Characterization of Solidification Microstructures in Vacuum Arc Remelted Nickel Alloy 718

Thomas A Ivanoff,Eric M Taleff,Trevor J Watt

doi:10.1007/s11663-019-01530-2

Abstract

Digital image analysis techniques were developed to autonomously characterize dendritic solidification features in microstructures of a nickel alloy 718 ingot produced by vacuum arc remelting (VAR). Automated macrophotography was implemented to image microstructures and produce large image montages of etched ingot cross sections. Two image analysis techniques, particle identification and two-point correlation, were applied to autonomously measure primary dendrite arm orientation and secondary dendrite arm spacing from the image montages. Particle identification measured individual dendritic features. The two-point correlation technique measured averaged feature values over defined image areas. These methods are described and compared. Melt pool profile and solidification time histories were reconstructed from primary dendrite arm orientation and secondary dendrite arm spacing measurements, respectively. These characterization techniques provide two new methods of reconstructing melt pool profiles and solidification times in cast ingots. The information produced is expected to be useful in the validation of computational models for solidification during VAR, similar remelting processes, and general casting processes that produce dendritic solidification features.

Highlights

This study proposes the use of microstructural features intrinsic to dendritic solidification to determine melt pool profiles and local solidification times as an alternative to marking techniques
Solidification microstructures in two slabs extracted from the midline cross-section of a nickel alloy 718 ingot produced by vacuum arc remelting (VAR) were imaged using automated macrophotography
A digital image montage was created for the prepared surface of each slab, covering its entire area

Summary

Introduction

CAST-AND-WROUGHT nickel alloy 718 is the most used superalloy in the aerospace industry[1,2,3] because of its useful combination of mechanical properties[4,5,6,7,8,9,10,11] and manufacturability.[9,11,12] If processed appropriately, this material can exhibit good creep strength, oxidation resistance, and microstructural stability up to 650 °C.[4,5,8,10] These mechanical properties are derived from a uniform distribution of coherent c¢¢ (Ni3Nb) intermetallic precipitates distributed throughout the c-nickel lattice.[4,5,13,14] Formation of the c¢¢ phase is promoted by niobium in alloy 718; the nominal composition of alloy 718 is provided in Table I.[5]. Defects formed during LMP often cannot be remediated by subsequent processing steps such as homogenization and mechanical deformation.[15,16,17,18,19,20,21] Some defects can cause non-uniform

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Discussion

Conclusion