6 Ti-6Al-4V 의 AM 에서기계적성질에미치는 Interpass Peening 의영향 ISSN 2466-2232 Online ISSN 2466-2100 변재규 * 이희준 ** 조상명 ***, * 부경대학교대학원신소재시스템공학과 ** 현대로템 ( 주 ) 중기사업본부 *** 부경대학교신소재시스템공학과 The Effect of Interpass Peening on Mechanical Properties in Additive Manufacturing of Ti-6Al-4V Jae-Gyu Byun*, Hui-jun Yi** and Sang-Myung Cho***, *Dept. of Materials System Engineering, Graduate School, Pukyong National Univ., Busan 48547, Korea **Defense Inducstrial Division, Hyundai Rotem Company, Changwon 51407, Korea ***Dept. of Materials System Engineering, Pukyong National University, Busan 48547, Korea Corresponding author : pnwcho@pknu.ac.kr (Received April 4, 2017 ; Revised April 19, 2017 ; Accepted April 24, 2017) Abstract Ti-alloys have high specific strength and are widely used for the filed of space aeronautics plant. However, it is difficult to process Ti-Alloys due to its high yield strength and it cannot raise the machining speed because it has a possibility of catching fire while processing. In order to reduce the number of processes for the Ti-alloys, the researches related to Additive Manufacturing(AM) have been actively carried out at the moment. As for the initial stage of AM market related to Ti-alloys, it started to use the raw material of powder metal, and it is currently being developed based on welding. In this study, Interpass reduced the size of the primary β grain in the z-axis direction, increased the nucleation site of α-colony, and decreased the length and width of α laths as though interpass rolling. Interpass leads to an increase in yield/ultimate tensile strength decrease elongation, resulting decrease in anisotropy of the material. Key Words : Additive manufacturing, 3D printing, Peening, Mechanical property, GTAW 1. 서론 Additive manufacturing(am) 은 CAD도면의정보로제품을얇은층의형태로연속적으로적층하여원하는형상의제품을만들어내것으로대중적으로는 3D printing 이라고불린다 1). 금속AM은첨가되는재료의형상에따라크게분말 / 와이어방식으로나누어지게되며, Table 1은금속AM 의분류를나타내고있다 2). 금속AM은제품의형상과비슷하게적층함으로서가공량 을최소화할수있으며, Ti, Inconel, STS 등고가의소재에적합하다. 특히 Ti합금은높은비강도를가져항공 우주, 플랜트, 자동차등의분야에널리사용되고있지만, 높은항복강도로인해절삭가공이어렵고가공시화재발생위험으로가공속도를높이지못해가공량이많아질수록제조원가는상승한다. 가공전제품무게와최종제품의무게의비를뜻하는 Buy To Fly(BTF) 가높아질수록가공량이많아져제조원가는증가한다. BTF 를줄이기위하여해외에서는난가공성소재인 Ti-6Al-4V 의 AM연 Journal of Welding and Joining, Vol.35 No.2(2017) pp6-12 https://doi.org/10.5781/jwj.2017.35.2.2
