Metal additive manufacturing technologies, such as powder bed fusion process, directed energy deposition (DED) process, sheet lamination process, etc., are one of promising flexible manufacturing technologies due to direct fabrication characteristics of a metallic freeform with a three-dimensional shape from computer aided design data. DED processes can create an arbitrary shape on even and uneven substrates through line-by-line deposition of a metallic material. Theses DED processes can easily fabricate a heterogeneous material with desired properties and characteristics via successive and simultaneous depositions of different materials. In addition, a hybrid process combining DED with different manufacturing processes can be conveniently developed. Hence, researches on the DED processes have been steadily increased in recent years. This paper reviewed recent research trends of DED processes and their applications. Principles, key technologies and the state-of-the art related to the development of process and system, the optimization of deposition conditions and the application of DED process were discussed. Finally, future research issues and opportunities of the DED process were identified.

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Directed Energy Deposition (DED) Process: State of the Art

Defect formation is a critical challenge for powder-based metal additive manufacturing (AM). Current understanding on the three important issues including formation mechanism, influence and control method of metal AM defects should be updated. In this review paper, multi-scale defects in AMed metals and alloys are identified and for the first time classified into three categories, including geometry related, surface integrity related and microstructural defects. In particular, the microstructural defects are further divided into internal cracks and pores, textured columnar grains, compositional defects and dislocation cells. The root causes of the multi-scale defects are discussed. The key factors that affect the defect formation are identified and analyzed. The detection methods and modeling of the multi-scale defects are briefly introduced. The effects of the multi-scale defects on the mechanical properties especially for tensile properties and fatigue performance of AMed metallic components are reviewed. Various control and mitigation methods for the corresponding defects, include process parameter control, post processing, alloy design and hybrid AM techniques, are summarized and discussed. From research aspect, current research gaps and future prospects from three important aspects of the multi-scale AM defects are identified and delineated. 

Multi-scale defects in powder-based additively manufactured metals and alloys

Topology-optimized lattice structures with simultaneously high stiffness and light weight fabricated by selective laser melting: Design, manufacturing and characterization

Additive manufacturing (AM) can be seen as a disruptive process that builds complex components layer upon layer. Two of its distinct technologies are Selective Laser Melting (SLM) and Electron Beam Melting (EBM), which are powder bed fusion processes that create metallic parts with the aid of a beam source. One of the most studied and manufactured superalloys in metal AM is the Ti–6Al–4V, which can be applied in the aerospace field due to its low density and high melting point, and in the biomedical area owing to its high corrosion resistance and excellent biocompatibility when in contact with tissues or bones of the human body. The research novelty of this work is the aggregation of all kinds of data from the last 20 years of investigation about Ti–6Al–4V parts manufactured via SLM and EBM, namely information related to residual stresses (RS), as well as the influence played by different heat treatments in reducing porosity and increasing mechanical properties. Throughout the report, it can be seen that the expected microstructure of the Ti–6Al–4V alloy is different in both manufacturing processes, mainly due to the distinct cooling rates. However, heat treatments can modify the microstructure, reduce RS, and increase the ductility, fatigue life, and hardness of the components. Furthermore, distinct post-treatments can induce compressive RS on the part’s surface, consequently enhancing the fatigue life.

A Review of Heat Treatments on Improving the Quality and Residual Stresses of the Ti–6Al–4V Parts Produced by Additive Manufacturing

High cycle fatigue performance of AlSi10mg alloy produced by selective laser melting

This work presents an analytical modeling methodology for the prediction of part distortion in powder bed metal additive manufacturing (PBMAM). The presented model consists of analytical thermal modeling, thermal stress modeling, residual stress modeling, and distortion modeling. It has promising short computational efficiency without resorting to finite element analysis or any iteration-based simulations. The temperature profile is calculated using a moving point heat source solution and heat sink solution with consideration of heat input from a moving laser and heat loss from boundary heat transfer. The thermal stress is calculated from the temperature calculation using a thermal stress model considering thermal load, surface tension, and hydrostatic pressure. The residual stress is calculated from the thermal stress calculation using an elastoplastic relaxation procedure. The residual stress–induced part distortion is finally calculated from the calculated residual stress and residual strain using a surface displacement model. The calculated part distortion was validated to experimental measurement on a twin-cantilever part produced by PBMAM of Ti6Al4V. Close agreements were observed. The computational time was recorded less than 10 s. The high predictive accuracy and high computational efficiency allow the process parameter planning for distortion control and elimination through inverse analysis.

Analytical modeling of part distortion in metal additive manufacturing

Existence of high tensile residual stress in the additively manufactured parts result in part failure due to crack initiation and propagation. Herein, a physics-based analytical model is proposed to predict the stress distribution much faster than experimentation and numerical methods. A moving point heat source approach is used to predict the in-process temperature field within the build part. Thermal stresses induced by steep temperature gradient is determined using the Green’s functions of stresses due to the point body load in a homogeneous semi-infinite medium. Then, both the in-plane and out of plane residual stress distributions are found from incremental plasticity and kinematic hardening behavior of the metal, in coupling with the equilibrium and compatibility conditions. Due to the steep temperature gradient in this process, material properties vary significantly. Hence, material properties are considered temperature dependent. Moreover, the specific heat is modified to include the latent heat of fusion required for the phase change. Furthermore, the multi-layer and multi-scan aspects of the direct metal deposition process are considered by incorporating the temperature history from the layers and scans. Results from the analytical residual stress model showed good agreement with X-ray diffraction measurements, which is used to determine the residual stresses in the IN718 specimens.

James R. Dobbs

Papers

Analytical modeling of part distortion in metal additive manufacturing

Analytical modeling of residual stress in direct metal deposition considering scan strategy

Analytical mechanics modeling of in-process thermal stress distribution in metal additive manufacturing