Direct metal laser sintering (DMLS) is an established technology in metal additive manufacturing. This complex manufacturing process yields unique as-built material properties that influence mechanical performance and vary with different machine parameters. Part porosity and residual stresses, which lead to part failures, and grain structure, as it relates to mechanical properties and anisotropy of DMLS parts, require investigation for different print settings. This work presents results for density, residual stress, and microstructural inspections on designed test artifacts for the benchmarking of 3D metal printers. Results from printing artifacts on two separate DMLS printer models with default parameters show highly dense parts for both printers, with relative densities above 99.5%. Characterization of residual stress through cantilevered deflection specimens indicates similar resulting thermal stresses developed in both build processes, with deflection averages of 32.48% and 28.09% for the respective machines. Additionally, properties of the test artifact printed after adjusting default machine parameters for equal energy density are characterized.

Computer simulations are increasingly being used in the automotive industry to evaluate the state of stress in cylinder blocks during casting and heat treat processes. With recent advancements, it is now possible to model casting and quenching processes as well as residual stress and high cycle fatigue. However, calculating the final stress in cylinder blocks requires the integration of several software tools with different meshing topologies, numerical methods, data structures, and post-processing capabilities. The intent of this research is to develop an integrated virtual engineering environment that combines casting simulation, computational fluid dynamics, and finite element methods in order to simulate the manufacturing process from the beginning of casting, through water quenching heat treatment, to engine dynamometer testing. The computational environment is built on three CAE tools, Magmasoft, AVL Fire, and Abaqus, and required considerable amounts of research and development to validate each numerical method and the tools that facilitate data exchange between them.

The cooling history of carburized heat-treated gears plays a significant role in developing microstructure, hardness, and residual stress in the tooth that influences the fatigue performance of the gear. Evaluating gear carburizing heat treatment should include a microstructure and hardened depth evaluation. This can be done on an actual part or with a test piece. The best practice for a test piece is to use a section size that closely approximates the cooling rate at the gear flank of the actual gear. This study furthers work already presented showing the correct test piece size that should be used for different gear modules (tooth thicknesses). Metallurgical comparisons between test pieces, actual gears, and FEA simulations are shown.

A gas quenching method was developed by DANTE Solutions, in conjunction with the U.S. Army Combat Capabilities Development Command Aviation & Missile Center (DEVCOM AvMC), to control distortion in difficult to quench geometries. This new method addresses the nonuniform cooling inherent in most gas quenching processes. A prototype unit was constructed and tested with the aim of controlling the martensite formation rate uniformity in the component being quenched. With the ability of the DANTE Controlled Gas Quenching (DCGQ) unit to control the temperature of the quench gas entering the quench chamber, thermal and phase transformation gradients are significantly reduced. This reduction in gradients yields a more uniform phase transformation, resulting in reduced and predictable distortion. Being able to minimize and predict distortion during gas quenching, post heat treatment finishing operations can be reduced or eliminated, and as such, fatigue performance can be improved. This paper will discuss the prototype unit performance. Mechanical testing and metallographic analysis were also performed on Ferrium C64 alloy steel coupons and will be discussed. The results obtained showed that the slower cooling rate provided by the prototype did not alter the microstructure, hardness, strength, ductility, toughness, or residual stress of the alloy.

Excessive distortion was observed in many small components made from 1080 steel that was neutral hardened following stamping. A study was then undertaken to determine how to reduce the distortion of the heat-treated parts while maintaining proper hardness and microstructure. A numerical simulation based on Simheat software was conducted to determine the effect of elevated temperature on the quenching oil used and its impact on distortion and microstructure. A second oil designed to operate at higher temperatures was also examined. Using Simheat software, the two oils were compared based on predicted distortion, hardness, and microstructure and the results were subsequently validated using empirical methods. It was concluded that a significant improvement in distortion could be achieved by using a different oil and higher quench temperatures.

