Laser forming is an emerging manufacturing process capable of producing either uncomplicated and complicated shapes by employing a concentrated heating source. The heat source movement creates local softening, and a plastic strain will be induced during the rise of temperature and the subsequent cooling. This contactless forming process may be used for the simple bending of sheets and tubes or fabrication of doubly-curved parts.

Sixty years have passed since the day Theodore H. Maiman built the first laser in 1960 at Hughes Research Laboratories. Since then, different types of laser were invented and improved for use in industries and laboratories. Two recent review papers are currently published and address various aspects of using laser technology. Dixit et al. [1] and Das and Biswas [2] published review papers on laser forming in 2015 and 2018, respectively. However, laser technology, applied in manufacturing technology, can be viewed from a completely different prism. The “LASER” acronym stands for Light Amplification by Stimulated Emission of Radiation. A single phase-coherent beam can be focused on a tiny spot, and according to the power of the laser source, different processes are developed. The low magnitude power leads to surface treatment and annealing of the workpiece; increasing the laser power leads to the melting of the workpiece and welding. High power lasers melt and evaporate the workpiece, and metal cutting happens. The number of photons controls the power of the laser and the frequency emitted from the light source. The ability for industrial applications varies from 100 W to more than 5 kW. Forming is another type of process, which can be implemented by laser technology. The workpiece can be a flat plate (sheet) or tube.

Laser beam irradiation induces thermal stresses into the worksheet. Temperature gradient mechanism (TGM), buckling mechanism (BM), and upsetting mechanism (UM) are three proposed mechanisms that happen during the laser beam irradiation [3,4]. The cooling of the sheet, at last, leads to bending the sheet due to residual stresses and corresponding strains. By increasing the temperature during laser beam irradiation, the physical properties of the sheet include yield strength, ultimate tensile strength, and the elastic (Young) modulus will also be decreased. The yield strength of the material decreases faster than the elastic modulus.

Consequently, the equivalent (von Mises, for instance) stress proportional to the elastic modulus overtakes the yield strength in some regions of the sheet. So, plastic deformation occurs over the irradiated zone. The amount of plastic deformation depends on both the pre-stress field and temperature distribution in the sheet. The more massive plastic strain will be obtained by higher peak temperatures and larger pre-stresses [5]. The laser beam specifications, the scan pattern, and the properties of the sheet determine the final shape of the workpiece. Reasonable attempts have been made by researchers in recent years to investigate the effect of laser irradiation path on the quality, productivity, and shape of the final workpiece. In the laser forming process, the heating of the workpiece is non-uniform and asymmetric. The beam irradiates from one point of the workpiece and scans along a defined path. The geometry condition is not constant along the heating line. So, an unwanted deformation will be created in the sheet called the “edge effect”. This undesirable deformation reduces the forming accuracy. By introducing the laser forming technology, new applications responded in various industries. Bending of small parts with little bending angle (less than 1°), bending of complicated parts for aerospace industries such as tailor machined blanks (TMB), bending of low diameter tubes with minimum ovality and thickness variations, forming of composite parts with little delamination, and forming of doubly curved specimens to obtain the irradiating patterns which can be used in the flame forming of the dome and saddle-shaped parts in ship bodies are examples of the industrial applications of laser forming.

2. Laser Forming of Composite Sheets

The effect of material type and layers configuration on the laser-formed sheets were discussed in previous sections. In this section, the laser forming of composite sheets will be discussed. The type of composite is essential. Fiber and matrix have different mechanical properties (yield strength, stress-strain behavior) and physical properties (melting point, density, conductivity). Seyedkashi et al. [38] investigated the laser forming of three-layered SUS430/C11000/SUS430 laminated composite sheets. The copper mid-layer has higher thermal conductivity than the stainless steel 430 sheets. The plastic strain, along with the thickness and the shear stress between the layers, is more complicated than a single monolithic layer. Warping is another problem in the laser bending of composite samples. During the laser forming of stainless steel-carbon steel composite plate in ANSYS software, the non-uniform heat distribution and different heat conductivity and heat loss produce a warping force which causes undesirable deformation of composite in addition to the edge effect [39]. The sequence of layers is also important and affects the bending angle of the multilayer composite samples [40].

