Additive manufacturing of metallic components – Process, structure and properties

Additive manufacturing of metals

Unlike conventional materials removal methods, additive manufacturing (AM) is based on a novel materials incremental manufacturing philosophy. Additive manufacturing implies layer by layer shaping and consolidation of powder feedstock to arbitrary configurations, normally using a computer controlled laser. The current development focus of AM is to produce complex shaped functional metallic components, including metals, alloys and metal matrix composites (MMCs), to meet demanding requirements from aerospace, defence, automotive and biomedical industries. Laser sintering (LS), laser melting (LM) and laser metal deposition (LMD) are presently regarded as the three most versatile AM processes. Laser based AM processes generally have a complex non-equilibrium physical and chemical metallurgical nature, which is material and process dependent. The influence of material characteristics and processing conditions on metallurgical mechanisms and resultant microstructural and mechanical properties of AM proc...

Laser additive manufacturing of metallic components: materials, processes and mechanisms

Fine-structured aluminium products with controllable texture by selective laser melting of pre-alloyed AlSi10Mg powder

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

Direct metal deposition technology is an emerging laser aided manufacturing technology based on a new additive manufacturing principle, which combines laser cladding with rapid prototyping into a solid freeform fabrication process that can be used to manufacture near net shape components from their CAD files. In the present study, direct metal deposition technology was successfully used to fabricate a series of samples of the Ni-based superalloy Inconel 625. A high power CO2 laser was used to create a molten pool on the Inconel 625 substrate into which an Inconel 625 powder stream was delivered to create a 3D object. The structure and properties of the deposits were investigated using optical and scanning electron microscopy, X-ray diffraction and microhardness test. The microstructure has been found to be columnar dendritic in nature, which grew epitaxially from the substrate. The thermal stability of the dendritic morphology was investigated in the temperature range 800–1200 °C. These studies demonstrate that Inconel 625 is an attractive material for laser deposition as all samples produced in this study are free from relevant defects such as cracks, bonding error and porosity.

Laser aided direct metal deposition of Inconel 625 superalloy: Microstructural evolution and thermal stability

Texture control during laser deposition of nickel-based superalloy

Laser melting of Al–Si alloys has been investigated extensively, however, little work on the microstructural evolution of laser deposited Al–Si alloys has been reported to date. This paper presents a detailed microstructural investigation of laser deposited Al–11.28Si alloy. Laser aided direct metal deposition (DMD) process has been used to build up solid thin wall samples using Al 4047 prealloyed powder. The evolution of macro- and microstructures of laser deposited Al–Si samples was investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and electron backscattered diffraction techniques. Microstructural observation revealed that the morphology and the length scale of the microstructures are different at different locations of the sample. A periodic transition of microstructural morphology from columnar dendrite to microcellular structure was observed in each layer. The observed difference in the microstructure was correlated with the thermal history of the deposit.

Evolution of microstructure in laser deposited Al-11.28%Si alloy

In the current investigation AISI 4340 steel was laser deposited on a rolled mild steel substrate by Direct Metal Deposition (DMD) technology. The microstructural investigation of the clad was performed using optical and electron microscopes and X-ray diffraction techniques. The microstructure consisted of ferrite, martensite and cementite phases. Two types of martensite, lathe-type and plate-type, were observed in the microstructure. Decrease in microhardness values from the top layer to the alloy layer proves that the degree of tempering of the martensite phase increases in the same direction. The lattice parameters of the identified phases were found to be shorter than those reported in literature. The reported parameters in literature are from samples processed under equilibrium conditions.

Microstructural evolution of AISI 4340 steel during Direct Metal Deposition process

Direct metal deposition (DMD) technology is a laser-aided rapid prototyping method that can be used to fabricate near net shape components from their CAD files. In the present study, a series of Al-Si samples have been deposited by DMD in order to optimize the laser deposition parameters to produce high quality deposit with minimum porosity and maximum deposition rate. This paper presents the microstructural evolution of the as-deposited Al 4047 sample produced with optimized process parameters. Optical, scanning, and transmission electron microscopes have been employed to characterize the microstructure of the deposit. The electron backscattered diffraction method was used to investigate the grain size distribution, grain boundary misorientation, and texture of the deposits. Metallographic investigation revealed that the microstructural morphology strongly varies with the location of the deposit. The layer boundaries consist of equiaxed Si particles distributed in the Al matrix. However, a systematic transition from columnar Al dendrites to equiaxed dendrites has been observed in each layer. The observed variation of the microstructure was correlated with the thermal history and local cooling rate of the melt pool.

Ashish K. Dasgupta

Papers

Laser aided direct metal deposition of Inconel 625 superalloy: Microstructural evolution and thermal stability

Texture control during laser deposition of nickel-based superalloy

Evolution of microstructure in laser deposited Al-11.28%Si alloy

Microstructural evolution of AISI 4340 steel during Direct Metal Deposition process

Microstructural Characterization of Laser-Deposited Al 4047 Alloy