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

Propulsion system development requires new, more affordable manufacturing techniques and technologies in a constrained budget environment, while future in-space applications will require in-space manufacturing and assembly of parts and systems. Marshall is advancing cuttingedge commercial capabilities in additive and digital manufacturing and applying them to aerospace challenges. The Center is developing the standards by which new manufacturing processes and parts will be tested and qualified. Rapidly evolving digital tools, such as additive manufacturing, are the leading edge of a revolution in the design and manufacture of space systems that enables rapid prototyping and reduces production times. Marshall has unique expertise in leveraging new digital tools, 3D printing, and other advanced manufacturing technologies and applying them to propulsion systems design and other aerospace materials to meet NASA mission and industry needs. Marshall is helping establish the standards and qualifications “from art to part” for the use of these advanced techniques and the parts produced using them in aerospace or elsewhere in the U.S. industrial base.

Additive manufacturing

The results of detailed experiments and finite element modeling of metal micro-droplet motion associated with metal additive manufacturing (AM) processes are presented. Ultra high speed imaging of melt pool dynamics reveals that the dominant mechanism leading to micro-droplet ejection in a laser powder bed fusion AM is not from laser induced recoil pressure as is widely believed and found in laser welding processes, but rather from vapor driven entrainment of micro-particles by an ambient gas flow. The physics of droplet ejection under strong evaporative flow is described using simulations of the laser powder bed interactions to elucidate the experimental results. Hydrodynamic drag analysis is used to augment the single phase flow model and explain the entrainment phenomenon for 316 L stainless steel and Ti-6Al-4V powder layers. The relevance of vapor driven entrainment of metal micro-particles to similar fluid dynamic studies in other fields of science will be discussed.

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Metal vapor micro-jet controls material redistribution in laser powder bed fusion additive manufacturing

The creation of an object by selective laser melting (SLM) occurs by melting contiguous areas of a powder bed according to a corresponding digital model. It is therefore clear that the success of this metal Additive Manufacturing (AM) technology relies on the comprehension of the events that take place during the melting and solidification of the powder bed. This study was designed to understand the generation of the laser spatter that is commonly observed during SLM and the potential effects that the spatter has on the processing of 316L stainless steel, Al-Si10-Mg, and Ti-6Al-4V. With the exception of Ti-6Al-4V, the characterization of the laser spatter revealed the presence of surface oxides enriched in the most volatile alloying elements of the materials. The study will discuss the implication of this finding on the material quality of the built parts.

A Study on the Laser Spatter and the Oxidation Reactions During Selective Laser Melting of 316L Stainless Steel, Al-Si10-Mg, and Ti-6Al-4V

Transient Dynamics of Powder Spattering in Laser Powder Bed Fusion Additive Manufacturing Process Revealed by In-Situ High-Speed High-Energy X-Ray Imaging

Relationship between spatter formation and dynamic molten pool during high-power deep-penetration laser welding

Real-time penetration state monitoring using convolutional neural network for laser welding of tailor rolled blanks

Laser-induced plasma in deep penetration laser welding is located inside or outside the keyhole, namely, keyhole plasma or plasma plume, respectively. The emergence of laser-induced plasma in laser welding reveals important information of the welding technological process. Generally, electron temperature and electron density are two important characteristic parameters of plasma. In this paper, spectroscopic measurements of electron temperature and electron density of the keyhole plasma and plasma plume in deep penetration laser welding conditions were carried out. To receive spectra from several points separately and simultaneously, an Optical Multi-channel Analyser (OMA) was developed. On the assumption that the plasma was in local thermal equilibrium, the temperature was calculated with the spectral relative intensity method. The spectra collected were processed with Abel inversion method to obtain the temperature fields of keyhole plasma and plasma plume.

Measurements of laser-induced plasma temperature field in deep penetration laser welding

In the laser welding process, weld defects are often caused by many factors, such as variations in the laser power, welding speed and gaps between two workpieces. In an auto-welding system, the on-line monitoring of the welding quality is very important in avoiding weld defects. In this paper, an on-line coaxial monitoring system with an auxiliary illuminant was built for the fibre laser welding of galvanised steel; images of the weld pool were taken during the welding process. Profiles of the weld pool and the keyhole were obtained by processing the images using the region-growing algorithm and the Canny algorithm. In this research, we used the on-line monitored weld pool width to monitor the weld surface width. The weld penetration status was divided into the three categories of incompletely penetrated, moderately penetrated and over-penetrated using the value of d (diameter at the bottom of the keyhole)/D (diameter at the top of the keyhole). Thus, the weld width and weld penetration status of fibre laser welding can be monitored on-line.

Coaxial monitoring of the fibre laser lap welding of Zn-coated steel sheets using an auxiliary illuminant

Experimental data is presented relating to hole quality and feasibility studies when using continuous wave (CW) fiber laser to cut CFRP laminates (6.0 mm diameter hole) with single- and multi-pass strategy. The effect of typical processing parameters including laser power and cutting speed on thermal defect was also investigated. The statistical significance of individual cutting parameters was determined using main effects plot together with ANOVA analysis. Three methods were proposed to characterize HAZ based on the feature of thermal defect. According to statistical analysis, both cutting speed and laser power were significant with respect to HAZ (recorded at hole exit). The minimal value of HAZ was recorded with laser power of 650 W and cutting speed of 1100 mm/min. Energy per unit length (El) should be set above 40 J/mm to ensure cutting through CFRP laminates using fiber laser in high efficient machining process. Multi-pass strategy was investigated without setting pause/break time between each pass in order to increase cutting efficiency. Results showed limited improvement in terms of the level of HAZ even with cutting speed up to 10,000 mm/min, while machining time was significantly reduced to cut 6.0 mm diameter hole compared with single-pass strategy. For machining small holes with diameter down to 2.0 mm, high power fiber laser was not preferred due to severe thermal defect observed at hole entry and exit.

Yi Zhang

Papers

Relationship between spatter formation and dynamic molten pool during high-power deep-penetration laser welding

Real-time penetration state monitoring using convolutional neural network for laser welding of tailor rolled blanks

Measurements of laser-induced plasma temperature field in deep penetration laser welding

Coaxial monitoring of the fibre laser lap welding of Zn-coated steel sheets using an auxiliary illuminant

Fiber laser cutting of CFRP laminates with single- and multi-pass strategy: A feasibility study