Additive manufacturing (AM), widely known as 3D printing, is a method of manufacturing that forms parts from powder, wire or sheets in a process that proceeds layer by layer. Many techniques (using many different names) have been developed to accomplish this via melting or solid-state joining. In this review, these techniques for producing metal parts are explored, with a focus on the science of metal AM: processing defects, heat transfer, solidification, solid-state precipitation, mechanical properties and post-processing metallurgy. The various metal AM techniques are compared, with analysis of the strengths and limitations of each. Only a few alloys have been developed for commercial production, but recent efforts are presented as a path for the ongoing development of new materials for AM processes.

The metallurgy and processing science of metal additive manufacturing

Wire-feed additive manufacturing (AM) is a promising alternative to traditional subtractive manufacturing for fabricating large expensive metal components with complex geometry. The current research focus on wire-feed AM is trying to produce complex-shaped functional metal components with good geometry accuracy, surface finish and material property to meet the demanding requirements from aerospace, automotive and rapid tooling industry. Wire-feed AM processes generally involve high residual stresses and distortions due to the excessive heat input and high deposition rate. The influences of process conditions, such as energy input, wire-feed rate, welding speed, deposition pattern and deposition sequences, etc., on thermal history and resultant residual stresses of AM-processed components needs to be further understood. In addition, poor accuracy and surface finish of the process limit the applications of wire-feed AM technology. In this paper, after an introduction of various wire-feed AM technologies and its characteristics, an in depth review of various process aspects of wire-feed AM, including quality and accuracy of wire-feed AM processed components, will be presented. The overall objective is to identify the current challenges for wire-feed AM as well as point out the future research direction.

Wire-feed additive manufacturing of metal components: technologies, developments and future interests

Laser-based additive manufacturing (LBAM) processes can be utilized to generate functional parts (or prototypes) from the ground-up via layer-wise cladding – providing an opportunity to generate complex-shaped, functionally graded or custom-tailored parts that can be utilized for a variety of engineering applications. Directed Energy Deposition (DED), utilizes a concentrated heat source, which may be a laser or electron beam, with in situ delivery of powder- or wire-shaped material for subsequent melting to accomplish layer-by-layer part fabrication or single-to-multi layer cladding/repair. Direct Laser Deposition (DLD), a form of DED, has been investigated heavily in the last several years as it provides the potential to (i) rapidly prototype metallic parts, (ii) produce complex and customized parts, (iii) clad/repair precious metallic components and (iv) manufacture/repair in remote or logistically weak locations. DLD and Powder Bed Fusion-Laser (PBF-L) are two common LBAM processes for additive metal part fabrication and are currently demonstrating their ability to revolutionize the manufacturing industry; breaking barriers imposed via traditional, ‘subtractive’ metalworking processes. This article provides an overview of the major advancements, challenges and physical attributes related to DLD, and is one of two Parts focused specifically on DLD. Part I (this article) focuses on describing the thermal/fluidic phenomena during the powder-fed DLD process, while Part II focuses on the mechanical properties and microstructure of parts manufactured via DLD. In this current article, a selection of recent research efforts – including methodology, models and experimental results – will be provided in order to educate the reader of the thermal/fluidic processes that occur during DLD, as well as providing important background information relevant to DLD as a whole. The thermal/fluid phenomena inherent to DLD directly influence the solidification heat transfer which thus impacts the part's microstructure and associated thermo-mechanical properties. A thorough understanding of the thermal/fluid aspects inherent to DLD is vital for optimizing the DLD process and ensuring consistent, high-quality parts.

An overview of Direct Laser Deposition for additive manufacturing; Part I: Transport phenomena, modeling and diagnostics

The mechanical behavior, and thus ‘trustworthiness’/durability, of engineering components fabricated via laser-based additive manufacturing (LBAM) is still not well understood. This is adversely affecting the continual adoption of LBAM for part fabrication/repair within the global industry at large. Hence, it is important to determine the mechanical properties of parts fabricated via LBAM as to predict their performance while in service. This article is part of two-part series that provides an overview of Direct Laser Deposition (DLD) for additive manufacturing (AM) of functional parts. The first part (Part I) provides a general overview of the thermo-fluid physics inherent to the DLD process. The objective of this current article (Part II) is to provide an overview of the mechanical characteristics and behavior of metallic parts fabricated via DLD, while also discussing methods to optimize and control the DLD process. Topics to be discussed include part microstructure, tensile properties, fatigue behavior and residual stress – specifically with their relation to DLD and post-DLD process parameters (e.g. heat treatment, machining). Methods for controlling/optimizing the DLD process for targeted part design will be discussed – with an emphasis on monitored part temperature and/or melt pool morphology. Some future challenges for advancing the knowledge in AM-part adoption are discussed. Despite various research efforts into DLD characteristics and process optimization, it is clear that there are still many areas that require further investigation.

