Microstructure and mechanical properties of molybdenum-titanium-zirconium-carbon alloy TZM processed via laser powder-bed fusion

https://doi.org/10.1016/j.ijrmhm.2020.105369Get rights and content

Highlights

  • TZM processed by LPBF for the first time.

  • High density of 99.7% achieved.

  • Avoidance of cracks by grain boundaries free of oxygen.

  • Oxygen bound by zirconium as ZrO2.

  • Oxygen dissolved in ternary molybdenum‑titanium carbide.

Abstract

Molybdenum, processed by laser powder-bed fusion (LPBF), is susceptible to hot cracking because segregated oxygen impurities significantly weaken grain boundaries through the formation of MoO2. The present study reports on the LPBF processing of the most important molybdenum alloy TZM, whose alloying elements—titanium, zirconium, and carbon—lead to particle and solid solution strengthening. Results of investigations into the resulting microstructure and mechanical properties when processing TZM by LPBF are presented. The alloying elements suppress the segregation of oxygen to the grain boundaries so that crack-free samples with a density of 99.7 ± 0.3% may be produced. The microstructure shows grains that are columnar due to epitaxial grain growth and a weak 〈111〉 fiber texture parallel to the building direction. Mo2C and ZrO2 particles with a size of <50 nm are precipitated in the grain interior. Oxygen is not only bound by ZrO2. Research has shown that the ternary molybdenum‑titanium carbide, which can be found in LPBF – TZM – as is the case with pressed/sintered TZM—can dissolve oxygen. While the bending strength is 591 ± 26 MPa for samples in which only pores with a diameter of <50 μm could be detected on the fracture surface, the bending strength drops to 267 ± 50 MPa for samples with defects of 400 μm. In both cases, the fracture mode is transgranular brittle.

Section snippets

State of the art

The successful use of materials in high-temperature applications – for example, in the aerospace and medical industries or for metal working tools – requires materials that have excellent high-temperature properties. Molybdenum has a high melting point, a low thermal expansion coefficient, a high thermal conductivity, and an excellent high temperature strength and creep resistance [1]. The molybdenum alloy TZM has demonstrated its potential for use in applications where robust high-temperature

Experimental procedure

Spherical, pre-alloyed TZM powder with metallic purity >99.9% produced by gas atomization was used in this study. The powder was fractioned by sieving with a mesh size of 63 μm. The particle size was measured using a MALVERN MASTERSIZER 3000 laser diffraction particle-size analyzer; the carbon content was measured by applying the combustion method using an LECO CS-230; the oxygen, hydrogen, and nitrogen contents were measured by carrier gas hot extraction using an LECO TC-500; and the titanium

Macro- and microstructure

Fig. 1 shows the morphology of the spherical, gas atomized TZM powder. Several particles exhibit satellites or are irregularly shaped. The powders' physical and chemical properties are listed in Table 2. The contents of the alloying elements titanium, zirconium, and carbon in the powder are 5045 ± 29 μg/g, 861 ± 23 μg/g, and 257 ± 2 μg/g, respectively. The gas contents in the powder are 388 ± 36 μg/g for oxygen, < 5 μg/g for nitrogen, and < 1 μg/g for hydrogen.

Table 3 shows the density and

Conclusions

The present study constitutes the first detailed analysis of processing TZM via LPBF. Dense and crack-free samples were produced. The following conclusions are drawn:

  • TZM was produced via LPBF with a density of 99.7% of the theoretical density and a crack-free microstructure.

  • Epitaxial grain growth leads to elongated grains toward the BD with an aspect ratio of 0.4 ± 0.17. A weak <111> fiber texture parallel to the BD is formed.

  • Mo2C precipitates are found in the grain interior.

  • Oxygen impurities

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank the Austrian Research Promotion Agency (FFG) for supporting the research [DB-NR: 235364].

References (28)

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