In the following discussion, it is necessary to examine the surface quality in terms of surface roughness, HAZ, hardness, and kerf quality in laser beam machining. Most of the study has been done on sections with relatively thin thicknesses (up to 5 mm), for which other approaches could also be used. But machining with a laser beam demonstrates their ability to cut those materials with better surface quality than other types that cut the same thickness.
Most of the done experimental studies based on laser beam cutting of hard-to-cut materials are primarily intended to investigate the impact of changes in input process parameters, such as laser power, cutting speed, assist gas pressure, standoff distance, nozzle diameter, pulse width, pulse frequency, lamp current, and focal point on output quality characteristics like HAZ, surface roughness, hardness, kerf width, kerf taper, roundness, and deviation where the experiments are carried out based on Taguchi, factorial, artificial neural network, or generic algorithm method.
In addition, there are many points which have been covered in this review paper such as the most recent papers in this field based on the study of three popular metals (titanium alloys, steel alloys, and aluminum alloys) which are used in most of the applications in the industrial field like aerospace industries, spaceship industries, automotive industries, sheet metal works, weldments work, hydraulic industries, and ship industries. On the other hand, the studied papers have been introduced in a way that makes the information easy to read, as they are arranged in a way which makes each point clear and complete to help the researchers get the information in an easy way.
Table 4 shows the effect of the input parameters on the response parameters for titanium alloys in detail by providing the thickness of the cut sheet, the assist gas type, the laser source, the input parameters, the output parameters, and the key factors for the selected papers. It illustrates that when a set of cutting speed, laser beam power, and gas pressure are used as an input parameters, the quality of the cut surface of Ti6Al4V alloy is depending on the speed of cut and the power of the laser beam with values equal 3000W and 2400 mm/min [77]. Also, it shows that the pulse width influences the improvement of the kerf width and kerf deviation with an overall value of 27.39% and the optimized kerf thickness and kerf taper values were discovered to be 0.033 mm and 0.001 mm, respectively, when used beside the power of laser and the speed of cut to machine Ti6Al4V alloy [78].
Table 4 also shows, when using different types of assist gas such as argon, nitrogen, and air to compare their effect on the surface roughness and HAZ of the cut surface, that argon is better than other types to get good surface roughness and small width of HAZ of TC1 alloy and to get thinner HAZ layer and use high pulse rates, medium pulse energies, high cutting speeds, and high-pressure values with argon as an assist gas [4]. The wait time was found to have the biggest effect for material removal rate (MRR) while laser power and pulse frequency are the dominant parameters for taper when these sets of parameters are used to cut Ti6Al4V alloy to see their effect on material removal rate and the taper of the cut kerf, which shows that due to increased thermal energy and better molten metal ejection in the machining area, increased pulse width and gas pressure result in increased material removal rate. Increased non-cutting time throughout the machining time span causes MRR to decrease as pulse interval time increases. Because of the rise in thermal energy and additional metal removal, further taper is caused by an increase in pulse power and frequency [79].
Table 4 also shows that if the speed of cut or the power of laser beam was used separately in a set of cutting parameters, it will be the dominant parameter for the quality of the cut surface. For example, using the speed of cut and the pressure of the assist gas as input parameters to see their effect on the roughness of the cut surface, it has been found that the speed of cut is the dominant parameters on the roughness of the cut surface when cutting a Ti6Al4V alloy. Also, when laser beam power is used with the assist gas pressure, the frequency, and the focal point as an input parameter to check their effect on the roundness of the cut path when cutting Ti6Al4V alloy, it has been found that laser power is the dominant parameter on the output [80, 81]. Also, when the speed of cut, the power of laser beam, and the defocusing amount have been used as an input parameters, to see their effect on the roughness of the cut surface and the width of the kerf of cut by cutting the pure VT1-0 alloy by fiber laser source, it has been found that cut at the highest cutting speed was between 100 and 200 Hz for optimal cutting. Gas pressure ought to be between 10 and 12 bar when cutting materials are up to 2 mm thick, and the focal spot should be positioned on the material’s surface [82]. Cutting speed and pulse frequency are the key factors affecting surface roughness, while assist gas pressure and pulse width are for kerf taper when Ti6AL4V alloy was cut. So, it is recommended to use lower values of frequency and pulse width, greater values of cutting speed, and a moderate nitrogen gas pressure to achieve satisfactory cutting quality in Ti6Al4V [83, 87]. When cutting a pure titanium and Ti6Al4V alloy by using a set of the power of laser beam, the speed of cutting, and the assist gas pressure as an input parameter to see their effect on the width of the kerf of cut, it was found that for both materials kerf width is inversely proportional with cutting speed and proportional with laser power [85].