Fiber–Metal Laminates (FML) are made of different layers of metal and composite material. The difference between the properties of fiber, matrix, and metal leads to complicated plastic deformation, bending, and unpredictable. The complicated deformation mechanisms, interfacial delamination, and thermal alteration of the layers are three main challenges in the laser forming of FMLs. However, using the experimental test and using an artificial neural network tool made it possible to predict the behavior of FMLs during the laser bending process [41,42]. Moreover, the deformation of FMLs can be predicted using Eigen-strain field prediction [43,44]. The composites are divided into three categories, polymer matrix composite (PMC), ceramic matrix composite (CMC), and metal matrix composite (MMC). By increasing the use of MMC parts and the easy fabrication of PMC parts, more study is needed to fabricate the MMC and PMC made parts by laser beam technology. The use of MMC materials is increasing rapidly in the aerospace industry.

3. Laser Tube Bending

Tube bending was always a challenge in conventional bending. Usually, an internal plug is inserted inside the tube, and bending happens. However, a laser beam can also be used for tube bending. The laser beam is irradiated on the tube surfaces and may result in 2D or 3D tube bending. The amount of research about tube bending is low in comparison to the laser sheet metal forming. The effects of the irradiating length and the number of irradiating passes on tube bending have been studied. Three primary defects of the laser tube bending are lateral bending angle (especially when the scanning path is complicated like spirals), ovality, and thickness variation. The tube bending angle increased by increasing the irradiation length and number of passes. Moreover, the ovality percentage and the thickness variation will be increased by increasing the irradiation length [54]. The circular scanning method can be used for 2D and 3D tube bending. A scanning strategy has been developed to determine the scanning path using direct and reverse solution [55]. The effect of process parameters is like sheet metal forming, and the bending angle of the tube increases by increasing the laser power and beam diameter and with a decrease in the travel speed [56]. Figure 3 shows two samples of laser tube forming. Laser beam absorption can also be performed to improve the obtainable bending angle [57]. Different materials, such as carbon steel, stainless steel, and nickel tubes, can be formed and used in various industries such as aerospace industries, engines, heat exchangers, and air conditioners [54,55,56,57,58,59,60]. Statistical tools, such as Particle Swarm Optimization (PSO), can predict the bending angle and compensation of springback [58]. The Taguchi design of experiment, Artificial Neural Networks (ANN), and genetic algorithm (GA) are standard statistical tools which can find the optimized condition to obtain maximum bending angle, minimum ovality, minimum thickening, and minimum forming energy consumption [60].

Figure 3. 2D and 3D tube bending [55]. (Adapted from [55], with permission from Elsevier, 2020).

The size of the tube is another essential process parameter. Normal sizes, such as ½ and ¾ inch tubes, can be formed using a laser beam without difficulties. However, the laser forming of micro-tubes (for example, a tube with 635 μm outer diameter) needs more precise tools and controlled conditions [59,61]. The micro-tubes are thin-walled structures, due to high thermal conductivity and quick heat dissipation from the irradiated zone, short pulse, and high power laser beams utilized for inducing plastic strain and tube bending. It seems that researchers only focus on the bending of round tubes. Rectangular tubes, especially the 3D bending of them, is an attractive subject of study. The ovality of 2D and 3D laser-formed tubes and thinning of the tube is also important and needs more investigation.

4. Other Applications of Laser Beam Technology

A short pulse laser beam can be employed to fabricate micro-sized parts like MEMS parts [110]. Metal foams can be formed by laser forming [111,112,113,114,115,116,117,118,119]. Metal foams are a new category of materials which has a low density and good geometrical flexibility. Instead of applying a mechanical load, the laser irradiation heat produces a temperature gradient along with the thickness. The results show that the temperature gradient mechanism (TGM) is the primary mechanism of the closed-cell metal foams [113]. Stiffened parts and reinforced panels can be formed by laser beam irradiation [120,121,122]. The laser beam can be used in various processes like laser peen forming (LPF) [123,124,125,126,127], laser shock peening (LSP) [128,129], Laser solid forming (LSF) [130,131], laser folding [132], modification of mechanically bent parts (correction of spring back, over-bending or distortion of sheet [133]) and measuring the bending angle [134].

The contents have been extracted from review paper: https://doi.org/10.3390/met10111472