An overview of Direct Laser Deposition for additive manufacturing; Part II: Mechanical behavior, process parameter optimization and control

A review of the wire arc additive manufacturing of metals: properties, defects and quality improvement

Laser-based direct metal deposition (DMD) is a solid freeform fabrication process capable of fabricating fully dense and metallurgically sound parts. The process has been greatly enhanced toward multi-directional deposition by the use of discontinuous radially symmetrical powder nozzles to supply the build material. Since many operational parameters depend on the gas–powder stream characteristics between the nozzles and the deposition point, an extensive understanding of the gas–powder flow is necessary. Three-dimensional (3D) multi-phase gas–powder flow structures of radially symmetrical nozzles are modeled using computational fluid dynamics methods. The obtained results are in good agreement with the experimental ones, and they provide a good insight into the process phenomena.

Numerical simulation and experimental investigation of gas–powder flow from radially symmetrical nozzles in laser-based direct metal deposition

Laser-based direct metal deposition (LBDMD) is a promising additive manufacturing technology that is well suited for production of complex metal structures, low-volume manufacturing, and high-value component repair or modification. It finds broad application in the automotive, biomedical, and aerospace industries. The Research Center for Advanced Manufacturing (RCAM) at Southern Methodist University is developing a robot controlled LBDMD system that couples a 6-axis robot arm with an additional 2-axis tilt and rotatory positioning system. The system simplifies the process planning of multiple-directional deposition for complex parts and reduces production time. This paper describes the printing process specific to complex revolved parts. Taking advantage of the coupled 2-axis tilt and rotatory system, a hybrid slicing method is developed to map the overhanging structures of a revolved part to be at a planar base. Consequently, the traditional path planning strategies are applicable to generate the tool-path for the mapped structures. The method is successfully applied to build a propeller. An 8-axis robotized laser-based direct metal deposition system is developed.The process planning is developed to build the complex revolved parts.The hybrid slicing method maps the overhanging structures to a planar base.A propeller was printed to verify the performance of the process planning.

Process planning for 8-axis robotized laser-based direct metal deposition system

https://www.smu.edu/~/media/Site/Lyle/RCAM/Publications/Automated%20Torch%20Path%20Planning%20Using%20Polygon.ashx

Automated torch path planning using polygon subdivision for solid freeform fabrication based on welding

A smart belt consists of a standard belt with an embedded sensor capable of wireless communication or visual indicators to notify maintenance for a need for maintenance or replacement. The sensor measures material properties, cycles of operation etc. using a self energizing power source. When the belt requires replacement or maintenance a mode of indication will notify proper personnel.

Method and apparatus for monitoring conveyor belts

Laser-based direct metal deposition has demonstrated the capability to deposit metal along multiple directions. Suitable control of parameters, namely, the metal-powder feed rate, the traverse speed, and the laser power allow fabrication of a desired shape for a large family of parts. The capability to deposit material along multiple directions eliminates the requirement for support structures. However, accessing the point of deposition and manipulating the direction of deposition require a coordinated control of two kinematical systems; one is related to the deposition platform, whereas the other is related to the deposition head. The identification of the key challenges, a mathematical basis of the process, possible solutions, the subsequent process planning, and experimental results are the subjects of this article.

Rajeev Dwivedi

Papers

Numerical simulation and experimental investigation of gas–powder flow from radially symmetrical nozzles in laser-based direct metal deposition

Process planning for 8-axis robotized laser-based direct metal deposition system

Automated torch path planning using polygon subdivision for solid freeform fabrication based on welding

Method and apparatus for monitoring conveyor belts

Process Planning for Multi-Directional Laser-Based Direct Metal Deposition: