# Evaluating the Reliability of Powder Bed Fusion for Biomedical Materials: An Experimental Approach
## 4 Discussions  
Scanning speeds in the range of 350-500 mm/s were used in this research because such values reduce the risk of micro-void formation [22,47]. Results previously published by other researchers show that a 20-40 \(\upmu\)m thickness of the powder layer leads to improved mechanical characteristics but diminishes productivity [7, 9, 22, 48]. Considering the manufacturing time and powder grain size, this paper presents the process parameters suitable for a 50 \(\upmu\)m layer thickness for pure Ti, Ti6 Al7 Nb, CoCr, and CoCrWMo powders. Tables 5-8 present the SLM process parameters suitable for each alloy, leading to SLM parts with lower residual stress levels. Typical manufacturing defects were identified as flowing: balling effect, micro-cracks, heat-affected zone, residual stress phenomena, etc. It is known that high laser power combined with a relatively low scanning speed may cause the material track to be completely melted or even broken due to excessive shrinkage and high residual stresses, resulting in many visible cracks [8]. The PBF process generates large temperature gradients near the exposure area, where there is high-energy density input [42]. If unmanaged, these gradients can lead to failures during manufacturing or undesirable residual stress artifacts (including distortion) and increased crack formation risks [1]. The microstructure evolution, micro-cracks and related boundary effects were analyzed and discussed on the basis of scanning electron microscopy (SEM) and metallographic analyses within our previews works [39]. The cracks appeared because, under the action of a moving a high-energy laser, the melting and solidification processes were completed in a short period of time, which induced a high-temperature gradient and high stress. As a result, the cracks tended to form to release the thermal stress. The recorded micro-cracks were irregularly distributed in the structure, having different lengths up to 100 \(\upmu\)m.  
Residual stress is attributed to temperature gradients due to the rapid heating and cooling rates during PBF [49]. The process parameters configured in this research produced parts with low deformations and limited residual stress levels. Specific process parameters were determined for each material to ensure good processability. Cracks or major defects are unacceptable in PBF-manufactured parts. The physical-mechanical characteristics were influenced by microscopic cracks and pores, attributed to liquid film interruption at grain boundaries in the solidification temperature range due to tensile stress [11]. Macroscopic cracks significantly impacted the processability rank and occurred due to the low ductility of the material and stress-induced crack propagation [11]. Tensile tests were preferred over compression tests due to their capability of capturing the influence of micro-voids on mechanical characteristics. The mechanical properties were determined using standard specimens and tensile loads applied along the directions scanned by the laser beam. Under these testing conditions, the orientation of micro-voids and the solidification direction did not significantly influence the experimental results [50]. The tensile test results showed that laser power influences the physical-mechanical characteristics. For the Ti6 Al7 Nb and CoCr alloys, the ultimate tensile strength increases with higher laser power. In contrast, for pure Ti and CoCrWMo alloys, the opposite effect was observed. The total elongation of the PBF-manufactured specimens was low. This characteristic can be improved by heat treatments (annealing, quenching, or aging), thermochemical treatments (nitriding or carburizing), or hot isostatic pressing, depending on the chemical composition of the powder and the final application of the parts [7, 16, 17, 18, 19].