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Comparative study of TIG welded commercially pure titanium
Journal of Manufacturing Processes 36 (2018) 155–163
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Comparative study of TIG welded commercially pure titanium a,⁎
a
a
a
b
Regita Bendikiene , Saulius Baskutis , Jolanta Baskutiene , Antanas Ciuplys , Tomas Kacinskas a b
T
Kaunas University of Technology, Faculty of Mechanical Engineering and Design, Department of Production Engineering, Studentu st. 56, LT-51424, Kaunas, Lithuania JSC “IREMAS”, Metal Testing Laboratory, Jonalaukis, Ruklos sen. Jonavos dist., LT-55296, Lithuania
ARTICLE INFO
ABSTRACT
Keywords: Titanium welding Shielding tool Non-destructive tests Destructive tests Weld quality
This work presents a comparative study and industrial scale application of three Tungsten Inert Gas (TIG) welding modes of commercially pure titanium: pulsed welding in shorts interrupted intervals under the controlled technological welding conditions (mode 2), traditional TIG welding process (mode 3). As welding of commercially pure titanium needs special attention particularly to proper shielding, tools which ensure full protection were designed to shield initial weld pool, weld and nearby base metal from contamination. Consequently, welding in the fully covered Argon (Argon box) gas using designed Track and Back shielding tools (mode 1) was suggested for the industrial scale application, tested and discussed. The comparative study includes evaluation of process stability as well. Non-destructive and destructive tests were accomplished according to the standardized procedures to ascertain the influence of welding modes on the quality and performance characteristics of the welds. Results of the liquid penetrant test, visual inspection, and radiographic inspection showed that the welds obtained using designed Track and Back shielding tools have met all the acceptance criterions. Destructive tests’ results confirmed presumptions of importance to use appropriate shielding for pure titanium welding.
1. Introduction
attention has to be taken in order to reduce weldments’ presence in the open air after cleaning and during welding. Usually, a shielding gas is used for the weld protection, besides it is necessary to ensure complete cover of front and back heat affected zones [5]. In the work [6] V-butt weld joint of titanium alloy was produced by Gas Tungsten Arc Welding (GTAW) inside a glove box to guarantee maximum protection. Li et al. [7] have designed two kinds of shielding gas nozzles: first consisted of a single pipe, the second one was combined with three pipes. The first pipe has prevented joint oxidation, other two served as oxidation prevention during the process of welding and cooling. Obtained results showed that the effective three-pipe nozzle shielding demonstrated better protection than single-pipe nozzle as well as better gas flow rate. Otani [8] suggested different arrangement of shielding device while welding of titanium. As sufficient effect cannot be reached by shielding along the torch axis, it was decided to add an after-shielding device and back-shielding device at the back side of torch to protect the weld. In this study welding torch was placed at the angle of 40°. As has been reported by [4] to get good quality in titanium welds, primary, secondary and back side protection are needed; primary shielding covered initial weld pool, secondary and back side shielding prevented weld and
Pure titanium possesses an excellent strength to density ratio and high corrosion resistance, which makes it suitable for the production of weight saving components, and for structures which require good corrosion resistance [1]. Due to the low thermal expansion, titanium structures has lower thermal stresses than other metallic materials, and also one more exceptional property widely used in medicine is its outstanding biocompatibility. Commercially pure titanium is rather characterized as a metal being difficult to weld, but paying special attention to few conditions, it doesn’t have to be unrealizable. These conditions are proper selection of filler metal, cleanness, and particularly usage of shielding gas [2,3]. Herewith titanium has two other features that considerably influence its weldability: great oxygen affinity and poor combining with any other chemical elements. Because of great oxygen affinity, titanium rapidly forms oxides’ layer in the open air; moreover, when heated to the high temperatures titanium oxides start to form even faster. For the worse, at the melting temperature of titanium (1668 °C), dissolve oxides contaminate weld pool which is the main reason of defective and very weak weld formation. Due to absorption of gases, hardness and brittleness increases, it can be noticed by colour change [4]. Consequently, special
Corresponding author. E-mail addresses: [email protected] (R. Bendikiene), [email protected] (S. Baskutis), [email protected] (J. Baskutiene), [email protected] (A. Ciuplys), [email protected] (T. Kacinskas). ⁎
https://doi.org/10.1016/j.jmapro.2018.10.007 Received 11 July 2018; Received in revised form 28 August 2018; Accepted 4 October 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 36 (2018) 155–163
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Table 1 Chemical composition and properties of sheets to be welded.
Table 2 TIG welding parameters.
Chemical composition, wt.%
Process parameters
Ti
O
Fe
C
N
H
Bal.
≤ 0.35
≤ 0.30
≤ 0.08
≤ 0.05
≤ 0.015
1 Welding current, A Voltage, V Welding speed, mm/s Number of passes Number of intervals Duration between intervals, s Duration of one interval, s Length of 1 interval, mm Welding speed (1 interval), mm/s Shielding medium Argon output from torch, l/min LST EN ISO14175:2008 [12] Argon output from track tool, l/min Argon from back tool, l/min Wire consumption, mm Argon consumption, l Weld geometrical parameters (Fig. 1) - weld width e, mm - backside weld width el, mm - weld height g, mm - backside weld height, gl, mm
Mechanical properties Hardness HB
0.2 % Yield strength Rp, N/ mm2
Tensile strength Rm, N/mm2
Elongation, %
Young’s modulus, GPa
≤ 170
≥ 380
≥ 450
≥ 18
≥ 105
Physical properties Density, g/ cm3
Specific heat capacity, J/kg K
4.51 520 Ti3/3.7055 (DIN 17860)
Thermal conductivity W/m·K
Electrical resistivity, Ω·mm2/m
20
0.52
Welding mode
nearby base metal from contamination. In this study the properties of welds were analysed considering what type of weld pool shielding was used. Three shielding modes were extremely analysed and tested, the results obtained in the three types of modes were compared, and application of the Track and Back weld protection tools was introduced for industrial application in one of the companies in Lithuania. Additionally, non-destructive and destructive tests of weld samples were accomplished to understand behaviour of welds in service.
2
3
0.1
0.45
38 60 8 18 – 19 2.5
18 5 30 31 – 33 1.1
10
10
20 27 1500 925
– –
– –
400
92.2
8.0 6.0 0.8 0.9
8.8 7.0 1.0 1.7
11.8 10.8 0.9 1.0
110 12 0.25 3 6 60 60 120 – 125 2 Argon 4.8 10
2. Materials and methods Sheets with a dimension of 1000 × 250 × 4 mm made of commercially pure titanium were used in this study; the chemical composition, mechanical and physical properties of primary sheets are listed in Table 1. Moderate amount of oxygen, which is acting as α phase stabilizer, is presented alongside to other impurities like Fe, C, N, and H. Billets for welding procedure were made from primary sheets with dimensions of 250 × 100 × 4 mm (6 billets from one primary sheet) selected according to the [9]. Fig. 1 displays the V grooved weldment dimensions: groove angle (α) – 60°, sheet thickness (t) – 4 mm, distance between sheets (b) – 2 mm, and welding sequence [10]: e – weld width, el – backside weld width, g – weld height, gl – backside weld height. The welding was conducted using Kemppi MasterTig MLS 3000 ACDC + MasterCool 30 (DC straight polarity) machine with water cooled torch Kemppi TTK 350 W, electrode WT 20 RED 2.4 × 175 mm [11], and filler metal BÖHLER: ER Ti 2-IG (AWS A5.16-04:ERTi2).
Fig. 2. Schematic illustration of welding mode 1 with Track and Back shielding tools.
Weld joints were produced manually joining two V grooved billets by three different weld shielding modes: 1 – welding in the fully covered Argon (Argon box) gas using designed Track and Back shielding tools; 2 – pulsed welding in shorts interrupted intervals under the controlled technological welding conditions; and 3 – traditional TIG
Fig. 1. Schematic illustration of billet and welding procedure: a – weldment dimensions; b – welding sequence and geometrical parameters of weld bead. 156
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Fig. 3. Tensile test samples.
Fig. 4. Bend test samples.
double shielded TIG could provide a more symmetrical and more powerful arc, which is also beneficial to improving the welding efficiency [13]. Unfortunately, for repair operations in the heavily accessible areas glove boxes cannot be used effectively. As single welds need to be welded qualitatively as well as welds in the massive production, it has been decided to design original shielding tools. Track shielding tool has been used alongside with torch, when the later one is inserted into the designed orifice. This is a rectangular shaped tool placed over the welding pool, which navigates after torch in such a way preventing covered liquid metal pool. Length of protected welding intervals depends on the length of Track tool. There is a dispensing tube inside the tool, through which argon is blown continuously to the weld; Ni-Cr mesh is placed below, it serves as filter preventing from the formation of air swirls. Scheme of shielding tools setting-up is depicted in Fig. 2. It was verified that Track tool does not ensure weld quality without blow around protection, therefore designed Back shielding is fixed over the back side of weld. This is a rectangular shaped box likewise in Track tool equipped with dispensing tool inside and Ni-Cr mesh below. No shielding tools have been used to accomplish welds of modes 2 and 3. After TIG welding procedure all weld samples were manually cleaned with Ni-Cr metal wire brush, subsequently have undergone non-destructive and destructive tests. Three methods of non-destructive testing were used in order to evaluate quality of welds: penetrant testing, visual inspection, and radiographic inspection. Liquid penetrant testing was done in accordance with a
Fig. 5. Sampling scheme of tensile and bend tests samples.
welding process. Details of all three modes are presented in Table 2. Argon glove boxes are the most effective means nowadays to protect the welds, but they are more suitable for massive production, when thousand welds are made; compared with the traditional TIG process, 157
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Fig. 6. Traces of the defects on the welds: a – front side of welding mode 2; b – back side of welding mode 3. 1, 3 – linear indications; 2 – point indications.
Fig. 7. Samples of visual inspection: a – 1 welding mode; b – 2 welding mode; c – 3 welding mode.
standardized procedure; penetrant material MR-Chemie GmbH Mr 68 C, solvent MR-Chemie GmbH MR 70, and cleaning agent MR-Chemie GmbH MR 79 were used. Tests were accomplished according to the requirements of LST EN standard [14]: lighting during examination must be more than 550 lx; process temperature cannot exceed range of temperatures −12 °C – 60 °C defined by manufacturer; duration of penetrant test should be in a range of 10–30 min. The penetrant material (MR 68 C) was spread over the welds and held on the surface for 10–15 min, then special sprayed cleaning agent (MR 79) was used after
excess of penetrant had been wiped away with the cloth. Spraying with solvent MR 70 was the last step of test after which all the defects were revealed in 10–15 min. Visual inspection is the most cost-effective method, which requires a few equipment’s. Aside from good eyesight and sufficient light, all it takes is a pocket rule, a weld size gauge, a magnifying glass, and possibly a straight edge and square for checking straightness, alignment and perpendicularity. The inspection was dispensed according to the standardised procedures [15] using INOX Vernier caliper 150 mm/ 158
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Fig. 8. Radiographic images: a – 1 welding mode; b – 2 welding mode; c – 3 welding mode. Table 3 Classification of geometric imperfections. Welding mode
1 2
3
Imperfections Number
Defect
– 2011 2015 2013 1021 3041 2011 2015 2013
– Gas pore (from ∅ 0.3 mm to ∅ 2 mm) Elongated cavities Clustered (localized) porosity Transverse crack Metallic inclusion – tungsten (∅ 2 mm) Gas pore (from ∅ 0.3 mm to ∅ 2 mm) Elongated cavities Clustered (localized) porosity
Sensitivity
Optical density
Evaluation according LST EN ISO 23277:2015
0.063
2.6
Acceptable Not acceptable
Not acceptable
0.05 mm, weld inspection device УШC-3 Model C0314, weld gauges Limit INOX No. MBL-02 and ESAB No. 40214, also magnifying glass VOGEL 4 × . Radiographic inspection of welds was performed with Eresco 42MF2 portable industrial device. The opaque inspected weld samples were placed between the rays of radioisotopes and photo sensitized film. The wavelengths of gamma rays irradiated from the radioisotopes Iridium 192, Cobalt 60, and Cesium 137 are shorter than X-rays, therefore with longer exposure times it able to penetrate deeper. Not all the radiation has passed through the weld sample; this amount depends on the density, thickness and atomic state of sample being tested. The
degree to which tested weld samples absorbed the rays showed the intensity of the rays penetrated through the weld. The image developed on the photo-sensitized film revealed thicker areas or areas with higher density in the sample, which have been absorbed more rays, that is why lighter areas were observed on the film. Considering the thickness (t = 4 mm) and composition of weld samples theses parameters have been chosen: focusing range – 530 mm, duration of exposition – 1 min, position of image quality indicators – from the source side, anodic current – 6.0 mA, anodic voltage – 80 kV, sensitivity of photo-sensitized film – 0.063, gauged optical density – 2.6. The destructive tests were performed and the mechanical properties 159
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Table 4 Results of tensile test. Welding mode
Number of sample
Length3 after brake, mm
Maximum force, kN
Ultimate tensile strength, MPa
Relative elongation, %
Brake zone
1
1.1 1.2 1.3 2.1 2.2 2.3 3.1 3.2 3.3
59.8 58.8 59.3 59.8 59.7 59.6 54.4 51.2 50.5
22.48 23.15 22.97 23.41 23.80 24.14 22.66 20.22 16.51
450 463 459 468 476 483 453 404 330
19.6 17.6 18.6 19.6 19.4 19.2 8.8 2.3 1.0
BM1 BM1 BM1 BM1 BM1 BM1 FZ2 FZ2 FZ2
2 3
1 2 3
base metal. fusion zone. initial length of test piece 50 mm.
