Microstructure evolution and mechanical properties of laser metal deposition of Invar 36 alloy

https://doi.org/10.1016/j.optlastec.2019.106037Get rights and content

Highlights

  • Fully dense Invar 36 alloy specimens were fabricated by LMD.

  • Microstructure evolution of two laser scanning patterns were investigated.

  • Phases of LMD Invar 36 alloys were detailed.

  • Mechanical properties better than forged components were obtained by LMD.

Abstract

Laser metal deposition (LMD) technology was used to fabricate single-track-multi-layers samples using Invar 36 alloy powder by two different deposition patterns: one-way laser scanning pattern and back and forth laser scanning pattern. The microstructure characteristics, grain morphology, formation mechanism and mechanical properties of the samples fabricated in two different deposition patterns were systematically investigated. It was shown that there was a thin layer of equiaxed grains in the top zone, an alternative arrayed grain morphology of columnar and cellular grains in the middle and periodic circular growth of small columnar grains at the bottom, which was generated by different temperature gradient in different location. The crystal orientation of columnar grains was distinct due to different deposition patterns. X-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS) showed that the deposited specimen was composed of austenite-single phase. Samples built by back and forth laser scanning pattern showed higher microhardness and tensile strength but lower elongation compared with samples built by one-way pattern, the horizontally built sample showed higher Ultimate tensile strength and yield strength but lower elongation compared with vertically built sample in the same laser scanning pattern. Laser metal deposited Invar 36 showed more excellent mechanical properties compared to the forged and casted counterparts.

Introduction

Invar alloy, which is based on Fe-(64–36 wt%) Ni, is considered as a revolution in cryogenic material since it was found in 1896, this type of alloy is characterized by low coefficient of thermal expansion (CTE) below its Curie temperature and excellent mechanical properties in cryogenic environment due to its 36% nickel containing [1], [2], [3], [4], [5]. Based on these features, Invar alloy is widely applied to the components where high dimensional stability is required. The applications include precision instruments, wireless radio tubes, optical gauges, seismic creep gauges, satellite orbit and television shadow mask-frames, etc. Nevertheless, the anisotropy of the coefficient of thermal expansion manufactured by cold deformation machining leads to inefficiency and high expenditure. Furthermore, the low volume-produce of Invar 36 alloy for complex and precise shapes also make it a particular challenge for manufacturing Invar 36 alloy [6].

Laser metal deposition (LMD) technology is widely used to fabricate some alloy materials due to the fine grain morphology of the deposition layer and low dilution rate in manufacturing process [7], [8]. There are more and more experimental researches on LMD in many universal materials such as titanium and its alloy [9], [10], [11], aluminium and its alloy [12] and stainless steel [13], etc. Researches on LMD of Invar 36 alloy were reported less than other universal materials. Zhan et al. [14] carried out quantitative research on microstructure and thermal physical mechanism in laser metal deposition for Invar alloy. The results showed that the epitaxial columnar grains could be found in the bottom of the deposition layers, which consisted of elongate cellular crystals with different orientation and regular cellular crystals. The complicated microstructure occurred at the top of the deposition layer were filled with regular cellular crystals. Qiu et al. [15] investigated the microstructure and mechanical properties of selective laser melting of Invar 36 alloy, they found that the as-fabricated microstructure of Invar 36 was dominated by columnar γ grains decorated by α precipitates. The low CTE materials such as Invar 36 alloy were considered as suitable powder materials for LMD [16], [17], although there were some research on single-track LMD Invar alloy, the structure integrity, evolution of microstructure especially mechanical properties of LMD Invar 36 alloy are still not well understood.

In this paper, a systematic study was conducted to investigate the microstructure and mechanical properties of LMD Invar36 alloy, the experiment indicated that the microstructure evolution of Invar36 alloy fabricated by LMD is complicated. Two distinct laser scanning patterns were conducted during the deposition process, and the microstructure differences between the two deposition patterns were revealed and the microhardness and tensile test were carried out to explore the principle of mechanical properties.

Section snippets

Experiment

The LMD experiment system was shown in Fig. 1, which mainly employed a YC52 coaxial laser cladding nozzle with cooling system and protection system (See Fig. 1 (a)) (Precitec Inc.), a MF-PF2/2 powder feeder (GTV Inc.) with powder feeding accuracy of 0.1 g/min (See Fig. 1 (b)); an IPG YLS-6000 fiber laser system with the maximum output laser power of 6000 W and fiber diameter of 600 nm (IPG Inc.) (See Fig. 1 (c)); Fig. 2 showed the schematic diagram of LMD experiment. The laser beam irradiated

Macrostructure

The macrostructure of LMD Fe-36Ni Invar alloy sample with back and forth laser scanning pattern is shown in Fig. 5, the equiaxed zone with height of about 250 µm could be observed on the top of the deposited sample, which could also be observed in other materials [18], [19]. Analogous grain morphology of columnar grains in the middle and bottom part of the deposited sample were observed, but the size of the columnar grains gradually decreased from the top to the bottom zone, small columnar

Conclusions

The characteristics of grain morphology, microstructure and mechanical properties of laser metal deposited invar 36 were systematic investigated in this paper, and the main findings of the research were summarized as follows:

  • 1.

    Fully dense Invar36 alloy specimens were fabricated by laser metal deposition process. There was a layer of equiaxed grain on the top of the sample, alternative arrayed columnar grains in the middle and periodic circular growth of columnar grains on the bottom. A re-melting

CRediT authorship contribution statement

Hang Li: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Supervision, Writing - original draft, Writing - review & editing. Bo Chen: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Validation, Writing - review & editing. Caiwang Tan: Funding acquisition, Project administration, Resources, Validation. Xiaoguo Song: Funding acquisition, Project administration, Resources, Validation. Jicai Feng: Funding acquisition,

Declaration of Competing Interest

The authors declared that they have no conflicts of interest to this work.We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

This work was supported by the National Key R&D Program of China, China under the Grant (No. 2018YFB1107900), Shandong Provincial Natural Science Foundation, China under the Grant (No. ZR2017MEE042), Shandong Provincial Key Research and Development Program, China under the Grant (No. 2018GGX103026).

References (29)

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