Ti-6Al-4V 의 AM 에서기계적성질에미치는 Interpass Peening 의영향 7 Table 1 Classification of metal AM process 2) PBF Material Power source Process Company Deposition rate SLS(Selective Laser Sintering) EOS, 3D systems, TPM, Farsoon, etc. 0.1~0.2kg/h Laser DMLS(Direct Metal Laser Sintering) EOS 0.1~0.2kg/h SLM Solutions, 3D SLM(Selective Laser Melting) systems, Realizer, 0.1~0.3kg/h Powder Concept laser, etc. based Electron beam EBM(Electron Beam Melting) ARCAM 0.1~0.2kg/h LENS(Laser Engineered Net Shaping) Optomec 0.1~2kg/h DED Solid filler based Laser Electron beam GTAW, GMAW arc GMAW arc Plasma arc DMD(Direct Metal Deposition) DM3D 0.1~2kg/h DMT(Direct Metal Tooling) InssTek 0.1~2kg/h CLAD(Construction Laser Additive Direct) BeAM 0.1~2kg/h EBAM(Electron Beam Additive Manufacturing) Sciaky ~9kg/h (Wire Arc Additive Manufacturing) Cranfield Univ. ~4kg/h DML(Direct Metal Lamination) MUTOH ~4kg/h ADED(Arc Directed Energy Deposition) EWI ~4kg/h IFF(Ion Fusion Formation) Honeywell ~3kg/h RPD(Rapid Plasma Deposition) Norsk titanium ~6kg/h GTAW arc STAM(Super-TIG Additive Manufacturing) Super-TIG welding ~7kg/h 구가활발히진행중이다 3-7). 금속AM 은용융풀이응고하면서열이전도되는방향과반대로조직이성장해나간다. 이러한 epitaxial grain growth 는이전층의 grain 이부분적으로재용융되어결정의성장방향을결정하는예비핵의역할을하고, 다음층의 grain 성장방향은이전층과동일하게된다. Fig. 1은 Fude Wang 등 8) 이 Wire+Arc Additive Manufacturing() 공정으로 Ti-6Al-4V 을적층 하였을때나타난단면으로서 z축방향으로길게성장한 primary β grain의주상조직을볼수있다. 이러한 epitaxial grain growh 로인하여적층제품은기계적성질의방향성이나타나게된다. 금속 AM에서발생하는상기와같은기계적성질의방향성을감소시킬수있는공정개발이필요하다. 본연구는용착속도 TIG 용접아크를열원으로한금속 AM에서 Ti-6Al-4V 의적층하여 z축방향의기계적성질을 x축방향과동등수준으로향상시키기위하여 interpass 의영향을검토하는것을목적으로한다. 2. 실험방법 2.1 적층방법 Fig. 1 Montage of macrostructure of pulse GTAW deposition Ti-6Al-4V wall 8) TIG 용접을이용한 Ti-6Al-4V 적층실험은기판모재 200mm(L) 100mm(W) 2mm(t) 의 ASTM Grarde 5 를사용하였으며, 실험에사용된와이어는 Φ1.2 ERTi-5 를사용하였다. 재료의화학성분은 Table 2와 Table 3에서표시하고있다. 적층실험은 Fig. 2의모식도와같이 1layer 1pass 방식으로비드폭은약 14mm, 층당높이는 1.5mm가 대한용접 접합학회지제 35 권제 2 호, 2017 년 4 월 115
8 변재규 이희준 조상명 Table 2 Chemical composition of base metal(astm Grade 5) wt.% Ti Al V Fe N C H O Grade 5 Bal. 5.5~ 6.75 3.5~ 4.5 0.40 0.05 0.08 0.015 0.20 Peening Table 3 Chemical composition of filler metal(aws ERTi-5) wt.% Grade 5 Ti Al V Fe N C H O Bal. 5.5~ 6.75 3.5~ 4.5 0.22 0.03 0.05 0.015 0.12~ 0.20 Fig. 3 Before/after bead appearance of as-deposited Ti-6Al- 4V by interpass 14.00 160.00 15.0 15.0 105.0 7.0 105.00 80.0 2.00 Fig. 4 Schematic diagram of collected location of tensile test specimen Fig. 2 Schematic of deposition 1layer 1pass Ti-6Al-4V 되도록설계하여층간온도를 150 로유지하면서적층하였다. 각시험편당최종높이가약 100mm가되도록설계하여 70layer 를적층하였다. AM 적층조건은 Table 4에나타냈다. 190A 의전류로약 1.0kg/h 의적층속도로적층을하였고벽면끝에서의용융풀흘러내림을방지하기위하여오실레이션을사용하였다. Ti-6Al-4V 적층시에자주발생하는표면산화방지를위하여 trail shielding 장치를사용하 Table 4 Condition of AM Base metal Ti Gr. 5 (100 200 t2) Filler metal ø1.2 ERTi-5 Stand off 4mm Current 190A Welding speed 18cm/min Shielding gas Ar100% (20l/min) Feed rate 331cm/min Deposition area 20.8mm 2 Deposition rate 1.0kg/h OS Frequency 1.8Hz Dwell time 0.1s Deposition method 1layer 1pass 였다. 