were assessed by tensile, 3-point bending, and hardness tests. 9 tensile test samples (Fig. 3) have been prepared representing each welding mode (b0 = 12.5 ± 0.1 mm; L0 = 50 mm; LC = 72 ± 0.3 mm; Lt = 120 + 5 mm; R = 4 ± 1 mm; b1 = 22 ± 1 mm; L1 = 20 ± 1 mm). The tensile tests were accomplished on testing machine 1958U-10-1 in the laboratory environment with maximum and minimum loads of 100 kN and 0.002 kN respectively, at the temperature from 15 °C to 30 °C. 12 bending test samples (Fig. 4) were tested using the same machine as for tensile test. Fig. 5 represents tensile and bend test samples sampling scheme. Hardness test of cross-section of the welds along the weld sample was accomplished on Innovatest Verzus 750CCD tester applying standard load of 49.03 N (HV5) with 5 s dwell. All test samples were cleaned before test.
titanium dioxide (rutile) indicating a degree of contamination of weld. With full protection or minimal oxidation, the weld colour will be silver or light or dark straw; when newly welded part is not protected its colour appears to be white or grey. Samples for visual inspection are presented in Fig. 7. Inspection under appropriate lighting and good eyesight was done on the samples of three modes to evaluate shielding influence on the weld quality. As can be seen in the Fig. 7 a there were no defects or any imperfections according to the standard [17] quality level B on the surface of the welding mode 1 sample. Colour of surface was silver which was good indicator of achieved acceptance criterion. Weld of welding mode 2 showed flaky deposits (Fig. 7b) but not unallowable defects which are acceptable according to the standard as well as the colour of surface – light straw confirmed shielding mode of later welding process. The last welding mode 3 has passed weld process requirements with no evidence of geometric unconformities could be seen, but unfortunately colour of weld surface did not satisfy the requirements for titanium welds (Fig. 7c). Grey colour cannot be accepted. Defects appeared during welding and allied processes are classified into the six groups according to the standard [18]. Radiographic inspection was done comparing visible defects on the photo-sensitized film with standardized defects types (Fig. 8). Inspection results are presented in Table 3. Radiographic inspection of the welds made it clear that just fully shielded welds can meet quality requirements.
3. Results and discussion 3.1. Non-destructive tests Liquid penetrant testing (PT) has been chosen as an effective way to detect the quality of weld samples surface [16]. Before penetrant test, complete set of surface cleaning process was done to obtain satisfactory results. The surfaces of weld samples were grinded and visually observed to ensure that samples were dry and free of any dirt and grease, weld spatters remains, oils that could hide surface defects and openings. Inspection under appropriate lighting was done on the both sides of weld samples of each welding mode. Both sides of mode 1 weld samples showed no indications of defects; these samples have passed the test. Different situation was observed when front surface of mode 2 has been examined. Front surface revealed traces of three types: 8 point indications with diameter of 1.0 mm, 2 linear indications of 10 mm length, and 1 linear indication of 5 mm length (Fig. 6a). A few pores localized on the surface would be acceptable, but these two notable breaks in the central part of weld and one on the left have failed acceptance of this mode. It was assumed that defects in the central part appeared because titanium had not been cooled to the safe temperature when arc for the next interval was struck, on the other hand, short breaks (Fig. 6 a (3)) are usual defect caused by crater piping in the titanium welds. Back side of this mode was without any indications, but overall validation could not be positive. Samples of last welding mode were tested in the similar way. Point indications of different diameters (0.5 mm, 0.1 – 0.3 mm) were seen on the both sides (Fig. 6b); despite the fact that the majority of individual defects did not reach unallowable limits, according to liquid penetrant testing the mode 3 did not conform to the requirement of standards, and such a weld seam should be rejected. Titanium welds differ from other welds by the possibility to be assessed by colour. Different colours of weld surface are not an issue for steel welds, but titanium colour indicates the defects and problems of the welding technique. Usage of shielding gas might influence colour change when titanium weld is induced by surroundings at the elevated temperatures. Colour has been changed because of formation of
3.2. Destructive tests Each welded joint basically has three structurally different zones: unaffected metal, partially melted heat affected zone, and fusion zone. These zone highly influence the strength of the weld [4]. Tensile tests were dispensed to define the ultimate tensile strength of weld under the static load. During the test relative elongation and contraction were recorded and evaluated as well. Tensile behaviour of samples produced under different shielding conditions (welding modes 1–3) was analysed, and detailed results are presented in Table 4. Weld samples of welding modes 1 and 2 have experienced break in the unaffected base metal zone. Length after brake for both modes was very similar from 59.3 to 59.7 mm with relative elongation from 18.6 to 19.4% respectively. None of the samples from 1 and 2 modes have been broken in the fusion or heat affected zone showing good quality of welds. Opposite situation was observed testing tensile strength of last welding mode. As can be seen in Table 4 all weld samples broke at fusion zone. Length after brake and relative elongation confirmed high brittleness of weld samples, particularly third sample of welding mode 3 showed just 1% of elongation. The highest ultimate tensile strength in average 472 MPa was obtained for the welding mode 2, followed by welding mode 1 with average tensile strength 457 MPa, and finally the lowest ultimate tensile strength was indicated testing samples of welding mode 3 – in average 160
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Fig. 9. Tensile curves of welding modes.
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Table 5 Results of bend test. Welding mode
Number of sample
Type of bend test
Bend angle, (α) °
Crack location
Evaluation according LST EN ISO 15614-1:2004
1
1.1 1.2 1.3 1.4 2.1 2.2 2.3 2.4 3.1 3.2 3.3 3.4
TFB1 TFB1 TRB2 TRB2 TFB1 TFB1 TRB2 TRB2 TFB1 TFB1 TRB2 TRB2
180 180 180 180 70 40 120 50 30 25 40 35
– – – – FZ3 FZ3 HAZ4 FZ3 HAZ4 HAZ4 FZ3 HAZ4
Acceptable Acceptable Acceptable Acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable
2
3
1 2 3 4
transverse face bend test. transverse root bend test. fusion zone. heat affected zone.
395 MPa. Tensile curves are presented in Fig. 9. In summary modes 1 and 2 have met requirements for the titanium welds, but welding mode 3 has failed tensile test and cannot be accepted. The bend test is a simple qualitative test on a butt weld to demonstrate soundness and ductility [19]. It was expected from bending test to clear up whether or not produced welds withstand transverse bending. Two samples of each welding mode were subjected to standardized transverse face bend test (TFBB) and transverse root bend (TRBB) tests; before test bending samples were grinded and polished. Different bend angles have been chosen for the experiment in order to evaluate bending limits of each mode. Table 5 and Fig. 10 present results and acceptance criterions of bending tests. Fig. 10 a clearly indicates high plasticity of welding mode 1 weld, carried out with double shielding tools. Welds in other two modes were obviously brittle (Fig. 10b and c). It is worth mentioning that samples of mode 2 showed different plasticity limits, with variation in bending angle from 40° to 120°. As it was mentioned above test welds of mode 2 were acceptable, because tension axis corresponded to the plane of uneven structure, whereas bending test direction was opposite which obviously caused the formation of cracks. Negative acceptance values were achieved while testing samples of mode 3: high brittleness of welds was caused by totally different structure of weld. Hardness of the base metal was 168 HV5, 169 HV5, and 155 HV5 respectively for modes 1–3 (Fig. 11). Base metal in mode 1 and mode 2 was practically unaffected by heat of fusion. Maximum achieved hardness of weld (mode 1) was 207 HV5, while heat affected zone showed variation of values from 164 HV5 to 177 HV5. Difference between hardness points of FZ and HAZ was 39 HV5. Such a similar distribution of values across weld confirms acceptable results of tensile and bending tests. Highest achieved hardness of FZ (peaks in Fig. 11) of mode 2 was 300 HV5, while in the centre of the weld hardness values dropped to 200 HV5, showing formation of brittle α’ between layers of the weld. Twice as base metal value, hardness of FZ of mode 3 was 343 HV5 proving formation of brittle phase in the whole volume of the weld, which corresponds to tensile and bending results of the following mode.
Fig. 10. Bent samples after test: a – 1 welding mode; b – 2 welding mode; c – 3 welding mode.
double shielding of the weld area, have met all the acceptance criterions of standardized non-destructive tests: liquid penetrant test, visual inspection, and radiographic inspection as well as have conformed all the requirements of tensile, bending, and hardness tests. 2 Pulsed welding in short interrupted intervals under the controlled technological welding conditions (mode 2) showed positive results of liquid penetrant test for back side of weld, unfortunately, overall validation could not be positive. Visual inspection showed a few but not unallowable defects which are acceptable. Radiographic inspection has revealed gas pores, elongated cavities, clustered (localized) porosity, transverse cracks, and tungsten metallic inclusions, which proved that results did not meet quality requirements. Behaviour of mode 2 samples is acceptable for tensile, but not for bending test due to the formation of brittle α’ phase. 3 Mode 3 is not suitable for joining of commercially pure titanium joints, because it has failed all the test performed and can not be acceptable for the industrial case applications. 4 Mode 1 using Track and Back shielding tools is suitable for the repair process.
4. Conclusions Comprehensive comparative study of the influence of welding modes on weld quality and acceptance criterions for the application in industry identifies the key findings: 1 Designed Track and Back tools have found their niche for industrial scale application. Samples produced according to the mode 1, using 162
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Fig. 11. Hardness of weld.
References
[18] LST ISO 652-1:2007. Welding and allied processes - Classification of geometric imperfections in metallic materials - Part 1: Fusion welding (ISO 6520-1:2007). 2007. [19] Moore P, Booth G. 9 - Mechanical testing of welds. In: Moore P, Booth G, editors. The Welding Engineer’s Guide to Fracture and Fatigue. Oxford: Woodhead Publishing; 2015. p. 113–41.
[1] Patnaik AK, Poondla N, Menzemer CC, Srivatsan TS. Understanding the mechanical response of built-up welded beams made from commercially pure titanium and a titanium alloy. Mater Sci Eng: A 2014;590:390–400. [2] Wang RR, Welsch GE. Joining titanium materials with tungsten inert gas welding, laser welding, and infrared brazing. J Prosthetic Dent 1995;74:521–30. [3] Saresh N, Pillai MG, Mathew J. Investigations into the effects of electron beam welding on thick Ti–6Al–4V titanium alloy. J Mater Process Technol 2007;192–193:83–8. [4] Karpagaraj A, Siva shanmugam N, Sankaranarayanasamy K. Some studies on mechanical properties and microstructural characterization of automated TIG welding of thin commercially pure titanium sheets. Mater Sci Eng: A 2015;640:180–9. [5] Atoui JA, Felipucci DN, Pagnano VO, Orsi IA, Nóbilo MA, Bezzon OL. Tensile and flexural strength of commercially pure titanium submitted to laser and tungsten inert gas welds. Braz Dent J 2013;24(6):630–4. [6] Kolli RP, Herzing AA, Ankem S. Characterization of yttrium-rich precipitates in a titanium alloy weld. Mater Charact 2016;122:30–5. [7] Li C, Li B, Wu Z, Qi X, Ye B, Wang A. Stitch welding of Ti–6Al–4V titanium alloy by fiber laser. Trans Nonferrous Metals Soc China 2017;27:91–101. [8] Otani T. Titanium welding technology. Nippon Steel Technical Report 2007;95:88–92. [9] LST EN ISO 15614-5:2004. Specification and qualification of welding procedures for metallic materials - Welding procedure test - Part 5: Arc welding of titanium, zirconium and their alloys (ISO 15614-5:2004). Lithuanian standards board 2004. [10] LST EN ISO 15609-1:2005. Specification and qualification of welding procedures for metallic materials - Welding procedure specification – Part 1: Arc welding (ISO 15609-1:2004). 2005. [11] LST EN ISO 6848:2016. Arc welding and cutting – Nonconsumable tungsten electrodes - Classification (ISO 6848:2015). 2016. [12] LST EN ISO 14175:2008. Welding consumables – Gases and gas mixtures for fusion welding and allied processes (ISO 14175:2008). 2008. [13] Li D, Lu S, Li D, Li Y. Principles giving high penetration under the double shielded TIG process. J Mater Sci Technol 2014;30:172–8. [14] LST EN ISO 3452-1:2013. Non-destructive testing - Penetrant testing - Part 1: General principles (ISO 3452-1:2013). 2013. [15] LST EN ISO 17637:2017. Non-destructive testing of welds - Visual testing of fusionwelded joints (ISO 17637:2016). 2017. [16] Guirong X, Xuesong G, Yuliang Q, Yan G. Analysis and innovation for penetrant testing for airplane parts. Procedia Eng 2015;99:1438–42. [17] LST EN ISO 5817:2014. Welding – Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) - Quality levels for imperfections (ISO 5817:2014). 2014.
Dr. Regita Bendikiene has been working in the field of steel surface strengthening by microalloying, and appli-cation of welding technologies (82 publications) since 1994. In 1999, she defended her PhD Digital Metallo-graphic Evaluation of Carbide Phase Morphology. She did 4 scientific internships: in Sweden, France, and Estonia. Since 2013 she is the chair of Technical Committee of Lithuanian Standards Board; a reviewer of International Scientific Journals; the chairman of International Conference Materials Engineering; Director of study programmes of Production Engineering Field. Dr. Saulius Baskutis is associate professor in the Department of Production Engineering at Kaunas University of Technology, a position which he has held since 1992. His principal interests are in welding processes, measurements of mechanical properties of materials, and district heating supply and distribution systems. He has written more than 80 articles and papers, several textbooks as well as six patents. Also he has more than twenty years’ experience in industry, working mostly as an engineer and projects’ manager. Dr. Jolanta Baskutienė. PhD in Mechanical Engineering (2008). Vice dean for Studies of the Faculty of Mechanical Engineering and Design at Kaunas University of Technology since 2012. Associated professor at the Department of Production Engineering since 2013. Teaching experience in study programmes of Industrial Engineering and Management, Mechatronics, Robotics and Mechanical Engineering study programs since 2008. Experience in the project related field: was involved in ECDEAST TEMPUS project, Engineering Curricula Design aligned with EQF and EUR-ACE Standards, (TEMPUS, EU/ EC, 2011/2013) as representative of Project Partner University. Dr. Antanas Ciuplys has been working in surface engineering field from 2000; 72 publications in the field of surface engineering and hardfacing technologies using different conventional welding technologies. He did internships at Moscow State Steel and Alloys Institute (Russia, 4 months, 2003); at Poznan technical university (Poland, 3 months, 2005). He is reviewer of International Scientific Journals. Tomas Kacinskas graduated from Kaunas University of Technology faculty of Mechanical Engineering and Design. Since graduation he has been working in one of the largest metal processing, manufacturing, erection, repair, design and maintenance company in Lithuania JSC IREMAS as senior engineer
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Comparative study of TIG welded commercially pure titanium a,⁎
a
a
a
b
Regita Bendikiene , Saulius Baskutis , Jolanta Baskutiene , Antanas Ciuplys , Tomas Kacinskas a b
T
Kaunas University of Technology, Faculty of Mechanical Engineering and Design, Department of Production Engineering, Studentu st. 56, LT-51424, Kaunas, Lithuania JSC “IREMAS”, Metal Testing Laboratory, Jonalaukis, Ruklos sen. Jonavos dist., LT-55296, Lithuania
ARTICLE INFO
ABSTRACT
Keywords: Titanium welding Shielding tool Non-destructive tests Destructive tests Weld quality
This work presents a comparative study and industrial scale application of three Tungsten Inert Gas (TIG) welding modes of commercially pure titanium: pulsed welding in shorts interrupted intervals under the controlled technological welding conditions (mode 2), traditional TIG welding process (mode 3). As welding of commercially pure titanium needs special attention particularly to proper shielding, tools which ensure full protection were designed to shield initial weld pool, weld and nearby base metal from contamination. Consequently, welding in the fully covered Argon (Argon box) gas using designed Track and Back shielding tools (mode 1) was suggested for the industrial scale application, tested and discussed. The comparative study includes evaluation of process stability as well. Non-destructive and destructive tests were accomplished according to the standardized procedures to ascertain the influence of welding modes on the quality and performance characteristics of the welds. Results of the liquid penetrant test, visual inspection, and radiographic inspection showed that the welds obtained using designed Track and Back shielding tools have met all the acceptance criterions. Destructive tests’ results confirmed presumptions of importance to use appropriate shielding for pure titanium welding.
1. Introduction
attention has to be taken in order to reduce weldments’ presence in the open air after cleaning and during welding. Usually, a shielding gas is used for the weld protection, besides it is necessary to ensure complete cover of front and back heat affected zones [5]. In the work [6] V-butt weld joint of titanium alloy was produced by Gas Tungsten Arc Welding (GTAW) inside a glove box to guarantee maximum protection. Li et al. [7] have designed two kinds of shielding gas nozzles: first consisted of a single pipe, the second one was combined with three pipes. The first pipe has prevented joint oxidation, other two served as oxidation prevention during the process of welding and cooling. Obtained results showed that the effective three-pipe nozzle shielding demonstrated better protection than single-pipe nozzle as well as better gas flow rate. Otani [8] suggested different arrangement of shielding device while welding of titanium. As sufficient effect cannot be reached by shielding along the torch axis, it was decided to add an after-shielding device and back-shielding device at the back side of torch to protect the weld. In this study welding torch was placed at the angle of 40°. As has been reported by [4] to get good quality in titanium welds, primary, secondary and back side protection are needed; primary shielding covered initial weld pool, secondary and back side shielding prevented weld and
Pure titanium possesses an excellent strength to density ratio and high corrosion resistance, which makes it suitable for the production of weight saving components, and for structures which require good corrosion resistance [1]. Due to the low thermal expansion, titanium structures has lower thermal stresses than other metallic materials, and also one more exceptional property widely used in medicine is its outstanding biocompatibility. Commercially pure titanium is rather characterized as a metal being difficult to weld, but paying special attention to few conditions, it doesn’t have to be unrealizable. These conditions are proper selection of filler metal, cleanness, and particularly usage of shielding gas [2,3]. Herewith titanium has two other features that considerably influence its weldability: great oxygen affinity and poor combining with any other chemical elements. Because of great oxygen affinity, titanium rapidly forms oxides’ layer in the open air; moreover, when heated to the high temperatures titanium oxides start to form even faster. For the worse, at the melting temperature of titanium (1668 °C), dissolve oxides contaminate weld pool which is the main reason of defective and very weak weld formation. Due to absorption of gases, hardness and brittleness increases, it can be noticed by colour change [4]. Consequently, special
Corresponding author. E-mail addresses: [email protected] (R. Bendikiene), [email protected] (S. Baskutis), [email protected] (J. Baskutiene), [email protected] (A. Ciuplys), [email protected] (T. Kacinskas). ⁎
https://doi.org/10.1016/j.jmapro.2018.10.007 Received 11 July 2018; Received in revised form 28 August 2018; Accepted 4 October 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
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Table 1 Chemical composition and properties of sheets to be welded.
Table 2 TIG welding parameters.
Chemical composition, wt.%
Process parameters
Ti
O
Fe
C
N
H
Bal.
