The purpose of this research work is to characterize and inform the design of (mechanical) property-graded bulk structures made from a single metallic alloy via a laser powder bed fusion (LPBF) process, with an end goal of creating repeatable/reproducible functionally-graded additively manufactured (FGAM) parts. This paper specifically investigates the manufacture of stainless steel (SS) 316L structures via a pulsed selective laser melting (SLM) process, and the underlying causes of property variations (within a functionally-acceptable range) through various material characterization techniques. For this, a design of experiments spanning the volumetric energy density (VED) based process parameter design space was utilized to investigate the range of functionally-acceptable physical/mechanical properties achievable in SS 316L. Five sample conditions (made via different process parameter combinations) were down-selected for in-depth microstructure analysis and mechanical/physical property characterization; these were suitably selected to impart a wide and controllable property range (209–318 HV hardness, 90–99.9% relative density, and 154–211 GPa modulus). It was observed that property variations resulted from combinations of porosity types/amounts, martensitic phase fractions, and grain sizes. Based on these findings, property-graded standard test specimens were designed and manufactured for further investigation—tensile specimens having a monotonic hardness change along its gauge length, four-point bending specimens with varying elastic moduli as a function of the distance from the neutral axis, and Moore’s rotating beam fatigue specimens with moduli variations based on the distance from the center. Altogether, this work lays the foundation for understanding and designing the local and global mechanical performance of FGAM bulk structures.

In part 1, traditional methods of tool wear characterization were qualitatively assessed, and consequently a volumetric approach of wear quantification was developed, standardized, and evaluated using a gauge R&R study. The objective of this paper is to experimentally investigate and validate this assessment methodology using the wear results from a series of controlled machining experiments on grade-5 titanium alloy. The traditionally difficult-to-machine alloy, Ti-6Al-4V, was specifically chosen as the work material in order to highlight how the use of this assessment methodology is necessitated especially because of (i) the pronounced complexities in the geometric profiles of typical cutting tools employed for machining Ti-6Al-4V and (ii) its nonconformity in behavior with standard tool wear models, such as the Taylor’s tool life model and its extensions. This assessment methodology is then validated through the simultaneous analysis and comparison of traditional flank wear and associated photomicrographs with volumetric wear and its evolution. Furthermore, the concept of the M-ratio and its derivatives are developed to quantify the efficiency of the cutting tool during each pass at a constant material removal rate (MRR).

The objective of this paper is to analyze the effect of different carbide grain sizes on the tool material volume worn away from straight tungsten–carbide–cobalt (WC–Co) turning inserts. A previously developed metrology method for assessing the tool material volume worn away from milling inserts is adapted for quantifying the volumetric tool wear (VTW) of turning inserts. Controlled turning experiments are conducted at suitable points in the feed-speed design space for two sets of uncoated inserts having different carbide grain sizes. Three levels of Ti–6Al–4V stock removal volumes (10-cm³ each) are analyzed. For each insert, the tool material volume worn away (in mm³) as well as the three-dimensional (3D) wear profile evolution is quantified after each run. Further, the specific volumetric wear rate and the M-ratio (volume of stock removed to VTW) are related to the material removal rate (MRR). The effect of carbide grain size on VTW is examined using scanning electron microscopy and elemental analysis. Finally, the inverse dependence of the M-ratio on MRR enables the definition of actual usable tool life in terms of its efficiency in removing stock, rather than being based on a tool geometry related metric.

The objective of this paper is to assess the correlation of volumetric tool wear (VTW) and wear rate of carbide tools on the material removal rate (MRR) of titanium alloys. A previously developed methodology for assessing the worn tool material volume is utilized for quantifying the VTW of carbide tools when machining Ti–6Al–4V. To capture the tool response, controlled milling experiments are conducted at suitable corner points of the recommended feed-speed design space, for constant stock material removal volumes. For each case, the tool material volume worn away, as well as the corresponding volumetric wear profile evolution in terms of a set of geometric coefficients, is quantified—these are then related to the MRR. Further, the volumetric wear rate and the M-ratio (volume of stock removed to VTW) which is a measure of the cutting tool efficiency, are related to the MRR—these provide a tool-life based optimal MRR for profitability. This work not only elevates tool wear from a 1D to 3D concept, but helps in assessing machining economics from a stock material-removal-efficiency perspective as well.