Additive manufacturing of metallic components – Process, structure and properties

A review of selective laser melting of aluminum alloys: Processing, microstructure, property and developing trends

Materials for additive manufacturing

Residual Stress in Metal Additive Manufacturing

Metal additive manufacturing (AM) has garnered tremendous research and industrial interest in recent years; in the field, powder bed fusion (PBF) processing is the most common technique, with selective laser melting (SLM) dominating the landscape followed by electron beam melting (EBM). Through continued process improvements, these methods are now often capable of producing high strength parts with static strengths exceeding their conventionally manufactured counterparts. However, PBF processing also results in large and anisotropic residual stresses (RS) that can severely affect fatigue properties and result in geometric distortion. The dependence of RS formation on processing variables, material properties and part geometry has made it difficult to predict efficiently and has hindered widespread acceptance of AM techniques. Substantial investigations have been conducted with regards to RS in PBF processing, which have illuminated a number of important relationships, yet a review encompassing this information has not been available. In this review, we survey and assemble the knowledge existing in the literature regarding RS in PBF processes. A discussion of background mechanics for RS development in AM is provided along with methods of measurement, highlighting the anisotropic nature of the stress fields. We then review modeling efforts and in-process experimental measurements made to advance process understanding, followed by a thorough analysis and summary of the known relationships of both material properties and processing variables to resulting RS. The current state of knowledge and future research needs for the field are discussed.

An overview of residual stresses in metal powder bed fusion

A multiscale modeling approach for fast prediction of part distortion in selective laser melting

Selective laser melting (SLM) is a promising technology to manufacture functional (end-use) metal parts with complex geometry directly from CAD data The process induced high tensile residual stress and part distortion due to the non-uniform heat input during a SLM process would detrimentally affect the part performance However, it is extremely challenging to predict distortion of a practical SLMed part if each single track is taken into account by using the conventional modeling methods The complex multiphysics phenomenon such as fluid flow in the melt pool, phase transformation during cooling, and resulted anisotropic properties further complicate this issue In this study, a temperature-thread multiscale modeling approach has been developed to effectively predict residual stress and part distortion of a twin cantilever An equivalent body heat flux has been proposed from the microscale laser scan model and imported as the “temperature-thread” to the subsequent mesoscale layer hatch model The hatched layer is then heated up by the equivalent body heat flux and used as a basic unit to build up the macroscale part in a layer by layer fashion The thermal history and residual stress fields of the twin cantilever during the SLM process were simulated The predicted cantilever distortion agrees with the measured data with a reasonable accuracy

Efficient predictive model of part distortion and residual stress in selective laser melting

Superalloy Inconel 625 has been widely used in selective laser melting (SLM). Since SLM-induced microstructure with columnar grains, strong texture, porosity, and undesired properties, heat treatment is often used to tune the microstructure and mechanical properties. However, the microstructure evolution of IN 625 from SLM to heat treatment is poorly understood. In this study, IN 625 samples were SLMed and then heat treated at elevated temperatures. Microstructure evolution characteristics of the processed IN 625 alloy have been characterized using optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction, and micro indentation. Fine dendrite microstructures with strong texture parallel to layer build-up direction was observed in the as-SLM samples due to rapid cooling and epitaxial growth. High dislocation density and high microhardness were found in the γ matrix which also contains high Z-contrast precipitates. After annealing at high temperatures, random grain growth accompanied by dislocation annihilation and twinning occurs. The decrease in lattice parameter and the prevalence of large grain boundary misorientation in the γ matrix suggests that SLM-induced residual stress be significantly reduced. In addition, the uncertainty of mechanical properties due to the process variations was quantified.

C. Li

Papers

Residual Stress in Metal Additive Manufacturing

A multiscale modeling approach for fast prediction of part distortion in selective laser melting

Efficient predictive model of part distortion and residual stress in selective laser melting

Microstructure evolution characteristics of Inconel 625 alloy from selective laser melting to heat treatment

Prediction of Residual Stress and Part Distortion in Selective Laser Melting