• Common defects and anomalies in powder bed fusion metal additive manufacturing. • Formation mechanism and practical mitigation strategies are discussed. • Defects/anomalies are classified as powder-related, processing-related, and post-processing related issues. • Properties such as mechanical behavior and corrosion resistance of defective parts are discussed. • Current challenges, gaps, and future trends are discussed. Metal additive manufacturing is a disruptive technology that is revolutionizing the manufacturing industry. Despite its unrivaled capability for directly fabricating metal parts with complex geometries, the wide realization of the technology is currently limited by microstructural defects and anomalies, which could significantly degrade the structural integrity and service performance of the product. Accurate detection, characterization, and prediction of these defects and anomalies have an important and immediate impact in manufacturing fully-dense and defect-free builds. This review seeks to elucidate common defects/anomalies and their formation mechanisms in powder bed fusion additive manufacturing processes. They could arise from raw materials, processing conditions, and post-processing. While defects/anomalies in laser welding have been studied extensively, their formation and evolution remain unclear. Additionally, the existence of powder in powder bed fusion techniques may generate new types of defects, e.g., porosity transferring from powder to builds. Practical strategies to mitigate defects are also addressed through fundamental understanding of their formation. Such explorations enable the validation and calibration of models and ease the process qualification without costly trial-and-error experimentation. 

Defects and anomalies in powder bed fusion metal additive manufacturing

The possibility of using a variety of sensor signals acquired during metal powder bed fusion processes, to support part and process qualification and for the early detection of anomalies and defects, has been continuously attracting an increasing interest. The number of research studies in this field has been characterised by significant growth in the last few years, with several advances and new solutions compared with first seminal works. Moreover, industrial powder bed fusion (PBF) systems are increasingly equipped with sensors and toolkits for data collection, visualisation and, in some cases, embedded in-process analysis. Many new methods have been proposed and defect detection capabilities have been demonstrated. Nevertheless, several challenges and open issues still need to be tackled to bridge the gap between methods proposed in the literature and actual industrial implementation. This paper presents an updated review of the literature on in-situ sensing, measurement and monitoring for metal PBF processes, with a classification of methods and a comparison of enabled performances. The study summarises the types and sizes of defects that are practically detectable while the part is being produced and the research areas where additional technological advances are currently needed.

https://iopscience.iop.org/article/10.1088/1361-6501/ac0b6b/pdf

In-situ measurement and monitoring methods for metal powder bed fusion: an updated review

Making sustainable aluminum by recycling scrap: The science of “dirty” alloys

Laser powder bed fusion (LPBF) is a key metal additive manufacturing process and has attracted increasing attention both in academia and industry. An essential physical issue influencing LPBF is metal vaporization, and there has been much dispute regarding the occurrence and influence of metal vaporization during LPBF. The latest in-situ X-ray imaging results directly demonstrated the occurrence of massive vaporization based on the widespread presence of keyholes under typical LPBF conditions. In this study, a comprehensive review of metal vaporization during LPBF was conducted, in terms of its influence and underlying mechanism, as well as numerical simulations. Metal vaporization was found to be primarily dependent on the temperature of the melt pool and the surrounding atmosphere, and to substantially influence the transfer of energy, momentum, and mass during the process. Critical formation problems, such as powder denudation, plume, spatter, lack of fusion, and porosity were closely related to metal vaporization. An adequate energy input and optimized shielding atmosphere were found to be necessary to inhibit the negative influence of metal vaporization. Moreover, the vaporization by-products could be used for quality monitoring, and the vaporization loss of elements could be quantitatively adjusted to regulate the compositional distribution. 

Metal vaporization and its influence during laser powder bed fusion process

Invar 36 (FeNi36) is a weldable nickel alloy, known for its low thermal expansion properties, that contains 36% of nickel and 64% iron. The laser-based powder bed fusion process is often associated with defects and flaws that should be controlled for production quality. Spatter formation and delamination are two phenomena that could be observed in thermal images taken during the laser melting process. This paper presents an in-situ monitoring setup for detecting spatters and delamination of Invar 36. The size, shape, count, and cooling rate of spatter particles are evaluated using a combination of infrared pyrometer and high-speed infrared thermography. Thermograms of Invar 36 are processed to determine the spatter characteristics. The influence of processing conditions on the spatter count, spatter size, and size of melt track is discussed. The effect of spatters on the surface morphology of Invar 36 parts is also investigated. The processing conditions that cause melt pool instabilities, excessive spattering, and delamination are presented. This paper concludes recommendations for avoiding delamination and excessive spatter formation during the laser-based powder bed fusion of Invar 36.

In-situ monitoring and detection of spatter agglomeration and delamination during laser-based powder bed fusion of Invar 36

Spatter causes defect formation, powder redistribution and contamination in laser powder bed fusion (LPBF) additive manufacturing process It is critical to distinguish different types of spatter and understand their features and formation mechanisms This work reveals the features and formation mechanisms of five unique types of spatter during the LPBF process by in-situ high-speed, high-energy x-ray imaging Spatters observed during LPBF testing are quantified by their speed, size, and direction Distinct quantifiable characteristics for each type of spatter are identified Effects of the laser power, scan speed, and ambient pressure on spatter formation and features are unraveled A spatter formation map for AlSi10Mg alloy is constructed

Zachary A. Young

Papers

Types of spatter and their features and formation mechanisms in laser powder bed fusion additive manufacturing process