Process Optimization Strategies for Achieving Microstructural Control and Enhanced Mechanical Performance in Advanced High Strength Steels

Abstract

Strategies were developed for processing advanced high strength steels (AHSSs) via severe plastic deformation (equal channel angular pressing, ECAP) and additive manufacturing (directed energy deposition, DED) techniques, with the goal of producing high performance materials via precise microstructural control. For ECAP process optimization, processing schemes were designed that produced significant grain refinement, resulting in enhanced strength and toughness combinations, while avoiding workpiece fracture due to high amounts of imposed strain for both AF9628 and Fe-Mn-Al-C. ECAP at elevated temperatures (> 950°C) significantly improved ultimate tensile strengths (σUTS) (AF9628: σUTS > 2.0 GPa; Fe-Mn-Al-C: σUTS > 1.6 GPa) and Charpy (-40°C) impact toughness energies (AF9628: > 55 J; Fe-Mn-Al-C: > 200 J).

For DED process optimization, two approaches were applied. First, a design of experiments (DOE) approach printing 316L was developed which broadly sampled the DED parameter space and identified optimal levels and ranges for them. Second, an empirically based DED process optimization approach was also applied to an AHSS, AF9628, following a parametric sampling of the power, speed, mass flow rate, and focal height spaces. Both approaches ultimately succeeded, producing high strength and respectable ductility (εf) (316L: σUTS > 600 MPa, εf > 30%; AF9628: σUTS > 1.2 GPa, εf > 10%) from fully dense (ρ > 99%) builds. AF9628’s σUTS was lower in DED builds than in the wrought product due to elemental evaporation losses caused by the high energy densities.

In depth microstructural and mechanical characterization of both AHSSs, AF9628 and Fe-Mn-Al-C, was also done. In AF9628, effects of hierarchical microstructures – from the prior austenite phase to the martensite transformation to the precipitation of carbides –on mechanical performance were investigated in depth. Likewise, in ECAP Fe-Mn-Al-C, effects of grain refinement and chemical segregation on strength, toughness, and anisotropy were assessed. Overall, the process optimization strategies for ECAP and DED of AHSSs take advantage of microstructural control in terms of maximizing grain refinement or minimizing porosity, respectively. These findings and processing strategies were developed to be easily applied to other high strength materials as well, and therefore they should propel the future development of high-performance alloys to enhanced performance.