96.7 % 91.5 % 92.4 % 96.3 % 95.5 % 97.9 %
Band Band mean contrast 60° contrast MAD peak Twin peak at value (Σ3) 0.5686 0.4737 0.5055 0.5113 0.5851 0.4772 0.5775 0.5913 0.5129 0.6679 0.6295 0.5927 0.5327 0.5367
99.5 110 104 112 93.5 110 109 111 103 97.5 88.5 109 107 107
17800 20400 19100 18200 20200 22000 19200 18300 19600 18800 18200 17900 15300 20100
0.48 % 0.31 % 0.66 % 0.28 % 0.49 % 0.29 % 0.81 % 0.13 % 0.20 % 0.10 % 0.01 % 0.05 % 0.05 % 0.05 %
Table 3. The results of EBSD studies for the tensile samples (continued). Abs. Abs. Abs. Grains occ. of occ. of occ. of Sample detecte 1.5° 2.5° 59.5° d misor. misor. misor. 00 01 02 03 04 05 06 07 08 09 10 11 12 13
34863 25260 33404 68672 74460 94280 65966 82827 183325 244286 279814 326352 378414 177045
3682 2965 4010 7455 5465 9474 4113 6257 21062 27277 39824 67335 98569 16891
238 177 390 483 410 329 557 87 301 238 64 195 224 150
130 133 160 156 173 143 132 167 220 349 315 684 1312 124
Grain size [um]
AMIS [°]
302.73 279.96 287.2 279.37 286.28 307.46 271.45 274.14 258.24 262.4 282.59 221.2 176.61 260.55
0.875 1.052105 0.955813 1.240256 1.145029 1.293357 1.819697 1.303413 2.070591 1.295444 1.827111 2.293705 2.335419 2.579597
Recrystalli Sub‐ Deform zed structured ed fraction fraction fraction 25.60 % 32.50 % 21.30 % 17.50 % 6.39 % 4.59 % 3.05 % 3.31 % 0.57 % 0.51 % 0.58 % 0.67 % 1.11 % 0.38 %
74.10 % 67.30 % 78.40 % 81.10 % 92.90 % 93.00 % 96.30 % 94.00 % 68.10 % 48.90 % 26.90 % 13.00 % 5.79 % 60.50 %
0.27 % 0.17 % 0.28 % 1.37 % 0.73 % 2.42 % 0.66 % 2.70 % 31.30 % 50.60 % 72.50 % 86.30 % 93.10 % 39.20 %
14
Figure 4. Recrystallized, substructured and deformed fractions of the tensile samples 00-12. 2.5
Hardness measurements of the tensile test samples
The hardness was measured across the whole tensile sample as HV1 values using the hardness tester Struers Duramin A-300. The distance between measurement points was 0.5 mm and total of 19-23 points were measured. The hardness measurement results are presented in Appendix K. From the hardness values measured, 15 measurement points in the center of the weld were selected to calculate an average hardness for each strained sample. The average hardness values are shown in Figure 5.
15
Figure 5. The hardness values ± standard deviation as a function of strain. 2.6
Verification of the reference curves with the confirmation test
According to [1] the quality of the obtained EBSD pattern deteriorates with increasing strain. The idea of this study was to construct reference curves based on the band contrast values. According to Brewer [1] also the average intra-grain misorientation AMIS is related to the strain. This was also studied. It was noticed from the collected EBSD data that the induced strain could not be related to the band contrast peak value, the band contrast peak location or the FWHM value of the band contrast. However, the number of grains detected in the map area and the values obtained from the recrystallized fraction map, the absolute occurrence of small angle grain boundaries (1.5° and 2.5°) and the AMIS values showed dependency on the strain, see Figures 6-8.
16
Figure 6. The recrystallized, substructured and deformed fractions together with the amount of grains detected as a function of strain in the samples 00-12.
Figure 7. Absolute occurrences of 1.5° and 2.5° misorientations as a function of strain.
