Selected properties of welded joints of 2 mm thick titanium alloy Grade 2 produced by different welding parameters of electron beam welding were investigated. The visual inspection, macro and micro analysis, tensile strength test and microhardness measurements were performed. Samples manufactured by appropriate welding parameters had no internal defects, heat-affected zone (HAZ) was narrow and transformation from α phase to β phase was observed in this area. The failure occurred in the base metal during tensile test, while maximum ultimate tensile strength reached 454,3 MPa

B. Šimekova

D. Marić

J. Barta

M. Maronek

English

Hrčak - Portal of scientific journals of Croatia

255METALURGIJA 58 (2019) 3-4, 255-258J. BÁRTA, M. MARÔNEK, B. ŠIMEKOVÁ, D. MARIĆANALYSIS OF WELD JOINTS MADE OF TITANIUM ALLOY GRADE 2 PRODUCED BY ELECTRON BEAM WELDINGReceived – Primljeno: 2018-11-20Accepted – Prihvaćeno: 2019-02-10Original Scientific Paper – Izvorni znanstveni radJ. Bárta (jozef.barta@stuba.sk), M. Marônek, B. Šimeková, Faculty of Materials Sceicne and Technology, Slovak University of Technology, Trnava, Slovakia.D. Marić, Faculty of Mechanical Engine ering, Slavonski Brod, Univer-sity of Osijek, Croatia.Selected properties of welded joints of 2 mm thick titanium alloy Grade 2 produced by different welding parameters of electron beam welding were investigated. The visual inspection, macro and micro analysis, tensile strength test and microhardness measurements were performed. Samples manufactured by appropriate welding parameters had no internal defects, heat-affected zone (HAZ) was narrow and transformation from α phase to β phase was ob-served in this area. The failure occurred in the base metal during tensile test, while maximum ultimate tensile strength reached 454,3 MPa.Keywords: titanium Grade2, electron beam welding, mechanical properties, structural analysisINTRODUCTIONTitanium is a light metal with a strength close to steel, but its weight in comparison to steel is 45 % low-er. Titanium and its alloys are most commonly used in the space, aerospace, chemical, food and petrochemical industries. Titanium has a high affinity for oxygen. Tita-nium oxide (TiO2) is rapidly formed on the surface of the metal even at room temperature. [1,2]From a welding point of view, it should be noted that alloys that can be heat-treated can also be strengthened during the thermal welding cycle. Such alloys during welding may require preheating and subsequent heat treatment. Heat treatment of titanium alloy welded joints can lead to dimensional stability and thus reduces cracking and corrosion cracking. Because of the high reactivity of titanium with oxygen at higher tempera-tures, it can be welded only in perfect gas protection (argon, helium) or in the vacuum chamber. [3] Electron beam welding uses the energy of concentrated electron beam in the process of joining metallic materials.This welding method is characterized by high energy density, which allows deep beam penetration into the welded materials. Electron beam welding is most often carried out in a high vacuum. The advantages of electron beam welding include the very good appearance of welds with a very fine surface structure, the minimum specific heat input leading to further advantages such as a narrow heat-affected zone (HAZ) and minimal deformations. [3]On the other hand, only a small percentage of the total time is used for welding due to the chamber deplet-ing. [4]ISSN 0543-5846METABK 58(3-4) 255-258 (2018)UDC – UDK 669.295:621.791.052-537.533:620.14:620.18=111Fusion welding and solid-state welding are used for welding titanium and its alloys as well. The results of the two-phase welding of Ti6Al4V alloy by the fiber, disk and Nd:YAG laser has also been published. The authors observed increased microhardness in weld metal as a re-sult of the martensitic structure in the weld [5-7].Information on electron beam welding of Ti alloys is rather insufficient. The published sources involve the data on the welding of the cast ZTC4 Ti alloy. Struc-ture of the weld joints was formed by martensite. Strength of the weld joints was higher compared to the parent metal [8].MATERIALS AND METHODSAs a base metal (BM), the commercial titanium al-loy Grade 2 was used. The size of samples was 100 × 100 × 2 mm. The chemical composition is shown in Table 1 and mechanical properties in Table 2.Table 2  Typical mechanical properties of titanium alloy Grade 2Yield strength / Rp 0.2 345 - 450 MPaTensile strength / Rm 485 MPaElongation / A5 28 %Reduction of area / Z 55 %Hardness / HV 160 - 200 Modulus of elasticity / E 103 GPaImpact toughness / KIC 40 - 82 JTable 1  Typical chemical composition of titanium alloy Grade 2 / wt. %O N C H Fe Other Ti0,25 0,03 0,08 0,015 0,3 0,5 Bal.256J. BÁRTA et al.: ANALYSIS OF WELD JOINTS MADE OF TITANIUM ALLOY GRADE 2 PRODUCED BY … METALURGIJA 58 (2019) 3-4, 255-258The objective of the experiment was to set up the appropriate welding parameters for electron beam weld-ing of titanium alloy Grade 2. Welding was realized at the MTF STU using the equipment shown in Figure 1.Prior to welding, the base material was cleaned from oxides. Consequently, penetration welds according to the parameters shown in Tables 3 and 4 were performed. The penetration welds are shown in Figure 2.Figure 1 Electron beam welding deviceIn the first step of the experiment, the welding was carried out at a constant welding speed of 30 mm/s, and the welding current was the variable parameter. The welding parameters at constant welding speed and ac-celerated voltage of 55 kV, are shown in Table 3. Based on the visual inspection, the welding current of 70 mA was evaluated as the best and set up as constant. In the second step, the welding speed varied. The welding pa-rameters for the constant welding current, accelerated voltage of 55 kV and varying welding speed are shown in Table 4.Table 4 Welding parameters at constant welding currentSample Iz/ mAIf/ mAvz/ mm/sP/ kJ/mm3.1 70 890 20 0,1733.2 70 890 40 0,0873.3 70 890 15 0,2313.4 70 890 35 0,0994.1 70 890 10 0,3464.2 70 890 50 0,06934.3 70 890 25 0,1384.4 70 890 45 0,077Iz– welding current, If – focusation current, vz – welding speed, P – specifi c heat inputTable 3 Welding parameters at constant welding speedSample Iz / mAIf/ mAvz/ mm/sP/ kJ/mm1.1 60 890 30 0,0991.2 80 890 30 0,1321.3 100 890 30 0,1651.4 40 890 30 0,0662.1 70 890 30 0,1152.2 90 890 30 0,1482.3 50 890 30 0,0822.4 30 890 30 0,049Iz– welding current, If – focusation current, vz – welding speed, P – specifi c heat inputFigure 2 Penetration weldsRESULTSVisual inspection revealed that the penetration welds were without undesired coloration, i. e. the vacuum pro-vided sufficient protection for the welded materials. Samples 1.2, 2.1, 3.2, 3.4, 4.2 and 4.4 were judged to be satisfactory by visual inspection. In all other samples, Figure 3  Macroscopic analysis a) 2.1; b) 3.2; c) 3.4; d) 4.2; e) 4.4  e) a) b) c) d)257J. BÁRTA et al.: ANALYSIS OF WELD JOINTS MADE OF TITANIUM ALLOY GRADE 2 PRODUCED BY …METALURGIJA 58 (2019) 3-4, 255-258the lack of penetration in the weld root was observed. Moreover, the samples 4.3, 4.1, 3.1 exhibited lack of fusion in the weld root and continuous undercut was observed in samples 4.1, 3.3 and 3.1. Based on these defects, all mentioned penetration welds were evaluated by visual inspection as unsatisfactory.Samples designated by visual inspection as satisfac-tory were further subjected to macroscopic analysis. Images from the macroscopic analysis showed that the examined samples had a narrow HAZ – typical for elec-tron beam welding. No internal defects such as cracks, cavities or pores had been observed in the welds (Fig-ure 3). The examined samples were therefore indicated as satisfactory.Microscopic analysis of the samples was carried out by a NEOPHOT 30 light microscope. Given that all samples had a similar microstructure, microscopic anal-ysis was performed only on sample 3.2.The microstructure of the base metal is shown in Figure 4 and consisted of α titanium with a fine-grained polyhedral structure with a grain size in the range of 10 - 40 μm.seconds. The measured values are plotted in Figure 7. The hardness of the weld metal and the HAZ was high-er than the hardness of the base metal.The observed microhardness of titanium alloy Grade 2 was between 135 - 203 HV. Hardness exceeding 200 HV might have been caused due to the presence of mar-tensite or measurement inaccuracy.The tensile test was performed on a Tinius and Ol-sen 300 ST instrument. Samples 2.4, 3.2 and 3.4 were used for the tensile test. Three test pieces were made Figure 4 Microstructure of base materialThe material had a monophase structure. The transi-tion from base material to HAZ (Figure 5) was narrow and grain coarsening arose here. In the transition region, the grains lost their polyhedral character and grains with acicular morphology were formed. This change was caused by α to β phase transformation at a tempera-ture of approximately 888 °C. This transformation was partly diffuse and partly diffusion-less.The weld metal (WM) microstructure is