단면에칭은 1.5%HF + 4.5%HNO 3 + 증류수혼합용액을사용하여 30초동안부식시켜미세조직을검토하였다. 2.2 Interpass 방법 Ti-6Al-4V을적층할때 hammer machine 으로층마다용접비드표면에 을적용하였다. Fig. 3과같이적층이끝나고나면표면산화없는반짝반짝하고매끄러운표면이나타나고소성변형으로인해울퉁불퉁하게되도록표면전체를 하여다음층을적층하였다. 2.3 시험편채취위치 Ti-6Al-4V 적층물의 Interpass 적용여부에따라서 Fig. 4의모식도와같이 x, z축으로각각인장시험편을채취하였다. 3. 결과및고찰 3.1 Ti-6Al-4V 적층물외관및단면 3.1.1 Ti-6Al-4V Peening 을적용하지않고 1layer 1pass 의방법으로 70 층을적층한적층물의외관과단면을각각 Fig. 5, 116 Journal of Welding and Joining, Vol. 35, No. 2, 2017
Ti-6Al-4V 의 AM 에서기계적성질에미치는 Interpass Peening 의영향 9 (a) Top view (a) Top view (b) Front view (c) Side view Fig. 5 Appearance of Ti-6Al-4V deposition (b) Front view (c) Side view Fig. 7 Appearance of Ti-6Al-4V deposition Fig. 6 Cross section and microstructure of Ti-6Al-4V deposition Fig. 6에나타내었다. 매크로단면사진에서주상정이 z축방향으로성장한모습을확인할수있다. 위치에관계없이전체적으로 Widmanstatten α상과길쭉한 colony α상이혼합되어나타는것을확인하였다. 3.1.2 Ti-6Al-4V 1layer 1pass로 interpass 을적용한적층물의외관과단면은 Fig. 7, Fig. 8에각각나타내었다. 폭이나높이에서큰차이가없으나단면에서거대한주상정이감소되고미세화된 β grain 을확인할수있었다. 미세조직은전체적으로위치에관계없이 Widmanstatten α상이나타났다. Fig. 8 Cross section and microstructure of Ti-6Al-4V deposition 3.2 Interpass plastic working 에따른 Ti-6Al-4V 적층물의기계적성질검토 Fig. 9에전체적인항복 / 인장강도를비교하여나타내었다. 항복 / 인장강도는 x축에비하여 z축이감소하는경향이나타났고, 이는 z축으로길게성장하는주상정의영향으로판단된다. Peening 을적용하지않은적층물의 x축시험편은항복 / 인장강도모두 AWS spec. 을만족했지만, z축시험편은항복 / 인장강도가현저히감소하여 AWS spec. 에만족하지못하였다. 반면에층마다 을적용한적층물의시험편은항복 / 인장강도가기존에비하여전체적으로증가하였고, z축에서의항복 / 인장강도가 x축과비슷하게유지되어 x축과 z축시험편모두 AWS spec. 을만족하였다. Fig. 10 에나타낸연신율의경우는모든시험편이 AWS 대한용접 접합학회지제 35 권제 2 호, 2017 년 4 월 117
10 변재규 이희준 조상명 Yield/Ultimate tensile strength (MPa) 1100 1050 1000 950 900 850 800 750 700 Yield strength Ultimate tensile strength AWS Spec. Yield strength AWS Spec. Ultimate strength Fig. 9 Comparison of yield/ultimate tensile strength by deposition method and axis Yield/Ultimate tensile strength (MPa) 1100 1050 1000 950 900 850 800 750 700 Spec. Wrought minima (bars) ASTM B265-09 Yield strength Ultimate tensile strength Elongation control (horizontal) Rolled at 50 kn (horizontal) Rolled at 75 kn (horizontal) control (vertical) rolled rolled at 50 kn at 75 kn (vertical) (vertical) Fig. 11 Comparison of rolled specimens against unrolled ones (error bars indicate standard deviation), specification minima, and wrought Ti-6Al-4V 9) 25 20 15 10 5 0 Elongation 20 18 16 AWS Spec. Elongation 3.3 Interpass plastic working 에따른 Ti-6Al-4V 적층물의미세조직검토 Elongation (%) 14 12 10 8 6 4 2 0 Fig. 10 Comparison of elongation by deposition method and axis spec. 을만족하였으며 을적용하지않은 z축의시험편에서가장높은연신율값을얻을수있었다. 