≤ 0.35
≤ 0.30
≤ 0.08
≤ 0.05
≤ 0.015
1 Welding current, A Voltage, V Welding speed, mm/s Number of passes Number of intervals Duration between intervals, s Duration of one interval, s Length of 1 interval, mm Welding speed (1 interval), mm/s Shielding medium Argon output from torch, l/min LST EN ISO14175:2008 [12] Argon output from track tool, l/min Argon from back tool, l/min Wire consumption, mm Argon consumption, l Weld geometrical parameters (Fig. 1) - weld width e, mm - backside weld width el, mm - weld height g, mm - backside weld height, gl, mm
Mechanical properties Hardness HB
0.2 % Yield strength Rp, N/ mm2
Tensile strength Rm, N/mm2
Elongation, %
Young’s modulus, GPa
≤ 170
≥ 380
≥ 450
≥ 18
≥ 105
Physical properties Density, g/ cm3
Specific heat capacity, J/kg K
4.51 520 Ti3/3.7055 (DIN 17860)
Thermal conductivity W/m·K
Electrical resistivity, Ω·mm2/m
20
0.52
Welding mode
nearby base metal from contamination. In this study the properties of welds were analysed considering what type of weld pool shielding was used. Three shielding modes were extremely analysed and tested, the results obtained in the three types of modes were compared, and application of the Track and Back weld protection tools was introduced for industrial application in one of the companies in Lithuania. Additionally, non-destructive and destructive tests of weld samples were accomplished to understand behaviour of welds in service.
2
3
0.1
0.45
38 60 8 18 – 19 2.5
18 5 30 31 – 33 1.1
10
10
20 27 1500 925
– –
– –
400
92.2
8.0 6.0 0.8 0.9
8.8 7.0 1.0 1.7
11.8 10.8 0.9 1.0
110 12 0.25 3 6 60 60 120 – 125 2 Argon 4.8 10
2. Materials and methods Sheets with a dimension of 1000 × 250 × 4 mm made of commercially pure titanium were used in this study; the chemical composition, mechanical and physical properties of primary sheets are listed in Table 1. Moderate amount of oxygen, which is acting as α phase stabilizer, is presented alongside to other impurities like Fe, C, N, and H. Billets for welding procedure were made from primary sheets with dimensions of 250 × 100 × 4 mm (6 billets from one primary sheet) selected according to the [9]. Fig. 1 displays the V grooved weldment dimensions: groove angle (α) – 60°, sheet thickness (t) – 4 mm, distance between sheets (b) – 2 mm, and welding sequence [10]: e – weld width, el – backside weld width, g – weld height, gl – backside weld height. The welding was conducted using Kemppi MasterTig MLS 3000 ACDC + MasterCool 30 (DC straight polarity) machine with water cooled torch Kemppi TTK 350 W, electrode WT 20 RED 2.4 × 175 mm [11], and filler metal BÖHLER: ER Ti 2-IG (AWS A5.16-04:ERTi2).
Fig. 2. Schematic illustration of welding mode 1 with Track and Back shielding tools.
Weld joints were produced manually joining two V grooved billets by three different weld shielding modes: 1 – welding in the fully covered Argon (Argon box) gas using designed Track and Back shielding tools; 2 – pulsed welding in shorts interrupted intervals under the controlled technological welding conditions; and 3 – traditional TIG
Fig. 1. Schematic illustration of billet and welding procedure: a – weldment dimensions; b – welding sequence and geometrical parameters of weld bead. 156
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Fig. 3. Tensile test samples.
Fig. 4. Bend test samples.
double shielded TIG could provide a more symmetrical and more powerful arc, which is also beneficial to improving the welding efficiency [13]. Unfortunately, for repair operations in the heavily accessible areas glove boxes cannot be used effectively. As single welds need to be welded qualitatively as well as welds in the massive production, it has been decided to design original shielding tools. Track shielding tool has been used alongside with torch, when the later one is inserted into the designed orifice. This is a rectangular shaped tool placed over the welding pool, which navigates after torch in such a way preventing covered liquid metal pool. Length of protected welding intervals depends on the length of Track tool. There is a dispensing tube inside the tool, through which argon is blown continuously to the weld; Ni-Cr mesh is placed below, it serves as filter preventing from the formation of air swirls. Scheme of shielding tools setting-up is depicted in Fig. 2. It was verified that Track tool does not ensure weld quality without blow around protection, therefore designed Back shielding is fixed over the back side of weld. This is a rectangular shaped box likewise in Track tool equipped with dispensing tool inside and Ni-Cr mesh below. No shielding tools have been used to accomplish welds of modes 2 and 3. After TIG welding procedure all weld samples were manually cleaned with Ni-Cr metal wire brush, subsequently have undergone non-destructive and destructive tests. Three methods of non-destructive testing were used in order to evaluate quality of welds: penetrant testing, visual inspection, and radiographic inspection. Liquid penetrant testing was done in accordance with a
Fig. 5. Sampling scheme of tensile and bend tests samples.
welding process. Details of all three modes are presented in Table 2. Argon glove boxes are the most effective means nowadays to protect the welds, but they are more suitable for massive production, when thousand welds are made; compared with the traditional TIG process, 157
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Fig. 6. Traces of the defects on the welds: a – front side of welding mode 2; b – back side of welding mode 3. 1, 3 – linear indications; 2 – point indications.
Fig. 7. Samples of visual inspection: a – 1 welding mode; b – 2 welding mode; c – 3 welding mode.
