PSI - Issue 77

Y. Bakir et al. / Procedia Structural Integrity 77 (2026) 639–648

646

8

Y.Bakir et al./ Structural Integrity Procedia 00 (2026) 000–000

Figure 10 demonstrates another form of output from the monitoring system. These images mainly aim to show maximum amount GV, therefore highest amount of radiation captured for each pixel. While the GV curves are result of averaging the values captured on a layer, in the images the local distribution of the intensity for single layers is given. Usually drawing a conclusion based on image of a single layer might not be correct way to evaluate the process, since there are differences in the scanning strategy between layers. However, the general trend with the mean GV discussed before, seems to be continuing here, the default strategy resulting in higher amount of radiation emitted from the melt pool. In addition, clusters of pixels with higher GV were observed on default process images. Idle time strategy seems to result in more uniform data across the layer. While the changes in standard deviation in between layers were visible in GV curve, seemingly deviation within a layer is also higher with default process. 3.3. Tensile Testing Tensile testing was done on parts that were produced using Default and Idle time strategies. Mechanical properties; 0.2% offset yield strength(Rp 0.2 ), the ultimate tensile strength(Rm), elongation at yield(A g ) and elongation at break(A 5 ) are given in the Tab.1. The average of the results is demonstrated. The differences on the results are not significant for the different strategies.

Table 1Tensile test results for specimens printed with two different strategies[4].

Strategies Default Idle Time

Rp 0.2 (MPa)

R m (MPa)

A g (%)

A 5 (%)

983.3 973.8

1064.2 1077

2.25

21.1 21.4

2.3

3.4. Residual Stress Temperature gradient mechanism(TGM) is one of the mechanisms that induce residual stress within parts that built with SLM technology. Expansion of heated material is resisted by previously printed layers. Similarly, contraction during solidification is also resisted. These mechanisms induce residual stress to the parts. If the material’s yield strength is reached, plastic deformation may occur. This mechanism is also used in shaping thin sheet metal parts[8].

Figure 11 Temperature Gradient Mechanism(TGM)[8].

Residual stresses were measured in order to observe the possible influence of the different scanning strategies. Introducing idle time between the exposure of subsequent hatching lines appeared to lead to separate melting tracks near the edges of the parts. Amount of melted material that is solidified per unit time decreases. Thus, the solidifying material per unit time also decreases. Idle time strategy decreases the productivity of the process. However, the idle time strategy may also decrease the stresses formed within the part according to the TGM. Residual stress was measured on 5 parts built for each strategy. Curves of the results for each strategy is given in the Figure 11. According to the graph the peak of residual stress was around 420 N/mm 2 and it was measured for the parts that are printed with default process settings. The peak value for idle time strategy was measured at around 215 N/mm 2 and the drilling depth was 0.2 mm more into the part in comparison to the peak of default strategy. On average there was not a difference in the results of residual stress for different strategies after around 0.65 mm into the parts.