17
Figure 8. The average intra-grain misorientation (AMIS) as a function of strain. The estimated values for the strain applied in the blind test (number 13) determined according to the curves above are shown in Table 4. The amount of grains detected was lower than in other samples and it could not be fitted on the curve. The AMIS value gave too high value for the plastic true strain, since it was known that this tensile test was carried out with the extensometer; the maximum strain of the extensometer is 15 %. These two values had to be left out of the verification calculations. Table 4. The strain in the blind test estimated using values from EBSD data. Tensile test 13 (X458A3) values obtained true strain, plastic (estimated from each reference curve) true strain, total (estimated from stress‐strain curve) eng. strain, total (calculated from true strain)
Abs. occ. Abs. occ. Re‐ Sub‐ Grains AMIS Deformed of 1.5° of 2.5° crystallized structured detected [°] fraction misor. misor. fraction fraction 177045
16891
0.07
0.065
*not in 0.1907 the curve
0.07117 0.06614
*not in the range
7.4 %
6.8 %
124
2.5796
0.38 %
60.5 %
39.2 %
0.07
0.08
0.08
0.07117
0.08128
0.08128
7.4 %
8.5 %
8.5 %
18 The hardness values showed clear dependency on the applied strain. In order to estimate the strain based on measured hardness, the curve of hardness as a function of strain was inverted, see Figure 9. The average hardness in the “blind test” was 60.63 HV1, which equals to plastic true strain εtr,pl=0.053767 according to the equation shown in Figure 9. Using Hooke’s law and the average elastic modulus determined earlier, the total true strain is εtr,tot=0.05475, which equals to total engineering strain εeng,tot=5.6 %.
Figure 9. True strain as a function of hardness. A polynomial fit is shown as black line. The average of all of these estimates for the strain is 7.4 %. Now it was revealed that the actual strain in this blind test was εeng,tot=6.66 % (εtr,tot=0.0645, εtr,pl=0.0634). According to these the absolute occurrence of 2.5° misorientations gave the best fit to the actual value. 2.7
The obtained reference curves
The blind test was also included in the test series since the strain was precisely known. The reference curves were then redrawn to include this sample with the actual strain used.
19
Figure 10. The reference curve based on the absolute occurrences of 1.5° and 2.5° misorientations.
Figure 11. The reference curve based on recrystallized, substructured and deformed fractions and grains detected.
20
Figure 12. The reference curve based on hardness.
21
3
STUDIES OF THE EB-WELDED PLATES
The samples, where the residual stresses were to be estimated, were divided into 2 sets of welds: the 1st set (welds X436-X440) for studying the effect of welding speed and cosmetic pass and the 2nd set (welds X453-X458) for studying the effect of annealing temperature and cosmetic pass. 3.1
Identification of samples
In Figure 13 the schematic centerline weld shape is illustrated with the terms used. Slope up means the area in which the starting procedure was conducted. Full penetration weld means the area in which the welding parameters were kept constant. Slope down means the area during which the ending procedure was conducted. Each of these areas is divided into three sub-areas: surface, base weld, and root. The surface refers to the area from z=0 mm to around z=10 mm; the root refers to the area 10-15 mm from the tip of the root and base weld is the area in between. Slope up
Full penetration weld
Root area
base weld material
Slope down
Surface area
Figure 13. Different areas of a weld centerline, welding direction is from left to right. (Figure by Timo Salonen) The plate welding experiments were marked with running numbers starting from one, and the number of an experiment was indicated with a three-number code, e.g. X436. For the metallographic studies, the welding samples were cut into smaller pieces, which were marked with identification letters. The zero points of the axes are located at the edge of the plate (not at the starting point of the welding). The X axis indicates the welding direction. The Y axis indicates perpendicular welding. The Z axis points to the depth of the welding. The transversal cross-section samples were cut by wire electric discharge machining WEDM from the plate containing the weld. The location of the cross-sections was x=240 – 250 mm. In the first drawing in Appendix A the location of the cross-section can be seen in the end of the 1st half of the plate.