standardized procedure; penetrant material MR-Chemie GmbH Mr 68 C, solvent MR-Chemie GmbH MR 70, and cleaning agent MR-Chemie GmbH MR 79 were used. Tests were accomplished according to the requirements of LST EN standard [14]: lighting during examination must be more than 550 lx; process temperature cannot exceed range of temperatures −12 °C – 60 °C defined by manufacturer; duration of penetrant test should be in a range of 10–30 min. The penetrant material (MR 68 C) was spread over the welds and held on the surface for 10–15 min, then special sprayed cleaning agent (MR 79) was used after
excess of penetrant had been wiped away with the cloth. Spraying with solvent MR 70 was the last step of test after which all the defects were revealed in 10–15 min. Visual inspection is the most cost-effective method, which requires a few equipment’s. Aside from good eyesight and sufficient light, all it takes is a pocket rule, a weld size gauge, a magnifying glass, and possibly a straight edge and square for checking straightness, alignment and perpendicularity. The inspection was dispensed according to the standardised procedures [15] using INOX Vernier caliper 150 mm/ 158
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Fig. 8. Radiographic images: a – 1 welding mode; b – 2 welding mode; c – 3 welding mode. Table 3 Classification of geometric imperfections. Welding mode
1 2
3
Imperfections Number
Defect
– 2011 2015 2013 1021 3041 2011 2015 2013
– Gas pore (from ∅ 0.3 mm to ∅ 2 mm) Elongated cavities Clustered (localized) porosity Transverse crack Metallic inclusion – tungsten (∅ 2 mm) Gas pore (from ∅ 0.3 mm to ∅ 2 mm) Elongated cavities Clustered (localized) porosity
Sensitivity
Optical density
Evaluation according LST EN ISO 23277:2015
0.063
2.6
Acceptable Not acceptable
Not acceptable
0.05 mm, weld inspection device УШC-3 Model C0314, weld gauges Limit INOX No. MBL-02 and ESAB No. 40214, also magnifying glass VOGEL 4 × . Radiographic inspection of welds was performed with Eresco 42MF2 portable industrial device. The opaque inspected weld samples were placed between the rays of radioisotopes and photo sensitized film. The wavelengths of gamma rays irradiated from the radioisotopes Iridium 192, Cobalt 60, and Cesium 137 are shorter than X-rays, therefore with longer exposure times it able to penetrate deeper. Not all the radiation has passed through the weld sample; this amount depends on the density, thickness and atomic state of sample being tested. The
degree to which tested weld samples absorbed the rays showed the intensity of the rays penetrated through the weld. The image developed on the photo-sensitized film revealed thicker areas or areas with higher density in the sample, which have been absorbed more rays, that is why lighter areas were observed on the film. Considering the thickness (t = 4 mm) and composition of weld samples theses parameters have been chosen: focusing range – 530 mm, duration of exposition – 1 min, position of image quality indicators – from the source side, anodic current – 6.0 mA, anodic voltage – 80 kV, sensitivity of photo-sensitized film – 0.063, gauged optical density – 2.6. The destructive tests were performed and the mechanical properties 159
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Table 4 Results of tensile test. Welding mode
Number of sample
Length3 after brake, mm
Maximum force, kN
Ultimate tensile strength, MPa
Relative elongation, %
Brake zone
1
1.1 1.2 1.3 2.1 2.2 2.3 3.1 3.2 3.3
59.8 58.8 59.3 59.8 59.7 59.6 54.4 51.2 50.5
22.48 23.15 22.97 23.41 23.80 24.14 22.66 20.22 16.51
450 463 459 468 476 483 453 404 330
19.6 17.6 18.6 19.6 19.4 19.2 8.8 2.3 1.0
BM1 BM1 BM1 BM1 BM1 BM1 FZ2 FZ2 FZ2
2 3
1 2 3
base metal. fusion zone. initial length of test piece 50 mm.
were assessed by tensile, 3-point bending, and hardness tests. 9 tensile test samples (Fig. 3) have been prepared representing each welding mode (b0 = 12.5 ± 0.1 mm; L0 = 50 mm; LC = 72 ± 0.3 mm; Lt = 120 + 5 mm; R = 4 ± 1 mm; b1 = 22 ± 1 mm; L1 = 20 ± 1 mm). The tensile tests were accomplished on testing machine 1958U-10-1 in the laboratory environment with maximum and minimum loads of 100 kN and 0.002 kN respectively, at the temperature from 15 °C to 30 °C. 12 bending test samples (Fig. 4) were tested using the same machine as for tensile test. Fig. 5 represents tensile and bend test samples sampling scheme. Hardness test of cross-section of the welds along the weld sample was accomplished on Innovatest Verzus 750CCD tester applying standard load of 49.03 N (HV5) with 5 s dwell. All test samples were cleaned before test.
titanium dioxide (rutile) indicating a degree of contamination of weld. With full protection or minimal oxidation, the weld colour will be silver or light or dark straw; when newly welded part is not protected its colour appears to be white or grey. Samples for visual inspection are presented in Fig. 7. Inspection under appropriate lighting and good eyesight was done on the samples of three modes to evaluate shielding influence on the weld quality. As can be seen in the Fig. 7 a there were no defects or any imperfections according to the standard [17] quality level B on the surface of the welding mode 1 sample. Colour of surface was silver which was good indicator of achieved acceptance criterion. Weld of welding mode 2 showed flaky deposits (Fig. 7b) but not unallowable defects which are acceptable according to the standard as well as the colour of surface – light straw confirmed shielding mode of later welding process. The last welding mode 3 has passed weld process requirements with no evidence of geometric unconformities could be seen, but unfortunately colour of weld surface did not satisfy the requirements for titanium welds (Fig. 7c). Grey colour cannot be accepted. Defects appeared during welding and allied processes are classified into the six groups according to the standard [18]. Radiographic inspection was done comparing visible defects on the photo-sensitized film with standardized defects types (Fig. 8). Inspection results are presented in Table 3. Radiographic inspection of the welds made it clear that just fully shielded welds can meet quality requirements.
3. Results and discussion 3.1. Non-destructive tests Liquid penetrant testing (PT) has been chosen as an effective way to detect the quality of weld samples surface [16]. Before penetrant test, complete set of surface cleaning process was done to obtain satisfactory results. The surfaces of weld samples were grinded and visually observed to ensure that samples were dry and free of any dirt and grease, weld spatters remains, oils that could hide surface defects and openings. Inspection under appropriate lighting was done on the both sides of weld samples of each welding mode. Both sides of mode 1 weld samples showed no indications of defects; these samples have passed the test. Different situation was observed when front surface of mode 2 has been examined. Front surface revealed traces of three types: 8 point indications with diameter of 1.0 mm, 2 linear indications of 10 mm length, and 1 linear indication of 5 mm length (Fig. 6a). A few pores localized on the surface would be acceptable, but these two notable breaks in the central part of weld and one on the left have failed acceptance of this mode. It was assumed that defects in the central part appeared because titanium had not been cooled to the safe temperature when arc for the next interval was struck, on the other hand, short breaks (Fig. 6 a (3)) are usual defect caused by crater piping in the titanium welds. Back side of this mode was without any indications, but overall validation could not be positive. Samples of last welding mode were tested in the similar way. Point indications of different diameters (0.5 mm, 0.1 – 0.3 mm) were seen on the both sides (Fig. 6b); despite the fact that the majority of individual defects did not reach unallowable limits, according to liquid penetrant testing the mode 3 did not conform to the requirement of standards, and such a weld seam should be rejected. Titanium welds differ from other welds by the possibility to be assessed by colour. Different colours of weld surface are not an issue for steel welds, but titanium colour indicates the defects and problems of the welding technique. Usage of shielding gas might influence colour change when titanium weld is induced by surroundings at the elevated temperatures. Colour has been changed because of formation of
3.2. Destructive tests Each welded joint basically has three structurally different zones: unaffected metal, partially melted heat affected zone, and fusion zone. These zone highly influence the strength of the weld [4]. Tensile tests were dispensed to define the ultimate tensile strength of weld under the static load. During the test relative elongation and contraction were recorded and evaluated as well. Tensile behaviour of samples produced under different shielding conditions (welding modes 1–3) was analysed, and detailed results are presented in Table 4. Weld samples of welding modes 1 and 2 have experienced break in the unaffected base metal zone. Length after brake for both modes was very similar from 59.3 to 59.7 mm with relative elongation from 18.6 to 19.4% respectively. None of the samples from 1 and 2 modes have been broken in the fusion or heat affected zone showing good quality of welds. Opposite situation was observed testing tensile strength of last welding mode. As can be seen in Table 4 all weld samples broke at fusion zone. Length after brake and relative elongation confirmed high brittleness of weld samples, particularly third sample of welding mode 3 showed just 1% of elongation. The highest ultimate tensile strength in average 472 MPa was obtained for the welding mode 2, followed by welding mode 1 with average tensile strength 457 MPa, and finally the lowest ultimate tensile strength was indicated testing samples of welding mode 3 – in average 160
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Fig. 9. Tensile curves of welding modes.
