Defect Formation Mechanisms in Selective Laser Melting: A Review
© Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2017
Received: 29 October 2016
Accepted: 28 March 2017
Published: 21 April 2017
The Erratum to this article has been published in Chinese Journal of Mechanical Engineering 2017 30:184
Defect formation is a common problem in selective laser melting (SLM). This paper provides a review of defect formation mechanisms in SLM. It summarizes the recent research outcomes on defect findings and classification, analyzes formation mechanisms of the common defects, such as porosities, incomplete fusion holes, and cracks. The paper discusses the effect of the process parameters on defect formation and the impact of defect formation on the mechanical properties of a fabricated part. Based on the discussion, the paper proposes strategies for defect suppression and control in SLM.
Additive manufacturing (AM) is an approach in which a part is manufactured layer by layer from the data of a 3D model. AM is a “bottom-up” approach as opposed to the traditional subtractive manufacturing that is often referred to as the “top-down” approach [1, 2]. The AM approach does not require the traditional tools, fixtures and complicated procedures. Therefore, it can offer an advantage of economically fabricating a customized part with complex geometries in a rapid design-to-manufacture cycle. With the development of high energy beams, it becomes possible to manufacture metal parts of high performance. Due to its unique advantages, the AM approach has been widely applied in many industries, such as aerospace, medical devices, military and automobile [3–5].
Although the SLM process offers a great advantage in manufacturing complex parts at a high material utilization rate , it is affected by many factors, such as laser energy input and scan speed, scan strategy, powder material, powder size and morphology. The SLM process consists of complicated physics, such as absorption and transmission of laser energy , rapid melting and solidification of material, microstructure evolution [17, 18], flow in a molten pool , and materials evaporation . The process is thus affected by the aforementioned factors to form defects of porosities, incomplete fusion holes, cracks, and impurities, etc. These defects are detrimental to a fabricated part in terms of its mechanical and physical properties, which in turn limits the application of SLM [21–24].
Since defect formation is a critical issue in an SLM process, research has been directed towards understanding and suppression of defect formation [7, 24–36]. This paper reviews the recent research outcomes on the types and formation mechanisms of the common SLM defects, such as porosities, incomplete fusion holes, and cracks. The paper also reveals how the SLM defects may affect the mechanical properties of a fabricated part. Other defects, such as metallic inclusions, segregations, residual stresses, metallurgical imperfections may also have a significant impact on the mechanical properties of a fabricated part, their respective formation mechanisms will be reviewed in a separate paper and published elsewhere. Finally, the paper provides a reference for defect suppression and control in the SLM processes.
2 Defect Types
Many parameters are involved in an SLM process, such as laser power, scan speed, hatch spacing, layer thickness, powder materials and chamber environment. Defects are inevitably introduced if any of these parameters are improperly chosen. The common defects are classified in three types: porosities, incomplete fusion holes, and cracks.
Firstly, if the packing density of metal powders is low, e.g., 50 percent, the gas present between the powder particles may dissolve in the molten pool. Because of the high cooling rate during the solidification process, the dissolved gas cannot come out of the surface of the molten pool before solidification takes place. Porosities are thus formed and remain in the fabricated part. Porosities may also be formed when metal powders of a hollow structure are utilized in an SLM process. On the other hand, the molten pool temperature is generally high due to the intense laser power. At this temperature, gas solubility in the liquid metal is high, making its enrichment easier. Furthermore, in the process of preparing powder materials, gas is inevitably introduced into the powder materials, especially the gas atomized powder materials in the scope of protection by an inert gas, such as argon or helium.
Qiu et al  observe that the porosities contain ridges in the internal surfaces and are thus probably associated with the incomplete re-melting of some local surfaces from the previous layers. The ridges form small volumes to which the molten metal is difficult to flow and penetrate. On the other hand, Gong et al  attribute these porosities to gas bubbles generated when a high laser energy is applied to the molten pool. Gas bubbles can be induced due to vaporization of low melting point constituents within an alloy. They can be far beneath the surface at the bottom of the molten pool. The high solidification rate of the molten pool does not give gas bubbles sufficient time to rise and escape from the surface. Thus, gas bubbles are trapped in the molten pool, resulting in defect inclusions of regular spherical porosities in the forming part.