The notion that compressive residual stresses can extend the service life of components subject to rolling contact fatigue is well documented. However, the exact nature of the relationship between effective case depth and the residual stress state is not well understood for components with case depths greater than 0.050 in. (1.27 mm). It is expected that compressive residual stresses gradually transition to tensile stresses as case depth increases beyond a threshold value. This study will measure the residual stress state of components with different case depths before and after simulated service in order to determine where the compressive to tensile transition occurs. It will also investigate the role of retained austenite and the effect of strain-induced transformation caused by rolling contact. Residual stress and retained austenite measurements will be conducted using X-ray diffraction.

Plain carbon steel cylinders were heat treated and quenched in order to study the effects of heat treating on residual stresses and microstructure. Residual stress measurements were obtained via X-ray diffraction using the sin 2 Ψ method and microstructure was characterized based on the associated peak widths. Measurements were made both at the surface and through depth following electropolishing. Triaxial stress gradients were observed in all test samples with concomitant varying microstructural characteristics. The method used to measure residual stresses in this study is typical and recommended for general practice.

Forging processes include various steps to attain favorable material properties such as heat treatment, rapid quench, cold work stress relieving, and artificial aging. These steps, however, also contribute to bulk residual stress. Excessive bulk residual stresses cause a wide of problems, including part distortion during machining and in use, reduced crack initiation life, increased crack growth rates, and an overall reduction in part life. This paper summarizes recent work aimed at measurement-based assessment of bulk residual stresses in cold-compressed aluminum die forgings. The results show that forging process induced residual stress is a repeatable phenomenon with RMS repeatability less than 5% of yield.

Undesired distortion can arise during machining of metals from two main mechanisms: 1) release of bulk residual stress in the pre-form, and 2) deformation induced by the cutting tool. The interaction between these two mechanisms is explored herein using aluminum plate-shaped samples that have a large surface with variations of bulk residual stress (BRS), where that surface is subsequently milled and we observe milling-induced residual stress (MIRS) and distortion. Plate samples are cut from two kinds of large blocks, one kind stress-relieved by stretching and a second kind that had been solution heat treated, quenched, and aged. MIRS is measured following milling using hole-drilling with fine depth increments. Distortions of thin wafers cut at the milled surfaces are used to show how the interactions between BRS and MIRS change milling-induced distortion. Data from the study show that the directions of MIRS and distortion relative to the milling direction are changed when milling in samples with high BRS magnitude (roughly ±100 MPa), with the direction of maximum curvature rotating toward or away from the milling direction depending on the sign and direction of BRS. High magnitude BRS increased distortion, nearly doubling the amount found compared to milling in samples free of BRS.

This paper provides an overview of the types of residual stress that engineers deal with and the tools and techniques available to measure it. It identifies the causes of manufacturing-induced residual stress along with their effects. It summarizes the practical aspects of a wide range of measurement methods, including hole drilling, layer removal, contour, lab XRD, and synchrotron analysis. It assesses current challenges and gaps in determining stress in relation to stress type, stress component, microstructure, specimen geometry, materials type, and measurement location.

In this work, the authors employ computer modeling to investigate a quenching process recently demonstrated at Karlsruhe Institute of Technology. A matrix of models was run to assess the effects of heat transfer and phase transformation kinetics on residual stress and microstructure in a relatively thick walled tube. The experiments at Karlsruhe were conducted using a high pressure water quench to produce martensite and residual compressive stress in the bore of a 4140 steel tube. Results show that the timing and rate of martensite formation and bainite kinetics have a significant effect on both the in-process and residual stress state.

This paper presents the preliminary results of experiments designed to mimic typical machining and thermal processing practices for aerospace titanium alloys. The most significant finding is that multiple side mill passes result in lower magnitude compressive stresses than a single side pass, which suggests that successive interactions with the milling tool serves to relieve residual stresses at the surface. The most likely mechanism for this is that Ti exhibits significant springback during machining, and multiple tool passes essentially remove the “springback” layer. Each successive removal of material allows stress relaxation in the remaining surface layer. By contrast, with only a single pass, surface residual stresses did not sufficiently relax.