22 3.2
Welding parameters
In all these experiments the welding beam current IB was 300 mA, the working distance was 300 mm and the accelerating voltage UB was 150 kV. In the tests X436-X440 the welding speed v was varied, see Table 5. In the tests X453-X458 the welding speed was 2.5 mm/s. These experiments were welded in medium vacuum (10−2 - 10−3mbar) with a vertical electron beam. The oscillation pattern was E32 in all welds with ACX value 4,2 and ACY 3,3. The focus current was kept constant of IL=2395 mA during full penetration area. The slope up started in x=30 mm and the slope down started in x=320 mm. The welded material was phosphorus micro-alloyed oxygen-free copper Cu-OFEP with P= 30 – 70 ppm, O < 5 ppm, H < 0,6 ppm and S < 8 ppm. Table 5. Effect of welding speed and the cosmetic pass on residual stresses.
Weld ID X436 X437 X438 X439 X440
Welding speed v [mm/s] 4.0 3.2 2.5 2.1 2.1
Cosmetic pass no no no no yes
Table 6. Effect of annealing temperature and the cosmetic pass on residual stresses, v=2.5 mm/s. Annealing temperature Cosmetic Weld ID [°C] pass X456 20 yes X458 20 no X453 170 yes X454 250 yes X455 200 yes X457 200 no Light blue= difference between plate size Blue= effect of the welding speed Red=effect of the annealing temperature green background = effect of the cosmetic pass yellow background= full factorial design: anneal on/off, cosmetic on/off
23 3.3
Sample preparation for macroscopic examination
The cross-sections were ground using P160 and P320 grinding papers to obtain sufficient smoothness of the surface. The samples were etched with technical grade nitric acid (HNO3). The samples were first “rinsed” with nitric acid by immersing them for 5−10 s, washed with water, and then actually etched by immersing for 30 s in fresh nitric acid. After etching the samples were washed with water and ethanol and dried with a drier. The etched samples were photographed using a Nikon D70s camera and a Nikon AF Micro-Nikkor 60 mm f/2.8D camera objective. The camera was attached to a Kaiser RS1 (5510) stand. The images from the backside of the cross-section samples were flipped, so that all the cross-section images show the cross-section as viewed from the start of the welding. The metallographic images of the cross-sections are presented in Appendix L. The cross-sections were examined using a stereomicroscope Leica MZ7.5. The imperfections found are listed in Appendix M. 3.4
EBSD studies of the cross-sections
After the macroscopic studies the cross-section samples were cut at the depths z=5 mm and z=15 mm. These 10 mm high samples were then prepared using nearly the same procedures as was used in the preparation of the tensile test specimens for the EBSD studies: only the used forces were slightly higher, which allowed a bit shorter preparation times. The settings for the EBSD measurements were the same. The EBSD maps were scanned in the center of these samples, thus the center of the EBSD maps is in the depth z=10 mm. The EBSD maps showing the band contrast together with small angle grain boundaries (blue), high angle grain boundaries (green) and twin boundaries (red) are shown in Appendix N. The Inverse Pole Figure (IPF) maps with orientation X0 are shown in Appendix O and the maps with Euler Angle coloring are shown in Appendix P. The maps showing recrystallized, substructured and deformed fractions are presented in Appendix Q. The local misorientation maps are shown in Appendix R. The maps showing the average of the mean intra-grain misorientation AMIS are presented in Appendix S. 3.5
Summary of the EBSD results of the cross-sections
The results are shown in Table 7 and Table 8.
24
Table 7. The results of EBSD studies for the cross-sections. Band Band abs. occ. EBSD 60° Sample contrast contrast of 1.5° indexing Twin (Σ3) peak at peak value misor. X436A X437A X438A X439A X440A X453A X454A X455A X456A X457A X458A
98.4 % 97.9 % 97.5 % 96.2 % 97.7 % 97.6 % 96.7 % 98.6 % 97.4 % 97.7 % 97.8 %
112 104 106 104 110 106 102 116 114 107 111
21100 18500 17400 20300 18100 21400 16900 21000 20000 22700 20900
0.52 % 1.53 % 0.44 % 0.64 % 0.37 % 0.27 % 0.61 % 0.65 % 1.12 % 1.01 % 0.71 %
32244 31828 37506 41421 40147 47605 49021 89717 32918 31961 46966
abs. occ. of 2.5° misor. 3828 3396 5760 2641 3281 3549 3060 5747 3404 4930 3325
abs. occ. of 59.5° misor. 307 865 344 341 225 179 301 559 492 541 491
Table 8. The results of EBSD studies for the cross-sections (continued). Sample AMIS [°] X436A X437A X438A X439A X440A X453A X454A X455A X456A X457A X458A
3.6
1.217 1.135 0.900 0.755 1.155 1.091 0.936 1.468 0.903 0.857 0.981
grains detected
Grain size [um]
182 170 184 195 122 150 138 153 198 146 143
237.7 263.7 170.5 158.2 278.1 259.5 255.6 254.2 196 213.7 239.9
mean recrystallized substructured deformed MAD fraction fraction fraction 0.5081 0.5253 0.5377 0.5619 0.5787 0.5467 0.6335 0.4584 0.5414 0.4904 0.6130
35.3 % 36.7 % 31.9 % 28.6 % 20.4 % 12.5 % 16.8 % 3.4 % 15.1 % 23.2 % 25.4 %
64.5 % 63.1 % 67.8 % 71.0 % 79.3 % 87.2 % 82.9 % 95.3 % 84.6 % 76.6 % 74.4 %
0.2 % 0.2 % 0.37 % 0.32 % 0.3 % 0.34 % 0.33 % 1.3 % 0.3 % 0.27 % 0.24 %
Hardness measurements of the cross-sections
The hardness was measured with 2 mm intervals outside the weld and with 0.5 mm intervals in the weld area. Otherwise the measurement was carried out the same way as were the hardness measurements from the tensile test samples. The results are shown in Appendix T. The hardness in the center of the weld (average of 15 measurement points in the center of the weld) is presented in Figure 14.
25
Figure 14. Hardness values ± standard deviations in the cross-sections.
26
27
4
DISCUSSION
The effect of welding speed, annealing temperature and cosmetic pass are discussed here. Also the residual stresses are estimated. The effect of the cosmetic pass could be analyzed together with the welding speed using samples from both sets of welds, and together with the effect of annealing temperature using the 2nd set of welds. 4.1
The effect of welding speed
In the 1st set of welds the welding speed was altered between the welds. The results of the samples with different welding speeds are listed in Table 9. The recrystallized, substructured and deformed fractions are shown in Figure 15. The absolute occurrence of 1.5º and 2.5º misorientation are presented in Figure 16, the average of mean intragrain misorientation angle AMIS in Figure 17 and hardness in Figure 18. Welding speed does not appear to have a measurable effect on the properties studied. Table 9. The results of the samples with different welding speeds. Sample speed abs. occ. abs. occ. AMIS recrystallized substructured deformed hardness [mm/s] of 1.5° of 2.5° [º] fraction fraction fraction HV1 misor. misor. X436A 4.0 32244 3828 1.217 35.3 % 64.5 % 0.25 % 41.7 X437A 3.2 31828 3396 1.135 36.7 % 63.1 % 0.21 % 41.2 X438A 2.5 37506 5760 0.900 31.9 % 67.8 % 0.37 % 41.3 X439A 2.1 41421 2641 0.755 28.6 % 71.0 % 0.32 % 43.5
28
Figure 15. Recrystallized, substructured and deformed fractions with various welding speeds.
Figure 16. Absolute occurency of 1.5º and 2.5º misorientation as a function of welding speed.
29
Figure 17. Average of mean intra-grain misorientation angle AMIS as a function of welding speed.
Figure 18. Hardness as a function of welding speed.