It is therefore understood that such spherical porosities are generally resulted from the entrapped gases in the molten pool due to the excessive energy input or unstable process conditions. The spherical porosities are randomly distributed in an SLM fabricated part, and difficult to eliminate completely.
2.2 Incomplete Fusion Holes
In the SLM process, a laser selectively melts the metal powders point by point, line by line, and layer by layer to complete the whole part. If the laser energy input is low, the width of the molten pool is small, which results in an insufficient overlap between the scan tracks. The insufficient overlap is a cause of formation of the un-melted powders between the scan tracks. In the deposition process of a new layer, it becomes difficult to fully re-melt these powders. As a consequence, incomplete fusion holes are formed and remain in the SLM fabricated part. Furthermore, if the laser energy input is too low to cause an enough penetration depth of the molten pool, LOF defects may be generated due to a poor interlayer bonding [24, 29, 37]. Therefore, LOF defects are usually distributed between the scan tracks and the deposited layers.
Moreover, in a location where defects have been generated, the surface of the location becomes rough. The rough surface directly contributes to the poor flow of the molten metal to form interlayer defects. The interlayer defects may gradually extend and propagate upwards to form large multi-layer defects in a continuous deposition process .
Element content analysis of the defect (wt. %)
For stainless steels and nickel-based superalloys, because of their low thermal conductivity and high thermal expansion coefficient, they are more vulnerable to generating cracks with high susceptibility to cracking in an SLM process [9, 27, 42, 43]. To solve this problem, pre-heating the substrate and improving the ambient temperature are recommended to reduce the cracks in the SLM fabricated parts [26, 27].
3 Effect of Process Factors on Defect Formation
3.1 Effect of Laser Energy Input
Laser energy input directly determines the melt condition of metal powders, the flow of molten metal, which has a significant impact on the type and size of the defects in an SLM process. The energy input in the material can be related to the main process parameters, such as laser power, scan speed, hatch spacing, and layer thickness.
At a relatively low scan speed and a high laser power, the energy input is high, more powders are melted at an elevated temperature, porosity defects are created. These defects can be attributed to the entrapped gas originated from the raw material powders in the SLM process as mentioned above. In addition, low melting point constituents, e.g., Al, Mg elements in the alloy, may evaporate into gas to form gas bubbles. During the rapid solidification process in SLM, the gas bubbles do not have sufficient time to escape from the molten pool to the pool surface. They remain within the molten pool to form porosity defects of a spherical shape [29, 44]. On the other hand, the molten pool becomes large if energy input is high, which causes powder denudation around the molten pool. The denudation process results in insufficient molten metal to fill the gap between the adjacent tracks. Large porosities are thus formed .
At a relatively high scan speed and a low laser power, the energy input is too low to fully melt the powders, generating a discontinuous molten pool. This makes it difficult to fully melt the powders between the adjacent tracks to form an effective overlap, resulting in the formation of incomplete fusion defects. In addition, if a large powder thickness causes an insufficient penetration of the laser energy input, an effective overlap may not be developed between layers, causing the formation of interlayer incomplete fusion defects [24, 29, 45, 46].
Therefore, as an integrated parameter, energy density represents the combined effect of the major process parameters on defect formation in an SLM process. Energy density is handy to use in selecting the appropriate laser power, scan speed, hatch spacing, layer thickness to minimize the defects and improve the manufacturing efficiency in the SLM process.
3.2 Effect of Powder Materials
The morphology and size of metal powders have a significant influence on the powder bed smoothness and powder flowability, thus are strictly required in an SLM process. Metal powders are produced in different methods, such as water atomization, gas atomization, plasma rotating electrode and electrolytic method, which has a diverse effect on defect formation [53–55]. In addition, the gas contained in the powders increases the probability of defect formation.
3.3 Effect of Scan Strategy
4 Influence of Defects on Mechanical Properties
Defects in an SLM process cause stress concentration in the fabricated part, which may lead to the part failure. When stress exceeds the material limit, a crack may form and gradually propagate in the part. The following Sections 4.1-4.2 are dedicated to discussing the influence of defects on the mechanical properties in the SLM parts.