shown in Figure 6 and consisted of coarse polyhedral grains with acicular morphology. The Widmannstätten structure and twinning of the grains which is characteristic of the martensitic structure was possible to rarely observe. In this case, martensite was formed by phase β oversatu-rated with β-stabilizing elements. The dark areas in the weld metal microstructure were acicular grains contain-ing very small (1 μm) polyhedral grains.The microhardness measurement was performed on a Bühler Indentament 1 105 microtest. The test was car-ried out with a load of 0,98 N. The load time was 10 Figure 5 Transition area microstructureFigure 6 Weld metal microstructureFigure 7 The course of microhardness258J. BÁRTA et al.: ANALYSIS OF WELD JOINTS MADE OF TITANIUM ALLOY GRADE 2 PRODUCED BY … METALURGIJA 58 (2019) 3-4, 255-258from each sample. The values obtained by the tensile test for individual test pieces are shown in Table 5.Most of the test pieces broke in the base metal, only the test pieces marked as 1.2 and 2.1 were broken in the weld. All test pieces after tensile test are shown in Fig-ure 8. Based on the tensile test results, it was found that the most appropriate welding parameters regarding the tensile strength were used in sample 3.2 where the fail-ure occurred in the base metal of all three test pieces.Table 5 Tensile test measured valuesSample Test pieceRm/ MPaA5/ % Failure2.4 1.1 434 17,6 BM1.2 451 13,3 Weld1.3 451 18,2 BM3.4 2.1 451 16,4 Weld2.2 466 19,9 BM2.3 446 17,7 BM3.2 3.1 442 18,7 BM3.2 443 18,4 BM3.3 457 20,0 WeldFigure 8  Test pieces made from samples 2.4, 3.4 and 3.2 after tensile testCONCLUSIONSThe experiment was focused on the set up of the ap-propriate welding parameters for electron beam weld-ing of titanium alloy Grade 2. The penetration welds produced at the various welding parameters were sub-jected to visual control, which was judged to be satis-factory for samples 2.1, 3.2, 3.4, 4.2 and 4.4 and there-fore subjected to further investigation. Subsequent mac-roscopic analysis showed that the penetration welds had a narrow HAZ and there were not observed any internal errors such as cracks, cavities or pores.The microstructural analysis of the penetration welds revealed that the grains of the base metal had polyhedral fine-grained structure. The material had a monophase structure. In the HAZ, the grains due to the temperature approximately 888 °C changed from α phase to β phase. The transformation from polyhedral to acicular grains was observed.The hardness of the samples continuously rose to-wards the weld metal, only in some cases it varied. The hardness in welding metal ranged from 150,6 to 219 HV.The highest average tensile strength 454,3 MPa was observed at sample 3.4. Tensile strength variance was in the range of 345 - 485 MPa, which was typical for tita-nium alloy Grade 2 titanium.Based on performed weld joint quality examination, the appropriate electron beam welding parameters of 2 mm thick titanium alloy Grade 2 were: accelerated volt-age 55 kV, welding current 70 mA, focusing current 890 mA and welding speed 40 mm/s.AcknowledgementThis work was supported by the Slovak Research and Development Agency under the contract No. APVV-15-0337.REFERENCES[1] I. Hrivňák: Welding and weldability of materials, Bratisla-va: STU 2009, ISBN 978-80-227-3167-6[2] J. Jozwik: Evaluation of Tribological Properties and Con-dition of Ti6Al4V Titanium Alloy Surface Technical Ga-zette 25(2018)1, 170-175[3] Matnet Slovakia: Electron Beam Welding, URL: http://www.matnet.sav.sk/index.php?ID=370 (10.12.2017)[4] Matnet Slovakia, Titanium URL: http://www.matnet.sav.sk/index.php?ID=492 (10.12.2017)[5] G. Casalino, M. Mortello, S. L. Campanelli: Ytterbium  Fiber Laser Welding of Ti6Al4V Alloy, Journal of Manu-facturing Processes 20 (2015) 1, 250-256[6] E. Akman, A. Demir, T. Canel, T. Sınmazçelik: Laser Wel-ding of Ti6Al4V Titanium Alloys, Journal of Materials Processing Technology 209 (2009) 8, 3705-3713[7] Lisiecki, A.: Laser Welding of Titanium Alloy Ti6Al4V using a Disk Laser. Available at: http://www.meching.com/journal/Archive/2012/7/MTM/27_Lisiecki.pdf[8] W. Bing, L. Jinwei, T. Zhenyun: Study on the Electron Beam Welding Process of ZTC4 Titanium Alloy, Rare Me-tal Materials and Engineering 43 (2014) 4, 786 - 790Note:  Responsible translator; Martina Šuto, University of Osijek, Osijek, Croatia.

Analysis of weld joints made of titanium alloy GRADE 2 produced by electron beam welding

Abstract

Similar works

Full text

Available Versions

Hrčak - Portal of scientific journals of Croatia