Effect of CO₂Laser‐Sustained Nitrogen Plasma on Heat and Mass Transfer During Laser‐Nitriding of Commercially‐Pure Titanium

Laser-Sustained Plasma (LSP) Nitriding of Titanium: A Review

“…Since calorimetric measurements showed that the nitrogen LSP absorbed about 38% (1.33 kW) of the 3.5 kW laser power sustaining it, it was concluded that the LSP re-radiated close to 75% of the power it absorbed from the laser, with the remaining 25% power used for heating the nitrogen gas exiting the LSP. Further, the presence of LSP was also found to increase the width of the nitrided trails and to reduce surface oxidation [109]. To quantify the effect of the LSP on nitrogen intake into the melt pool, Kamat et al [109] measured the weight increase of the titanium coupons after nitriding in the presence and absence of a nitrogen LSP, i.e., during LSP and conventional laser nitriding, respectively; the processing conditions (laser power, scan speed, OFD) were maintained the same to ensure a fair comparison.…”

“…Such rapid growth of TiN crystals on the CP-Ti substrate without any direct radiation from the CO2 laser beam demonstrated the utility of the nitrogen LSP as a high-energy source of active nitrogen species. To study the effects of the nitrogen LSP on heat transfer to the substrate, the temperature of the CP-Ti substrate (3.175 mm thick) was measured in both Nassar et al's perpendicular (Figure 6a) [109] and Black et al's parallel (Figure 6c) [113] configurations using K-type thermocouples welded to the back of the substrate. In the perpendicular configuration, Kamat et al [109] conducted nitriding experiments in the presence and absence of a nitrogen LSP (i.e., LSP nitriding and conventional laser nitriding, respectively) and found that at similar processing conditions (8 mm OFD and 90 mm/s scan speed), the peak temperatures measured by a thermocouple located directly beneath the laser trail (on the backside of the CP-Ti coupon) were similar in both cases; moreover, the peak temperature recorded by a thermocouple located 13 mm away from the laser trail (again, on the back face of the substrate) was higher in the presence of the LSP.…”

“…Since calorimetric measurements showed that the nitrogen LSP absorbed about 38% (1.33 kW) of the 3.5 kW laser power sustaining it, it was concluded that the LSP re-radiated close to 75% of the power it absorbed from the laser, with the remaining 25% power used for heating the nitrogen gas exiting the LSP. Further, the presence of LSP was also found to increase the width of the nitrided trails and to reduce surface oxidation [109]. between the LSP and the substrate and estimated that the nitrogen LSP was equivalent to a heat source of 998 Watts within an error bar of ±10% for a laser power of 3.5 kW.…”

“…While the dilution of nitrogen with argon was found to be necessary to eliminate pore and crack formation at the top surface, it led to a drastic reduction in nitrogen uptake (e.g., an 11 vol.% dilution of nitrogen in the LSP reduced the nitrogen intake into the melt pool by 70%) due to reduced Marangoni convection in the melt pool [110]. To quantify the effect of the LSP on nitrogen intake into the melt pool, Kamat et al [109] measured the weight increase of the titanium coupons after nitriding in the presence and absence of a nitrogen LSP, i.e., during LSP and conventional laser nitriding, respectively; the processing conditions (laser power, scan speed, OFD) were maintained the same to ensure a fair comparison. It was found that the nitrogen uptake was consistently higher in the case of LSP nitriding for all the tested conditions; a microstructural examination of the top surface of the nitrided layers indicated that LSP nitriding produced a more porous surface consisting of "expulsion sites" that were absent in conventional laser nitriding (Figure 9).…”

“…The role played by near-surface plasma during CW CO 2 laser nitriding of commercially pure titanium (CP-Ti) was investigated in great detail in the Center for Multi-scale Wave-Material Interaction (CMWMI) at the Pennsylvania State University [100,106,[108][109][110][111][112] in recent years, resulting in the development of the so-called "LSP nitriding" process that was capable of improving the wear resistance of CP-Ti in open and uncontrolled atmosphere with minimal surface oxidation. Using a combination of charge-couple device (CCD) imaging and optical spectroscopy, Nassar et al [108] characterized the near-surface plasma formed during the laser nitriding process as a function of scanning speed and off-focal distance (OFD).…”