Microstructure and mechanical properties of Nd:YAG laser welded Invar 36 alloy. Yue Zhao. 1, a. , Aiping Wu. 1. , Wei Yao. 2. , Zhimin Wang. 2. , Yutaka S. Sato.
Materials Science Forum Vols. 675-677 (2011) pp 739-742 Online available since 2011/Feb/21 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.675-677.739
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Invar 36 alloy is increasingly used as a structural material for manufacture of liquefied natural gas (LNG) transporters and storage tanks. However, the conventional arc welding of Invar 36 alloy has high susceptibility of hot-cracking. As a high-energy-beam welding process, laser welding could be effective for producing defect-free Invar 36 weld. In the present study, defect-free Invar 36 weld was successfully produced by Nd:YAG laser welding. The microstructure and mechanical properties of weld were also tested.
Invar 36 alloys, which also named Fe-36wt.%Ni alloy, exhibits very low thermal expansion coefficient (TEC) below its Curie temperature and excellent mechanical properties in cryogenic environment. It’s usually used as highly reliable and precision materials. Especially with the development of energy industry, Invar 36 alloy is increasingly applied as one of the vital structural materials for LNG ship, storage tank and pipeline [1]. It has been reported that the conventional arc welding of Invar 36 alloy often causes problems like hot-cracking, reheat cracking and porosity [2,3], due to the excessive high heat input introduced by arc welding. Laser welding is a high-energy-beam welding, which can effectively prevent the problems associated with conventional high-heat-input arc welding [4]. Therefore, laser welding could be an effective welding method to produce defect-free weld of Invar alloys. There have been few reports on laser welding of Invar alloy. Wu and his colleagues [5] reported laser welding of 0.85 mm-thick Invar 36 sheet. They studied the effect of parameters on the weld formation and tested the mechanical properties of laser weld. However, the report of microstructure and relationship between microstructure and mechanical properties of laser welded invar alloy is insufficient. In the present study, after investigating the feasibility of laser welding for 3mm-thick Invar 36 alloy, the microstructure, mechanical properties of the welds and relationship between the microstructure and mechanical properties were also examined.
The base material used in this study was 3mm-thick Invar 36 alloy sheet. Chemical composition (wt.%) of the base material is Fe-36.0Ni-0.3Mn-0.19Si-0.03C-0.001S-0.002P. The bead-on-plate laser welding was performed on a Nd:YAG industry laser welding machine. Rated power of the laser generator is 4000 W, and the spot diameter of welding laser is 0.4 mm. All welding processes were shielded by high-purity Argon gas. The defocusing amount of laser beam was 0 mm, i.e., the laser beam was focused on the surface of plate. The laser beam power was varied between 1800W and 2800W, and the welding speed was varied between 1.2 m/min and 1.5 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 166.111.49.64-08/08/11,09:27:30)
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m/min. In this paper, the weld produced by a laser beam power of A W and welding speed of B m/min is expressed as “A W, B m/min weld”. Defect of weld was checked by an X-ray inspection system. Microstructure was characterized by optical microscopy (OM) and Joel 6301 field emission scanning electronic microscopy (SEM). Specimens for OM and SEM were cut perpendicular to the welding direction, mechanically polished with 3µm and 1 µm diamond paste respectively, and then chemically etched in a 60vol% nitric acid + 40vol% acetic acid solution. Vickers hardness test and transverse tensile test were employed to examine mechanical properties of weld. Vickers hardness profile was measured with a load of 1 kN and a dwell time of 10 s on the cross section of weld. The transverse tensile specimens were cut perpendicular to the welding direction, and dimension of the transverse tensile specimen is shown in Fig.1.
Welding speed and laser beam power were controlled to acquire defect-free weld. The present parameter window and formation of the welds are shown in Fig.2. Compared with welding speed, laser beam power had stronger influence to the formation of welds in the present study. Relatively low power (below 2000W) easily led to porosity problem. As shown in Fig.3, the porosities always had irregular shape, and scour mark were observed on the surface of porosities. Consequently, it can be inferred that these porosities were produced by landslide of unstable keyhole during laser welding process [6], i.e. keyhole-induced porosity. Keyhole-induced porosity could be prevented by using proper welding parameter, i.e. relatively higher power in the present study. While, too high power (over 2800W) resulted in spatter during welding process and excessive-penetrated weld. Defect-free welds were produced by the power between 2250W to 2500W and the welding speed between 1.2 to 1.5m/min. The heat input to acquire defect-free weld was between 940J/cm and 1250J/cm.
