## 2 Classification of LPBF Defects
### Mechanical Defects  
Thermal stresses induce mechanical defects; mechanical stresses are induced by temperature gradients in a material. In LPBF, the part undergoes thermal stresses during scanning or shortly after several subsequent scans, producing a large temperature gradient. The high thermal energy gradient induced by the laser beam causes the top layers of the solid to expand and induce elastic compressive stresses thermally. Once the yield stress point is exceeded, plastic deformation of the top layers occurs. As the plastically deformed layers rapidly cool down, the printed material layers contract and bend oppositely. It converts the compressive stresses into residual tensile stresses that can induce cracking. Shortly after scanning, the tensioned layer cools, which causes the layer to shrink faster due to the slight differences in thermal conductivity, causing a thermal contraction by the lower, colder layers. Due to the solid metallurgical connection of both layers, contraction is  
Figure 8: X-ray radiographic images highlighting gas pore’s locations [23].  
Figure 9: X-ray imaging highlighting the location and shape of keyhole pores [25].  
inhibited, causing tensile stresses to the upper and compressive stresses in the lower layers, as shown in Figure 10[28]. In extreme cases, high thermal stress can cause part distortion, shrinkage, cracking, warping, and delamination.  
**Liquation cracking** mainly depends on the stability of the liquid film of the melt pool and local stress concentration [29]. High laser energy and lower powder thermal conductivity contribute to liquid film instability and rapid thermal cycling. Thermal cycling induces tensile stress and causes cracking. Introducing some alloying element that can limit the solidification temperature would alter the chemical composition of the molten pool to prevent cracking [30]. Figure 11a,b shows the geometry of liquidon cracks in an IN718 LPBF-printed part [31].  
**Solidification cracking** is also known as cold cracking, a type of crack that occurs in the latter stages of the solidification process. Near the end of solidification, dendrites have almost fully grown into equiaxed grains. These dendrites are separated by a slight residual liquid strip [32]. Insufficient convection in the diminishing liquid region causes weak spots to form and causes the formation of micro-cracks that can propagate and be enlarged. Alloys with a wide solidification temperature range are more susceptible to solidification cracking than alloys that solidify over a narrow temperature range due to the accumulated thermal strain proportional to the temperature range [27]. Solidification cracks can be eliminated by adopting a re-melting strategy or post-build machining. A sure way to reduce solidification cracking is to avoid rapid cooling. Figure 11c shows the geometry of solidification cracks in an IN718 LPBF-printed part [31].  
**Delamination** is a unique cracking behavior observed in AM LPBF builds. Delamination is caused by macroscopic residual stresses originating from spatial thermal gradients [33]. The uneven expansion and shrinkage of the part during thermal cycles results in non-uniform inelastic strain and, hence, delamination [34]. Delamination appears as cracking between deposition layers, often resulting in complete part failure. Figure 12 highlights delamination cracking in an SS304L LPBF-printed part [35].  
Figure 11: Comparison of liquidon (**a,b**) and solidification (**c**) cracking in an IN718 LPBF-printed part [31].  
Figure 10: Unique residual stress distribution (compression and tension) seen in AM, showing the cool-down phase inducing residual stress developed at the solid layer connection during (**a**) heating and (**b**) cooling [28].