The influence of building direction and conventional heat treatment on the microstructure, physical, and mechanical characteristics of direct metal laser sintered Inconel 718 alloy

ABSTRACT

DMLS fabricated IN718 alloy samples with various build orientations are analysed with heat-treatment dependent mechanical and microstructural properties. The multiple heating and cooling cycles formed a complex micro-heterogeneous microstructure composed of columnar and equiaxed subgrains in the alloy. Solution treatment resulted in a partially dissolved δ phase, and the metastable γ” phase while ageing treatment eliminated the dendritic substructure. In contrast, δ-phases precipitated mainly in the interdendritic regions, and significantly eliminated the micro-segregation and Laves phases in the γ matrix after homogenised (HSA). . The tensile test showed that tensile strength decreased with increasing building orientation but ductility increased. Heat-treatment significantly improved the microhardness, ultimate tensile strength, and yield strength compared to the as-fabricated samples, but their ductility decreased. The tensile properties recorded an improvement of nearly 31.32% as compared to 0° as-built and 0° heat-treated direct aged samples. The higher microhardness (507.25 HV) was observed for the direct aged 0° build samples. The microhardness values were increased by nearly 45% after heat-treatment comparing to as-build 0° and 0° DA treated samples. The density was observed to be about 99.52% of the as-built samples, but performing the heat-treatment, no significant improvement in the density was observed and it was found similar to the as-fabricated sample.

KEYWORDS:

• Additive manufacturing
• direct metal laser sintering
• inconel 718 alloy
• microstructure
• mechanical and physical properties

Acknowledgments

Sample fabrication was done at the central tool room in Bhubaneswar, heat-treatment at NIT and BIT Patna, tensile testing at NIT Patna, Microhardness MNNIT Allahabad, and IIT Patna, EDS, and SEM tests at IIT Patna, XRD at AKU Patna.

Disclosure statement

The authors declare that they have no known competing financial interests

Important abbreviation

DMLS Direct metal laser sintered

AB as-build, as-manufactured, as-fabricated, as-printed

AM Additive manufacturing

AMS Aerospace material specifications

BCC Body centred cubic

CS Current study

D Diagonal/diagonally

DA Direct ageing

El Elongation

EDS Energy-dispersive X-Ray spectroscopy

FCC Face centred cubic

H Horizontal/horizontally

HAZ Heat affected zone

HIP Hot isostatic pressing

H₂O₂ Hydrogen peroxide

HCL Hydrochloric acid

HSA Homogenization solution ageing

HT Heat-treatment/treated

HV Hardness value

IN718 Inconel 718

SA Solution ageing

SEM Scanning electron microscope

SLM Selective laser melting

UTS Ultimate tensile strength

V Vertical/vertically

W Wrought

YS Yield strength