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Table 5 Results of bend test. Welding mode
Number of sample
Type of bend test
Bend angle, (α) °
Crack location
Evaluation according LST EN ISO 15614-1:2004
1
1.1 1.2 1.3 1.4 2.1 2.2 2.3 2.4 3.1 3.2 3.3 3.4
TFB1 TFB1 TRB2 TRB2 TFB1 TFB1 TRB2 TRB2 TFB1 TFB1 TRB2 TRB2
180 180 180 180 70 40 120 50 30 25 40 35
– – – – FZ3 FZ3 HAZ4 FZ3 HAZ4 HAZ4 FZ3 HAZ4
Acceptable Acceptable Acceptable Acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable Not acceptable
2
3
1 2 3 4
transverse face bend test. transverse root bend test. fusion zone. heat affected zone.
395 MPa. Tensile curves are presented in Fig. 9. In summary modes 1 and 2 have met requirements for the titanium welds, but welding mode 3 has failed tensile test and cannot be accepted. The bend test is a simple qualitative test on a butt weld to demonstrate soundness and ductility [19]. It was expected from bending test to clear up whether or not produced welds withstand transverse bending. Two samples of each welding mode were subjected to standardized transverse face bend test (TFBB) and transverse root bend (TRBB) tests; before test bending samples were grinded and polished. Different bend angles have been chosen for the experiment in order to evaluate bending limits of each mode. Table 5 and Fig. 10 present results and acceptance criterions of bending tests. Fig. 10 a clearly indicates high plasticity of welding mode 1 weld, carried out with double shielding tools. Welds in other two modes were obviously brittle (Fig. 10b and c). It is worth mentioning that samples of mode 2 showed different plasticity limits, with variation in bending angle from 40° to 120°. As it was mentioned above test welds of mode 2 were acceptable, because tension axis corresponded to the plane of uneven structure, whereas bending test direction was opposite which obviously caused the formation of cracks. Negative acceptance values were achieved while testing samples of mode 3: high brittleness of welds was caused by totally different structure of weld. Hardness of the base metal was 168 HV5, 169 HV5, and 155 HV5 respectively for modes 1–3 (Fig. 11). Base metal in mode 1 and mode 2 was practically unaffected by heat of fusion. Maximum achieved hardness of weld (mode 1) was 207 HV5, while heat affected zone showed variation of values from 164 HV5 to 177 HV5. Difference between hardness points of FZ and HAZ was 39 HV5. Such a similar distribution of values across weld confirms acceptable results of tensile and bending tests. Highest achieved hardness of FZ (peaks in Fig. 11) of mode 2 was 300 HV5, while in the centre of the weld hardness values dropped to 200 HV5, showing formation of brittle α’ between layers of the weld. Twice as base metal value, hardness of FZ of mode 3 was 343 HV5 proving formation of brittle phase in the whole volume of the weld, which corresponds to tensile and bending results of the following mode.
Fig. 10. Bent samples after test: a – 1 welding mode; b – 2 welding mode; c – 3 welding mode.
double shielding of the weld area, have met all the acceptance criterions of standardized non-destructive tests: liquid penetrant test, visual inspection, and radiographic inspection as well as have conformed all the requirements of tensile, bending, and hardness tests. 2 Pulsed welding in short interrupted intervals under the controlled technological welding conditions (mode 2) showed positive results of liquid penetrant test for back side of weld, unfortunately, overall validation could not be positive. Visual inspection showed a few but not unallowable defects which are acceptable. Radiographic inspection has revealed gas pores, elongated cavities, clustered (localized) porosity, transverse cracks, and tungsten metallic inclusions, which proved that results did not meet quality requirements. Behaviour of mode 2 samples is acceptable for tensile, but not for bending test due to the formation of brittle α’ phase. 3 Mode 3 is not suitable for joining of commercially pure titanium joints, because it has failed all the test performed and can not be acceptable for the industrial case applications. 4 Mode 1 using Track and Back shielding tools is suitable for the repair process.
4. Conclusions Comprehensive comparative study of the influence of welding modes on weld quality and acceptance criterions for the application in industry identifies the key findings: 1 Designed Track and Back tools have found their niche for industrial scale application. Samples produced according to the mode 1, using 162
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Fig. 11. Hardness of weld.
References
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Dr. Regita Bendikiene has been working in the field of steel surface strengthening by microalloying, and appli-cation of welding technologies (82 publications) since 1994. In 1999, she defended her PhD Digital Metallo-graphic Evaluation of Carbide Phase Morphology. She did 4 scientific internships: in Sweden, France, and Estonia. Since 2013 she is the chair of Technical Committee of Lithuanian Standards Board; a reviewer of International Scientific Journals; the chairman of International Conference Materials Engineering; Director of study programmes of Production Engineering Field. Dr. Saulius Baskutis is associate professor in the Department of Production Engineering at Kaunas University of Technology, a position which he has held since 1992. His principal interests are in welding processes, measurements of mechanical properties of materials, and district heating supply and distribution systems. He has written more than 80 articles and papers, several textbooks as well as six patents. Also he has more than twenty years’ experience in industry, working mostly as an engineer and projects’ manager. Dr. Jolanta Baskutienė. PhD in Mechanical Engineering (2008). Vice dean for Studies of the Faculty of Mechanical Engineering and Design at Kaunas University of Technology since 2012. Associated professor at the Department of Production Engineering since 2013. Teaching experience in study programmes of Industrial Engineering and Management, Mechatronics, Robotics and Mechanical Engineering study programs since 2008. Experience in the project related field: was involved in ECDEAST TEMPUS project, Engineering Curricula Design aligned with EQF and EUR-ACE Standards, (TEMPUS, EU/ EC, 2011/2013) as representative of Project Partner University. Dr. Antanas Ciuplys has been working in surface engineering field from 2000; 72 publications in the field of surface engineering and hardfacing technologies using different conventional welding technologies. He did internships at Moscow State Steel and Alloys Institute (Russia, 4 months, 2003); at Poznan technical university (Poland, 3 months, 2005). He is reviewer of International Scientific Journals. Tomas Kacinskas graduated from Kaunas University of Technology faculty of Mechanical Engineering and Design. Since graduation he has been working in one of the largest metal processing, manufacturing, erection, repair, design and maintenance company in Lithuania JSC IREMAS as senior engineer
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