Table 4 also shows that cast layer thickness is proportional with power and inversely proportional with cutting speed; also, kerf width is inversely proportional with cutting speed and proportional with power when Ti6Al4V alloy was cut using the power of the laser beam and the speed of cutting as an input parameters to see their effect on the quality of the cut surface and the width of the cut kerf [86]. It also shows when a set of assist gas pressure, pulse frequency, and lamp current are used to machine a different small thicknesses of Gamma-titanium aluminide alloy to investigate their effect on the width and the taper of the kerf of cut, lamp current was found the dominant parameter on outputs. Pulse frequency influences hole diameters. Thickness and assist gas pressure influence the exit hole diameter and hole taper [69].
Table 5 reveals the effect of the input parameters on the response parameters for steel alloys in detail by providing the thickness of the cut sheet, the assist gas type, the laser source, the input parameters, the output parameters, and the key parameters for the selected papers. For example, when a set of laser beam power, the velocity of the cutting, and defocusing amount are used to machine a 22MnB5 ultra-high-strength steel alloy to investigate their effect on the width and the HAZ of the kerf of cut, laser power and speed of cut were found the dominant parameters on outputs. Optimum values for minimum kerf and HAZ happened at 895W, 1923.44 mm/min at 100% of duty cycle. Minimum kerf taper happened at 779W, and 2268.65 mm/min at 100% of duty cycle [88]. Also, when a set of laser beam power, the velocity of the cutting, the pressure of the assist gas, and the defocusing amount are used as an input parameters to machine a set of different steel alloys to investigate their effect on the roughness of the cut surface and the width of the cut kerf, it has been found that cutting speed, defocusing, laser power, and assist gas pressure all have an impact on kerf width. Cutting speed, then laser power, has an impact on surface roughness. For minimum kerf width, optimum values were 420 W for laser power, 90 mm/min for speed, 10 bar for pressure, and − 1 mm for defocusing. For minimum surface roughness, optimum values were 420 W for laser power, 60 mm/min for speed, 8 bar for pressure, and − 1 mm for defocusing [106]. Also, when a set of laser beam power, the velocity of the cutting, the pressure of the assist gas, and the focal point are used as an input parameters to machine a set of different steel alloys to investigate their effect on the roughness of the cut surface, it has been found that the high quality of cutting happens at 0.7–0.8 mm of sheet thickness at high cutting speeds, low power with medium cutting speeds, medium power with high cutting speed, and medium assist gas pressure, and the focal position ought to be near the sheet lower surface [107].
Table 5 also reveals when a set of an AISI beam power, the velocity of the cutting, and pressure of the assist gas are used to machine a AISI 316L alloy to investigate their effect on the width of the kerf of cut, the speed of cut was found the dominant parameter on output. Optimum values for minimum value of surface roughness happened at 600W, 900 mm/min, and 6 bar [89]. But when the alloy has been changed to Strenx 900 steel plate alloy and the same set of laser beam power, the speed of cut, and the pressure of the assist gas were used, it was found that the laser power has the most influence on surface roughness over the other parameters, and the optimum values for minimum value of surface roughness happened at 600W, 900 mm/min, and 6 bar [90].
Furthermore, it also shows that when cutting Hardox 400 alloy by using oxygen as an assist gas with laser power, speed of cut, and pressure of oxygen as an input parameters to see their effect on the width of the cut kerf, it has been discovered that using a laser with a power of 5000 W, an auxiliary gas pressure of 0.50 bar, and a constant cutting speed of 1900 mm/min gets a fine cut. Kerf width depends on the amount of heat which is based on laser power and cutting speed, so low power and high cutting speed will lead to small kerf width [101]. It also shows that when cutting AISI 304 alloy using nitrogen as an assist gas with laser power, speed of cut, and pressure of nitrogen as an input parameters to see their effect on the width of the cut kerf, it has been discovered that using a laser with a power of 2571 W, an auxiliary gas pressure of 8.99 bar, and a constant cutting speed of 5498 mm/min gets a fine cut. Kerf width depends on amount of heat which is based on laser power and cutting speed, so low power and high cutting speed will lead to small kerf width, so cutting speed and laser power are the dominant parameters for small and fine kerf width [102].
