14 - 18 October 2018

Bilbao, Spain

HIP and Heat Treatment of IN718-Microstructure and Mechanical Property Relationships of HIPed and Heat Treated IN718 Powder

Session Chairs

Susan Davies

James Shipley

Dr Susan Davies (Bodycote Hot Isostatic Pressing AB, United Kingdom)
Mr James Shipley (Quintus Technologies AB, Sweden)

Introduction

The encapsulation and HIP technology has developed over the years to become a high-performance, high-quality and cost-effective process forthe manufacture of a wide range of components such as simple shape billets for the machining of high performance PM HSS cutting tools, andthe manufacture of large near net shape stainless parts for the oil and gas industry. The versatility and flexibility of the HIP process, combinedwith superior material properties, make it an ideal choice for new applications, among other powder metallurgy technologies.

This Special Interest Seminar will present the influence of the postprocessing after HIP to optimize the material properties of IN718 for both traditional PM HIP applications and Additive Manufactured parts.

Presentations

Microstructure and Mechanical Property Relationship of HIPed and Heat-Treated IN718

Dr Kumar Kandasamy, Dr Olivier Messier (Oerlikon, Switzerland)

Nickel base superalloy is a class of alloys with numerous applications in harsh environment characterised by high temperature and high stresses. As such their properties have been thoroughly investigated with conventional manufacturing routes. However, the recent introduction of additive manufacturing (AM) has raised several issues. These have limited the introduction of AM components in the aerospace sector. One of the by-products of such process is the occurrence of internal defects (i.e. pores) which are detrimental for high temperature cyclic conditions.

As such hot isostatic pressing (HIP) has been introduced to alleviate and remove defects in the parts. However, most conventional HIP systems are characterised with low cooling rate which necessitate additional heat-treatment to regularise the microstructure. Recently developed novel HIP systems enable high cooling rate and can be utilised to combine HIP cycle and heat-treatment (HT) cycle.

This study aims to assess the impact of combined HIP-HT cycle on laser powder bed fusion (LPBF) parts and compared it to other HTs using a common nickel base superalloy, IN718. Microstructure and high temperature mechanical performance are used to evaluate specimens having sustained combined HIP-HT cycle and HTs cycle results.

Surface Chemistry of IN718 powder and its Changes during HIP Processing

Mr Maheswaran Vattur Sundaram (Chalmers University of Technology, Sweden)

Metal powders are characterized by the large surface area that results in high surface reactivity of the powder. This is especially important in case of complex alloys containing elements with high sensitivity to oxygen (Cr, Mn, Si, V, Zr, etc.) such as tool steels, stainless steels and especially Ni-base super-alloys. The residual surface oxides hinder the metallic bonding between the powder particles and remain as crack propagation sites. Therefore, knowledge concerning the initial state of the powder before HIP as well as oxide transformation during the HIP process is of vital importance to assure defect-free manufacturing of HIP components.

In this study, the effect of the surface oxide composition in In718 powder is being examined. The oxides present on the initial powder surface are examined by means of X-ray photoelectron spectroscopy (XPS), Auger Spectroscopy and high-resolution scanning electron microscopy (HRSEM+EDX). Results indicate that the base powder is covered by a heterogeneous surface oxide layer, formed by thin nickel oxide layer (~2 nm) with the presence of fine flake-like particulates of thermodynamically stable oxide, rich in Al. The mechanical properties of the HIPed material are studied and the results are discussed with regard to the influence of residual surface oxides on the mechanical behaviour. Based on the experimental finding and thermodynamic simulation of the oxide stability, a generic model of the oxide distribution and its changes during HIP processing is developed.

Effects of Post-Fabrication Treatments on the Microstructure and Fatigue Crack Growth Properties of Inconel 718 Manufactured by Laser Engineered Net Shaping

Dr Diana Lados, Dr Yuwei Zhai (Worcester Polytechnic Institute (WPI), USA)

Laser Engineered Net Shaping (LENS®) is an Additive Manufacturing (AM) technique that selectively deposits metallic powders fully melted by a high-energy laser beam onto a substrate. The layer-wise manufacturing method, directional cooling, and rapid solidification rates during the LENS® process often result in heterogeneous microstructures and processing defects in the fabricated materials. This hinders the application of LENS® in critical fields such as aerospace, defense, and medical, where fatigue and fracture resistance of components is imperative. Unfavorable microstructural features and processing defects can usually be modified using post-fabrication treatments. However, directly applying traditional treatments to AM materials may yield unexpected results due to their unique microstructures. This study examines and compares the effects of several heat treating and HIP procedures on the microstructures, as well as room and high temperature fatigue crack growth properties of LENS®-fabricated Inconel 718. Among the studied post-fabrication treatments, the HIP process effectively improves the fatigue crack growth resistance of this alloy by significantly reducing the porosity level and dissolving detrimental Laves phases. Detailed observations and design considerations will be systematically discussed in this manuscript from the perspective of the materials’ use in high-integrity applications.

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