이는 z축의항복강도저하에의한연성증가로나타난결과라고판단된다. x축의경우는 을적용한시험편에서연신율이소폭증가하였다. Fig. 11은 Martina 등 9) 에의해 rolling 하중을 0, 50kN, 75kN 세가지조건으로적층한 Ti-6Al-4V 적층물의기계적성질을비교한그래프이다. Vertical (z-axis) 방향과 horizontal(x-axis) 방향모두 rolling 하중이증가할수록항복 / 인장강도가증가하는경향을보였다. 연신율은 horizontal 방향의경우하중 50kN에서소폭감소하였다가 75kN 에서는다시증가하였고, Vertical 방향의경우 rolling 하중이증가할수록연신율이감소하는것을볼수있다. Vertical 방향과 horizontal 방향의기계적성질을비교해보면 rolling 을하면항복 / 인장강도, 연신율이거의비슷한값을가지면서본연구의 interpass 과유사하게재료의이방성이없어지는것을볼수있다. 3.3.1 Primary β grain size 비교 Ti-6Al-4V의금속 AM에서 Alphons Anandaraj 등 10) 은 Fig. 12 와같이 interpass rolling 하중에따른미세조직을관찰하였고, Colegrove 등 11-12) 은 Table 5와같이 rolling 하중에따른 primary β grain 크기와 α lath 길이를측정하였다. Fig. 13은본연구에서 primary β grain 크기를비교하기위하여광학현미경으로촬영한 Ti-6Al-4V 적층물의미세조직이다. Table 6은 β grain 크기를나타낸것이며, Ti-6Al-4V 금속 AM시에 interpass peen- Fig. 12 Optical and SEM microstructure of as deposited Ti-6Al-4V from (a) as-built, (b) rolled at 50kN, (c) rolled at 75kN 10) Table 5 Primary grains size and alpha laths length and width by interpass rolling load 11) As-built 50kN 75kN Primary β grains 3 30mm 124μm 89μm α laths length 21.1 μm 15.5 μm 7.7 μm α laths width 1.2 μm 1.0 μm 0.7 μm 118 Journal of Welding and Joining, Vol. 35, No. 2, 2017
Ti-6Al-4V 의 AM 에서기계적성질에미치는 Interpass Peening 의영향 11 (a) Without ing을미적용하면 6*19.1mm, 적용하면 460μm로 primary β grain 의크기가현저히감소되었다. 이현상은 interpass rolling 을적용했을때 9) 와같이 interpass 을적용하면소성변형으로인하여내부에 dislocation density 가증가하게된다. 다음층을적층할때가해진열이재결정을발생시키고, 소성변형으로재료내부의저장된에너지가증가하여핵생성을더욱활발하게일으키고, primary β grain 의수가증가하여 β grain 의사이즈가감소하게된다. 3.3.2 α laths 의길이와폭비교 Fig 14는광학현미경으로 Ti-6A-4V 적층물을 800배로확대한미세조직이다. α laths 의길이와폭을측정하였고그결과를 Table 7에나타내었다. Interpass 미적용시험편에서 α laths 의길이는 25.5 μm, α laths 의폭은각각 1.4 μm으로나타났다. Interpass 적용시험편은 α laths 의길이는 14.8 μm, α laths 의폭은 0.8 μm로 interpass 을적용하면 α laths 의길이와폭이모두감소하는경향을보인다. 위의결과는 interpass rolling 의하중이증가할수록 α laths 의길이와폭이감소하는것과같은경향으로 interpass 을적용하면 primary β grain 크기가감소되고, α colony 의핵생성 site 가증가하여 α 상의성장이서로방해됨으로서 α lath 의길이와폭이모두감소되는것으로판단된다. 4. 결론 (b) With Fig. 13 Optical microstructure of Ti-6Al-4V deposition TIG welding(16x) Table 6 Primary β grain size of Ti-6Al-4V deposition interpass Without With Primary β grain size 6 19.1mm 460 μm Ti-6Al-4V 을 TIG 용접아크를열원으로한금속AM 장치로적층하여 interpass 이적층물의기계 (a) Without 적성질에미치는영향에대하여검토한결과다음과같은결론을얻었다. 1) Ti-6Al-4V의금속 AM에서 interpass plastic working 없이적층할경우 epitaxial grain growth 로인하여 z축의항복 / 인장강도는감소하게되며, 최종적층물은기계적성질의이방성이발생한다. 2) Interpass 을진행할경우 interpass rolling 과마찬가지로 z축방향의 primary β grain 크기는감소되고, α colony 의핵생성 site 가증가하며, α laths 의길이와폭이감소된다. 이로인해조직미세화로연신율의감소없이항복 / 인장강도가증가하여재료의이방성이감소하게된다. 3) 본연구의 interpass 방식은 portable 방식으로 interpass rolling 과비교하여금속 AM의현장에서좀더유연하게적용이가능할것으로판단된다. 또한곡선과다양한벽두께를가지고있는적층물의제작에서 interpass rolling 에비해 interpass 의적용이유리할것으로판단된다. 