Fig.1 Dimension of the transverse tensile specimen
Fig.2 Parameter window and formation of the welds
Cross-sectional overview of the defect-free weld was shown in Fig.4 (since similar with other defect-free welds, the results of 2250, 1.2m/min weld were applied in this part). Neither crack nor porosity was observed in the weld. The Invar 36 laser weld could be clarified to three regions: fusion zone, heat-affected zone and base material. The fusion zone was clearly visible around the weld beam center. There was no distinct boundary between heat-affected zone and base material.
Materials Science Forum Vols. 675-677
Fig.3 Porosity in 1800W, 1.2m/min weld
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Fig.4 Typical cross-sectional overview of defect-free weld
Optical micrographs of the base material, fusion zone and border region of the fusion zone were shown in Fig.5. The base material was seen to have a single austenitic equiaxed microstructure with an average grain size of approximately 10µm. The heat-affected zone contained austenitic equiaxed microstructure with obviously coarse grains than the base material. Average grain size of the heat-affected zone was about 100µm. The fusion zone contained austenitic columnar microstructure with the average grain size in lengthwise direction over 300µm. (a)
(b)
(c)
Fig.5 Optical micrographs of (a) base material, (b) fusion zone and (c) border region of fusion zone
Regular-shape pits were found in the fusion zone of Invar 36 laser welds, as shown in Fig.6. Most of these pits existed in the sub-grain boundary in the fusion zone with average size of approximately around 2µm. These pits should be traces of some kind of particle produced by laser welding process. However, no related report of precipitate in Invar 36 laser weld was found.
Fig.6 Regular-shape pits in fusion zone (should be the traces of precipitates)
Vickers hardness profiles of the base material and one defect-free weld were shown in Fig.7. The distribution of hardness in fusion zone and heat-affected zone is homogenous. The average hardness of fusion zone (130Hv) is 83% of that of base material (157Hv). Reduction of hardness could be attributed to the grain growth in fusion zone, which is corresponding with the grain boundary strengthening theory. Transverse tensile properties of the base material and two defect-free welds were shown in Fig.8. All welds were broken in the fusion zone during tensile test, because the fusion zone has the lowest hardness in the weld. Welding parameters have no obvious effect on tensile properties in these two welds. Average ultimate tensile strength and yield strength of the welds are 440MPa and 300MPa, respectively, which are 85% and 87% of those of the base material, respectively. The decline of ultimate tensile strength and yield strength are also resulted by the bigger grain size in the
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weld. The transverse tensile elongation of weld is 12%, which is only 36% of that of base material. The welds exhibit lower elongation than that of base material, because the fusion zone preferentially yield and then failed during transverse tensile test, i.e., the inhomogeneity of deformation of the weld causes lower elongation of the weld. There is no evidence indicating any effect of the precipitates on the mechanical properties in fusion zone.
Fig.7 Vickers hardness profiles of the base material and 2250W, 1.2m/min defect-free weld
Fig.8 Transverse tensile properties of the base material and two defect-free welds
1) Defect-free welds could be produced in 3mm-thick Invar 36 alloy at the welding speed between 1.2 and 1.5m/min and laser beam power between 2250 to 2500W. Improper welding parameters tended to lead problems of spatter or porosity in the fusion zone. 2) The base material was consisted of single austenitic equiaxed grains, while the fusion zone exhibited relatively coarser austenitic columnar grains. Small amount of rhomb-shape precipitates were found in the grain boundary of fusion zone. 3) For the defect-free welds, average hardness of the fusion zone is 130Hv, which is 83% of that of the base material. Average ultimate tensile strength, yield strength and elongation acquired by transverse tensile test are 85%, 87% and 36% of those of the base material, respectively. Different laser welding parameters have no obvious effect on mechanical properties. The grain growth in fusion zone cause the reduction of the mechanical properties in the fusion zone
[1] M. Shiga, Current Opinion In Solid State & Materials Science 1 (1996), p.340. [2] J. L. Corbacho, J. C. Suarez, and F. Molleda, Materials Characterization 41 (1998), p.27. [3] T. Ogawa, Welding Journal 65 (1986), p. s213 [4] K.Y. Benyounis, A.G. Olabi, and M.S.J. Hashmi, Journal of Materials Processing Technology 164-165 (2005), p.978 [5] D. Wu, B. Yin, Q. Zhou, X. Wang, and Z. Jin, Optics and Precision Engineering 17 (2009), p.557. [6] X. Zhang, Study on Formation and Prevention of Keyhole-induced Porosity, Doctoral thesis, Tsinghua University, ID 2110302004210292 (2006).
Advanced Material Science and Technology doi:10.4028/www.scientific.net/MSF.675-677 Microstructure and Mechanical Properties of Nd:YAG Laser Welded Invar 36 Alloy doi:10.4028/www.scientific.net/MSF.675-677.739
