LIST OF TABLES
CHAPTER 2: LITERATURE REVIEW ON LASER BENDING PROCESS
2.8 Effect on Mechanical and Micro-structural Properties
Laser scanning generates high temperature and localized deformation in the heated region due to which strain hardening, dynamic recrystallization, and phase transformation occur in the material. This changes mechanical and metallurgical properties of the heated region. Literature depicts studies on the effect of process parameters on micro-structural and mechanical properties of the laser deformed workpiece.
2.8.1 Studies on microstructural properties
Literature reveals a number of attempts on the microstructural studies during laser forming of the workpiece. Hu et al. (2001) analyzed the surface of the laser irradiated stainless steel plate and observed that the laser does not adversely affect the microstructure, and does not produce any cracks or porosity in the plate. The recrystallized grain size and the orientations of the grain were quite different from the base material. However, the authors expressed the possibility of a few scattered micro-cracks on the laser scanned surface, which may be associated with high cooling rate (Yilbas et al. 2012). Paunoiu et al. (2008) observed that the microstructure of stainless steel AISI 304 is affected by the laser beam. The annealing twins were appeared on the concave side of the laser scanned specimen. The annealing twins, density of the annealing twins, and the depth of their penetration increased with laser power. Yang et al. (2010) found that the overlapping of two laser pulses affect the interaction time between laser and material.
It was noted that when the overlapping exist, the case-hardening of the heat affected zone occurs. Hardness and anti-corrosion properties were found to be increased in the irradiated zones.
Bartkowiak (2004) studied laser forming of Ti-6Al-4V sheets using continuous wave and pulsed Nd:YAG lasers, and found that the depth of the oxidation zone increases with the number of scans. They also found that the depth of heat affected zone after 15 and 30 scans was about double in the pulsed laser system as compared to that in continuous wave mode.
Chen et al. (2004a) analyzed the microstructures of laser irradiated Ti–6Al–4V alloy and did not observe any obvious difference compared to the microstructure of the original material.
The scanning at high laser energy density separated out the α-phase from β-grain boundaries, and therefore, β-grain size was increased. Fan et al. (2007) studied phase transformations in heat affected zone of AISI 1010 stainless steel. The grains were distinctly refined due to the phase transformation and recrystallization in the heat affected zone. After irradiation, a substantial amount of martensite was formed due to the high cooling rate.
Topić et al. (2007) observed that the residual strains vary in both the transverse (tensile in nature) and the longitudinal (compressive in nature) directions of the scanning path.
Martensite phase transformation occurred due to rapid heating and cooling of the irradiated region. Grain refinement was observed at approximately 1.5 mm below the surface irradiated by the laser beam. Singh et al. (2013a) studied the microstructure of multi-scan laser bent mild steel. They observed that the average grain size in scanned region gradually decreased from the bottom to the top surface. The micro-structures at the top and the bottom surfaces are shown in Figure 2.8 (a) and Figure 2.8 (b). The reformation from coarse grain to fine grain was more at the top surface in comparison to that at the bottom surface. It was due to higher temperature and higher strain hardening at the top surface.
(a) (b)
Figure 2.8. Micro-structure after 20 laser scans (Laser power 400 W, Scan speed 300 mm/min) (a) at top surface, (b) at bottom surface (with e-mail consent of the author, Copyright © Singh et al. 2013a).
2.8.2 Studies on mechanical properties
Laser bending process involves strain hardening, compressive and tensile deformations, and generation of residual stresses in the heated region. These factors affect the mechanical properties like tensile strength, fatigue strength, hardness and ductility of the material (Merklein et al. 2001).
McGrath and Hughes (2007) reported that the fatigue life of workpiece materials enhanced after laser scanning. The endurance limit of the laser bent specimen was also increased. It was due to laser-hardening mechanism and compressive residual stresses induced due to laser beam irradiations. Thomson and Pridham (2001) studied the effects of laser scanning on the material properties of the mild steel. They found that the performance of laser formed parts was good. Due to laser scanning, the strength of the scanned region increased, the
ductility decreased and strain ageing apparently increased in laser forming of mild steel. Shen and Yao (2009) studied tensile behavior of laser bent low carbon steel workpiece, and observed improvements in the yield strength and tensile strength but reduction in the total elongation.
Walczyk and Vittal (2000) studied material properties of laser bent titanium sheets and observed that the fracture toughness and fatigue strength of titanium deteriorates due to laser forming.
Singh et al. (2013a) carried out a 3-point flexure test on laser bent specimen. The flexural stiffness of laser bent specimen was more as compared to mechanically bent workpiece. It was found that the Young’s modulus of elasticity was same for both the laser bent and mechanically bent workpiece, but the elasto-plastic stiffness of laser bent workpiece was more than that of the mechanically bent workpiece. Cheng and Yao (2001) observed that the intermediate cooling between laser scans moderately decreased the ductility of stainless steel. It was due to application of repeated work hardening cycles after respective softening of the work material. The hardening occurred due to plastic deformation and softening occurred because of recovery and recrystallization accompanied by each laser scan.
Majumdar et al. (2004) studied laser bending of AISI 304 stainless steel sheets and observed that the micro-hardness of the irradiated zone is increased by 1.5 to 2 times as a result of grain refinement. Fan et al. (2007) bent AISI 1010 stainless steel and noted that the micro- hardness decreases along the thickness of laser bent specimen. The hardness was influenced by both phase constitution and work hardening. Due to high cooling rate, a substantial amount of martensite formed at the top surface leading to high hardness. Kgomari and Mbaya (2010) compared the laser treated and mechanically formed high strength low alloy steels. It was observed that the phase changes from a fine-grain ferrite to predominantly coarse-grain ferrite at the laser scanned surface, and therefore, the hardness decreases significantly. Singh et al.
(2013a) studied laser bending of mild steel sheet and found that the hardness increased with laser power due to higher peak temperature.
Carey et al. (2010) investigated the use of laser forming to shape thermosetting glass fibre based fibre metal laminates. It was found that TGM is the only mechanism which gives an out-of-plane bend without considerable damage to the laminate. BM dominated process parameters caused the delamination of the structure and the heat damage to the composite layer.
UM could not be produced in fibre metal laminates.
Observations
The laser bending process involves localized high temperature and plastic deformation which significantly affect the mechanical and microstructural properties. Variation in these properties depends on process conditions and the type of workpiece material. Investigation on these properties is important to assess the feasibility of application of laser bending for various materials.