30 4.2
The effect of annealing temperature
The welds in the 2nd set were annealed at various temperatures. The results of the samples with different annealing temperature are listed in Table 10. The recrystallized, substructured and deformed fractions are shown in Figure 19. The absolute occurrence of 1.5º and 2.5º misorientation are presented in Figure 20, the average of mean intragrain misorientation angle AMIS in Figure 21 and hardness in Figure 22. Table 10. The results of the samples with different annealing temperature. Sample T [°C] abs. occ. abs. occ. AMIS recrystallized substructured deformed Hardness of 1.5° of 2.5° [º] fraction fraction fraction HV1 misor. misor. X453A 170 47605 3549 1.091 12.5 % 87.2 % 0.34 % 47.3 X454A 250 49021 3060 0.936 16.8 % 82.9 % 0.33 % 46.4 X455A 200 89717 5747 1.468 3.4 % 95.3 % 1.27 % 48.8 X456A 20 32918 3404 0.903 15.1 % 84.6 % 0.33 % 46.6
The values of the sample X455A (annealed at T=200 ºC) seem to differ quite much from the other annealed samples. When looking at the EBSD maps, especially the map with local misorientations, it is clear that the surface in X455 was not as good as in the other samples. This sample should be treated as an outlier. The hardness measurement diagonals were ca. 200 µm, which mean that the penetration depth of the hardness indenter was ca. 40 µm. This means that in this case the hardness is a better measure of the properties of the weld. But also the hardness in X455A was higher than would be expected. It raises a question if this sample has experienced some other divergent treatment. When the X455 is not considered, the annealing temperature appears to have no measurable effect on the occurrence of small angle misorientations, average of mean intra-grain misorientation, recrystallized, substructured or deformed fractions or hardness.
31
Figure 19. Recrystallized, substructured and deformed fractions with various annealing temperatures.
Figure 20. Absolute occurrence of 1.5º and 2.5º misorientation as a function of annealing temperature.
32
Figure 21. Average of mean intra-grain misorientation angle AMIS as a function of annealing temperature.
Figure 22. Hardness as a function of annealing temperature.
33 4.3
The effect of cosmetic pass and annealing
The effect of the cosmetic pass could be analyzed together with the effect of annealing temperature using the 2nd set of welds. Again, the results of X455A should be considered with care. The results of the samples with/without annealing (in 200 ºC) and with/without a cosmetic pass are listed in Table 11. The effect of cosmetic pass and annealing on recrystallized, substructured and deformed fractions, on the absolute occurency of 1.5º and 2.5º misorientation, on the average of mean intra-grain misorientation angle AMIS and on hardness are shown in Figure 23. Table 11. The results of the welds with/without annealing (in 200 ºC) and with/without a cosmetic pass. Weld Anneal. Cosm. ID temp. pass [ºC] X458A 20 no X457A 200 no X456A 20 yes X455A 200 yes
abs. occ. abs. occ. AMIS Recrystallized Sub‐ Deformed Hardness of 1.5° of 2.5° [°] fraction structured fraction HV1 misor. misor. fraction 46966 3325 0.981 25.4 % 74.4 % 0.24% 40.6 31961 4930 0.857 23.2 % 76.6 % 0.27% 40.5 32918 3404 0.903 15.1 % 84.6 % 0.33% 46.6 89717 5747 1.468 3.43% 95.3 % 1.27% 48.8
It seems that due to the cosmetic pass the recrystallized fraction decreases and the substructured fraction increases. The hardness is higher in the weld with cosmetic pass than without cosmetic pass.
34
Scatterplot of abs. occ. of; abs. occ. of; ... vs annealing te 0 abs. occ. of 1.5° misor.
6000
100
200
abs. occ. of 2.5° misor.
AMIS [°]
cosmetic 0 1
1.4
80000 5000
1.2
60000 4000
40000 Recrystallized_fraction
96.00%
24.00%
88.00%
16.00%
80.00%
8.00%
1.0 Substructured_fraction
Deformed_fraction 1.20% 0.80% 0.40%
Hardness_HV1
0
100
200
48
44
40 0
100
200
annealing temperature [ºC]
Figure 23. The effects of cosmetic pass (0=no cosmetic, black; 1= cosmetic, red) and annealing (20 ºC or 200 ºC) on the abs. occurencies of 1.5º and 2.5º misorientation, the average of mean intra-grain misorientation AMIS, the recrystallized, substructured and deformed fractions and hardness. The point at T=200 ºC with cosmetic pass should be considered with care. 4.4
The effect of cosmetic pass and welding speed
The effect of the cosmetic pass could be analyzed together with the welding speed using samples from both sets of welds. The only difference between the welds X438 and X458 was the plate size: the plate in X438 was 500 mm x 500 mm and the plate in X458 was 500 mm (length) x 440 mm (width). The results of X438 were selected for the Minitab analyses. The results of the samples with welding speeds 2.1 mm/s and 2.5 mm/s and with/without a cosmetic pass are listed in Table 12. The effect of cosmetic pass and annealing on recrystallized, substructured and deformed fractions, on the absolute occurency of 1.5º and 2.5º misorientation, on the average of mean intra-grain misorientation angle AMIS and on hardness are shown in Figure 24. It seems that due to the cosmetic pass the recrystallized fraction decreases and the substructured fraction increases. The hardness is higher in the weld with cosmetic pass than without cosmetic pass.