4.1 Tensile Properties
Furthermore, the SLM process has a directional effect on the properties of the forming parts due to its basic deposition principle. The directional effect is a direct cause of the severe anisotropy in the mechanical properties of the fabricated part. For a part fabricated based on the orthogonal scan strategy shown in Fig. 13, defects may be formed and distributed in the horizontal direction, resulting in the obvious reduction of the load-bearing cross-section area in the fabricated part . If the loading direction coincides with the building direction, the part is more susceptible to failure, leading to a low strength of the part [24, 37, 67]. In addition, because of the epitaxial growth in the SLM process, the elongated columnar grains in the fabricated part also aggravate anisotropy of the part [28, 34].
4.2 Fatigue Properties
For an SLM fabricated part, defects are more detrimental to its fatigue strength due to the points of stress concentration. A defect often serves as a source of crack initiation and propagation, which may greatly reduce the fatigue strength of the part. The stochastic distribution of the defects also aggravates the scattering of fatigue life, which may severely restrict the application of the SLM fabrication.
The morphology, number, size and location of defects all have a significant influence on the fatigue life of the SLM fabricated parts. Generally, the spherical defects have less influence on the fatigue life of a part due to their regular shapes and small size. On the other hand, the defects of an irregular shape (e.g., an incomplete fusion hole) promote stress concentration of a part so as to seriously reduce fatigue strength of the part because of the irregular shapes and larger sizes of the defects [37, 47].
Leuders et al [52, 63, 73] studied the mechanical properties and the growth mechanisms of fatigue cracks in the SLM titanium parts. Their results indicated that defects had a major impact on the fatigue life of the parts, especially at the stage of fatigue crack initiation. Due to the presence of defects, stress concentration could occur, causing crack initiation and consequently a decrease in fatigue strength. Leuders et al also analyzed the effect of defect location on the fatigue strength in their research. When a defect was located near the surface of a part, its fatigue life was shorter in comparison with that located far from the surface, indicating that defect location is critical to the fatigue strength of the part. Surface treatment, such as machining and shot peening, can be adopted to suppress or eliminate the near-surface defects so as to enhance part fatigue strength.
However, since it is difficult to accurately control the type, number, and location of a defect in a fabricated part, the fatigue strength of a part can be in jeopardy. Therefore, the fatigue strength of an SLM fabricated part is still questionable and needs to be improved.
5 Strategies for Defect Suppression
Defect suppression is a challenging issue in the SLM process. Currently, there are two major strategies to suppress defect formation in the SLM processes, namely online detection and numerical simulation, in addition to machining to reduce or eliminate defects.
Clijsters et al  designed a high-speed and real-time molten pool monitoring system, consisting of four modules, namely optical set-up, data processing, reference data and quality estimation. For each layer of deposition, the information of molten pool in the form of a light signal was collected by sensors, then transferred to a data processing module to establish the molten pool image, then used to analyze the location and size of defects compared with reference data to get the characteristics of molten pool to deduce defect formation information. Finally, the analysis results were used for the feedback control to optimize the process, and to reduce defect formation in the SLM fabricated parts.
Panwisawas et al  established a mathematical model of thermal fluid dynamics to better understand the morphological evolution of porosity during an SLM process. According to the deposition mechanism of the heating-melting-solidification cycles of metal powders, a thermal fluid dynamics model based on the Navier-Stokes equation, surface tension, capillary force, and Marangoni effect was introduced to explore the evolution of porosity as the scan speed increased. The results showed that for a fixed laser input power, increasing scan speed reduced energy input density, resulting in serious unfused defects in the interlayers.
The common defects are three types, namely spherical porosities, irregularly incomplete fusion holes, and cracks. Spherical porosities are randomly distributed, while incomplete fusion holes are generally distributed between the tracks and layers.
Many process parameters, such as laser power, scan speed, hatch spacing, layer thickness, and scan strategy, have significant influences on the formation of defects. Energy density is an integrated parameter for controlling defect formation; scan strategy has a significant influence on the location distribution of defects, most of the defects distribute at both ends of scan tracks and in between two adjacent tracks.
Defect formation has a significant influence on the mechanical properties of the SLM fabricated parts, especially fatigue strength. Defects play a prominent role in fatigue crack initiation, directly reduce the fatigue life of a part, which restricts the application of the SLM technique.
The quality control in an SLM process relies on defect detection and elimination. For high quality SLM fabrications, defect monitoring, simulation and modeling, as well as real-time defect elimination become necessary. Defect-free SLM fabrications are anticipated in the near future.
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