후 기 본연구는부경대학교자율창의학술연구비로연구되었습니다. References (b) With Fig. 14 Optical microstructure of Ti-6Al-4V deposition TIG welding(800 ) Table 7 α laths length and width of Ti-6Al-4V deposition interpass Without With α laths length 25.5 μm 14.8 μm α laths width 1.4 μm 0.8 μm 1. ASTM, F2792-12a, Standard Terminology for Additive Manufacturing Technologies 2. Jae-Gyu Byun, Sang-Myung Cho, Trend of Metal 3D Printing by Welding, J. of Welding and Joining, 34(4) (2016), 1-8 (in Korean) 대한용접 접합학회지제 35 권제 2 호, 2017 년 4 월 119
12 변재규 이희준 조상명 3. Kang, Min-Cheol, Dea-Hee Ye, and Geun-Ho Go, International Development Trend and Technical Issues of Metal Additive Manufacturing, J. of Welding and Joining, 34(4) (2016), 9-16 (in Korean) 4. Horii, Toshihide, Soshu Kirihara, and Yoshinari Miyamoto, Freeform fabrication of Ti-Al alloys by 3D microwelding, Intermetallics, 16(11) (2008), 1245-1249 5. Ma, Yan, et al., The effect of location on the microstructure and mechanical properties of titanium aluminides produced by additive layer manufacturing using in-situ alloying and gas tungsten arc welding, Materials Science and Engineering, A 631 (2015), 230-240 6. Baufeld, Bernd, and Omer Van der Biest, Mechanical properties of Ti-6Al-4V specimens produced by shaped metal deposition, Science and technology of advanced materials, 10(1) (2009), 015008 7. Szost, Blanka A., et al., A comparative study of additive manufacturing techniques, Residual stress and microstructural analysis of CLAD and printed Ti-6Al-4V components, Materials & Design, 89 (2016), 559-567 8. Wang, Fude, Stewart W. Williams, and M. T. Rush, Morphology investigation on direct current pulsed gas tungsten arc welded additive layer manufactured Ti6Al4V alloy, Int J Adv Manuf Technol, 57 (2011), 597-603 9. Martina, Filomeno, et al., Investigation of the benefits of plasma deposition for the additive layer manufacture of Ti-6Al-4V., Journal of Materials Processing Technology, 212(6) (2012), 1377-1386 10. Antonysamy, Alphons Anandaraj, Microstructure, texture and mechanical property evolution during additive manu facturing of Ti6Al4V alloy for aerospace applications, University of Manchester for the degree of Doctor of Philosophy in the faculty of Engineering and Physical Sciences, (2012) 11. Colegrove, Paul, and Stewart Williams, High deposition rate high quality metal additive manufacture using wire+ arc technology, (2012) 12. Donoghue, J., et al., The effectiveness of combining rolling deformation Wire-Arc Additive Manufacture on β-grain refinement and texture modification in Ti-6Al-4V, Materials Characterization, 114 (2016), 103-114 120 Journal of Welding and Joining, Vol. 35, No. 2, 2017