Table 5 also displays that, as it has been mentioned in titanium alloys, when the power of laser beam or the speed of cutting is used separately with a set of other parameters, it becomes the most effective parameters of cutting over the other parameters. For example, when the speed of cut, the pressure of the assist gas, the focal point, and the pulse width are used as input parameters to see their effect on the width of the kerf of cut and the material removal rate by cutting high silicon-alloy steel, it has been found that using hybrid TMRSM makes an improvement in both MRR and kerf width (KW) over Taguchi method. KW is significantly influenced by cutting speed, pulse width, the square of the pulse width, and the interplay between the pulse frequency and the cutting speed. MRR is significantly influenced by cutting speed, frequency, pulse width, and the square of cutting speed [3]. Another trial happened when the speed of cut, the pressure of the assist gas, the standoff distance, and the defocusing amount are used as input parameters to see their effect on the taper of the kerf of cut for cutting mild steel; it has been found that the cutting speed is the effective parameter over defocus amount, stand-off distance, and assist gas pressure. For thick mild steel plates, the oxygen-assisted laser cutting process window is comparatively small [64].
Moreover, when the speed of cut, the assist gas pressure, the focal point, and the standoff distance are used as an input parameters to see their effect on the roughness of the cut surface and the width of the HAZ layer by cutting two different materials which are 1.4828 (X15CrNiSi20-12) and 1.4571 (X6CrNiMoTi17-12–2) alloys with nitrogen as an assist gas, it has been found that nitrogen gives a better surface quality. For minimum surface roughness, use high cutting speed with focus position above the cut surface. The optimum values of parameters are 1000 mm/min, 15 bar, and + 1 for focus position for 1.4828 alloy. Surface quality is achieved better in small thickness and also cutting speed is inversely proportional with thickness [111]. But one of the set which were velocity of cut, the pressure of the assist gas, and the focal point to cut a SS304 presented a different result which was that the focal point is the dominant parameter on surface roughness not the cutting speed and the optimum values for minimum value of surface roughness happened at 2400–2600 mm/min, 11.5–12 bar, and pulse frequency − 2.2 and − 2.5 mm [92].
Table 5 also shows, when a set of the power of laser beam, the speed of cut, the pressure of the assist gas, and the focal point as an input parameters to see their effect on the roughness of the cut surface and the width of the kerf of cut by cutting SS316 alloy, that the two main factors affecting surface roughness are laser power and focal point position. Cutting speed has the most influence when determining upper kerf width, followed by assist gas pressure, and lower kerf width is mostly affected by laser power followed by cutting speed [94]. Also, it shows that when a study has been done on AISI 304 and St37-2 with different sheet thicknesses by using a set of parameters to see their effect on the roughness of the cut surface and the properties of the kerf of the cut, it was found that the workpiece thickness has an impact on the top kerf width. Material type, gas pressure, and thickness all had an impact on the bottom kerf width. The taper angle is influenced by material type. Material type and cutting speed have an impact on the surface roughness [95].
Table 5 also shows, when a set of cutting speed and laser power were used to cut AISI 304 to see their effect on surface roughness and the width of the kerf of cut by using two different sources of laser which are CO2 and fiber laser to cut a range of thicknesses from 1 to 10 mm, that surface roughness increases in fiber laser between 4- and 6-mm thickness. Surface roughness increases in CO2 laser cutting between 8- and 10-mm thickness. Max. cutting speed was around 7000 mm/min. There is some difference in kerf profile [96]. Also, when a set of cutting speed and laser power were used to cut Hardox 400 and Hardox 450 alloys to see their effect on the width of the kerf of cut, longitudinal, surface, and volume energies by using two different thicknesses, it has been found that laser power and cutting speed have the largest influence on LSE. Thickness is the most effective parameter for SSE. Low thickness, high speed, and medium power are the effective mix for VSE [98]. Also, when a set of the power of laser beam and the speed of cut are used to investigate the roughness of the cut surface, the width of HAZ layer, and the width of the cut kerf of 4130 steel alloy, it has been found that low laser power and high cutting speed lead to a small kerf width, width of HAZ, and minimum roughness. Cutting speed is the next most important factor in determining kerf width after laser power. Cutting speed is the dominant parameter on surface roughness followed by laser power [1].
Table 5 also shows, when using two different types of assist gas which are nitrogen and oxygen to compare the effect of them on the surface roughness, and the width of the kerf of the cut by cutting the stainless-steel alloy, that for minimum surface roughness and kerf width, use low values of laser power, high cutting speed, high frequency, and low assist gas pressure. For smooth cut and small kerf width, using N2 is preferred over using O2, but it is not economical [5]. It also shows when using two different types of assist gas which are nitrogen and oxygen to compare the effect of them on the surface roughness; the width and the taper of the kerf of the cut by cutting the AISI 304 and St37-2 alloys at different thicknesses which are 1, 3, and 6 mm for AISI 304 alloy; and 2.5, 5, and 6 mm for St37-2 alloy, it has been found that the workpiece thickness has an impact on the top kerf width. Material type, gas pressure, and thickness all had an impact on the bottom kerf width. The taper angle is influenced by material type. Material type and cutting speed have an impact on the surface roughness [95]. It also shows, when using the same assist gases which are nitrogen and oxygen to compare the effect of them on the surface roughness and the width of the kerf of the cut by cutting the AISI 304 and St37-2 alloys at different thicknesses which are 1, 2, 3, and 4 mm, that nitrogen as an auxiliary gas gives a good surface quality compared to oxygen but cut thinner parts than oxygen, and also oxygen leads to bigger kerf width than nitrogen as it adds an extra heat to the cut point. Cut quality is low as well as kerf width is larger when using low cutting speed with O2 as an auxiliary gas. Also, when employing oxygen as an auxiliary gas instead of nitrogen, the initial hole diameter is greater [100].