35 Table 12. The results of the welds with welding speeds 2.1 mm/s and 2.5 mm/s and with/without a cosmetic pass. Weld Weld. Cosm. ID speed pass [mm/s] X439A 2.1 0 X438A 2.5 0 X458A 2.5 0 X440A 2.1 1 X456A 2.5 1
abs. occ. abs. occ. AMIS recrystallized Sub‐ deformed Hardness of 1.5° of 2.5° [º] fraction structured fraction HV1 misor. misor. fraction 41421 2641 0.755 28.6% 71.0% 0.32% 43.5 37506 5760 0.900 31.9% 67.8% 0.37% 41.3 46966 3325 0.981 25.4% 74.4% 0.24% 40.6 40147 3281 1.155 20.4% 79.3% 0.26% 46.7 32918 3404 0.903 15.1% 84.6% 0.33% 46.6
Scatterplot of abs. occ. of; abs. occ. of; ... vs welding spee 2.10 abs. occ. of 1.5° misor.
40000
2.25
2.40
abs. occ. of 2.5° misor.
1.2
average of mean intra-grain mis
5000 1.0
cosmetic 0 1
4000
36000
0.8
3000 32000
recrystallized fraction
32.00%
substructured fraction
deformed fraction
84.00% 0.35% 78.00%
24.00%
0.30% 72.00% 16.00%
0.25%
hardness
2.10
2.25
2.40
46 44 42 2.10
2.25
2.40
welding speed [mm/s]
Figure 24. The effects of cosmetic pass (0=no cosmetic, black; 1= cosmetic, red) and welding speed (2.1 mm/s or 2.5 mm/s) on the abs. occurencies of 1.5º and 2.5º misorientation, the average of mean intra-grain misorientation AMIS, the recrystallized, substructured and deformed fractions and hardness.
36
Contour Plot of hardness vs cosmetic; speed 1
cosmetic
hardness < 42 42 – 43 43 – 44 44 – 45 45 – 46 46 – 47 > 47
0 2.1
2.2
2.3 speed
2.4
2.5
Figure 25. The effects of cosmetic pass (0=no cosmetic, 1= cosmetic) and welding speed (2.1 mm/s or 2.5 mm/s) on hardness
4.5
Estimated residual stresses in the cross-sections
Based on the reference curves determined earlier, the amount of plastic true strain was estimated. The estimated values for each weld are shown in Table 13 according to different reference sources, i.e. misorientation angles, recrystallization/deformed fractions and hardness. From the number of grains detected (Figure 11) it can only be estimated that the plastic true strain in all of the cross-sections is below 0.065. The plastic strain in the welds appears to be fairly low. Also, the amount of strain estimated based on results of the EBSD measurements correspond well with the amount calculated based on hardness. Except for the case of X455A, the values based on EBSD give higher strains than those based on hardness: EBSD, however, gives information from the depth of only a few microns, while the hardness measurement indentation in this study extended ca. 40 µm beneath the sample surface. It was clear from the EBSD maps that sample preparation in the sample X455A was not sufficient, which explains the deviating EBSD results for this sample.
37 Table 13. The plastic true strain in cross-sections estimated with the EBSD data and the hardness. Values in italic represent the estimated maximum strain. TRUE STRAIN, PLASTIC [mm/mm] ESTIMATED ACCORDING TO: Sample
abs. abs. occ. of occ. of recrystallized substructured deformed AMIS 1.5° 2.5° fraction fraction fraction misor. misor.
Hardness
X436A
