Defect Formation Mechanisms in Selective Laser Melting: A Review
Chin. J. Mech. Eng.
Defect Formation Mechanisms in Selective Laser Melting: A Review
Bi Zhang 0 1
Yongtao Li 0 1
Qian Bai 0 1
0 Department of Mechanical Engineering, University of Connecticut , Storrs, CT 06269 , USA
1 Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology , Dalian 116024 , China
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. Supported by National Natural Science Foundation of China (Grant No. 51605077), Science Challenge Project (Grant No. CKY2016212A506-0101) and Science Fund for Creative Research Groups of NSFC (Grant No. 51621064).
Selective laser melting; Process parameters; Defect; Mechanical properties
1 Introduction
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
].
Selective Laser Melting (SLM) is one of the additive
manufacturing processes. It is relatively mature and has been
a research focus in manufacturing metallic parts [
6
]. A
schematic layout of a typical SLM setup is presented in Fig. 1
[
7
]. During the SLM process, data is provided from a CAD
model which is then sliced into thin layers. Each sliced layer
is further developed with the appropriate scan paths. Through
the scanner mirrors, a laser beam selectively scans and melts
the powders that are previously paved on the substrate
according to the developed scan paths. After a layer is
finished, the building platform is lowered by an amount equal to
the layer thickness, and a new layer of powders is paved. The
process repeats until the completion of the whole part. To
date, the SLM process is able to fabricate metallic parts from
different material powders, such as titanium alloys [
7, 8
],
nickel-based superalloys [
9, 10
], aluminum alloys [
11, 12
]
and stainless steels [
13, 14
].
Although the SLM process offers a great advantage in
manufacturing complex parts at a high material utilization
rate [
15
], 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 [
16
], rapid melting and
solidification of material, microstructure evolution [
17, 18
],
flow in a molten pool [19], and materials evaporation [
20
].
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 (...truncated)