Table 5 also shows, when comparing the types of laser sources such as comparing the fiber laser with CO2 when cutting AISI 304 with 6-mm and 10-mm thickness, that industrial practice agrees that CO2 has a superior cut quality than fiber lasers when cutting sheets that are 6 mm thick. However, in the case of 10-mm thickness, neither technology stands out to demonstrate superior quality [43]. Also, when comparing the fiber laser with CO2 and diode lasers when cutting AISI 304 with 1-mm thickness, it has been found that diode speed is less than half of fiber laser speed, but higher than CO2 speed. Diode is less bright than fiber laser in thin metals. Diode quality is less than fiber in thin metals but good as fiber in thick metals [99].
Table 6 displays the effect of the input parameters on the response parameters for aluminum alloys in details by providing the thickness of the cut sheet, the assist gas type, the laser source, the input parameters, the output parameters, and the effective parameters for the selected papers. For example, when a set of laser beam power and the velocity of the cutting are used to machine Al-2024-T3 alloy to investigate their effect on the roughness, the hardness, and the HAZ of the cut surface, the results were introduced as follows: cutting did not happen at 1500W and 3500 mm/min, optimum values for minimum value of surface roughness happened at 2600 W and 5000 mm/min, and HAZ is proportional with power and inversely proportional with cutting speed, but hardness did not affect by cutting speed or power [72]. The same set was used to cut a sheet of Al2O3 alloy to investigate their effect on the surface roughness and the width of the kerf of cut. It has been found that kerf width is inversely proportional with cutting speed and proportional with power, kerf width increasing percent is low, and cast layer thickness is proportional with power and inversely proportional with cutting speed [86].
The speed of cut was found the dominant parameter on the width of the kerf of cut and the roughness of the cut surface when a set of laser power, speed of cut, and the pressure of the assist gas were used to cut a sheet of Al 6351 alloy; the optimum values for minimum value of surface roughness and kerf width happened at 3200W, 7390 mm/min, and 7.7 bar [112]. The cutting speed was again the dominant parameter when the same set was used to cut SiCp/Al composite alloy to see their effect on the width and taper of the kerf of cut. For efficient MRR, use low cutting speed with 3500W and high cutting speed with 4500W, and the minimum profile contour happened at 4000W, 2500 mm/min, and 8 bar [115]. Also, when the power of laser beam and the speed of cutting is used with a set of other parameters, they become the most effective parameters of cutting over the other parameters. For example, when a set of the power of laser beam, the speed of cut, the standoff distance, and the nozzle diameter are used as an input parameters to investigate their effect on the roughness of the cut surface and the width of the kerf of cut by machining the Al-5052 alloy, it has been declared that the primary factors for the output values are the laser power and cutting speed. Turbulence is produced by larger nozzle diameters, which lowers cut quality. It is evident from the microstructural investigation that turbulence has an impact on the quality of cut materials [70].
Moreover, when a set of the power of laser beam, the speed of cut, the pressure of the assist gas, and the frequency are used as an input parameters to investigate their effect on the roughness of the cut surface, the width, and the taper of the kerf of cut by machining the Al6061/SiCp/Al2O3 composite alloy, it has been discovered that the optimal cutting parameters are 2970.94W, 1196.4 mm/min, 8.42 Hz, and 12 bar, which mean that the power and the speed of cut have the most effect on the output [117]. Also, when a set of the power of laser beam, the speed of cut, the pressure of the assist gas, and the frequency are presented as an input parameters to investigate their effect on the roughness of the cut surface, the width of the HAZ layer, and the width of the kerf of cut by machining the AA5083 alloy, it has been found that the laser power and cutting speed are the dominants for the output values. The surface roughness values depend on the assist gas pressure and pulse frequency. HAZ and kerf width are inversely proportional with cutting speed and proportional with power. Assist gas higher pressure value will lead to high melted MRR [118].
Furthermore, when a set of laser beam power, cutting speed, assist gas pressure, and standoff distance are presented as input parameters to investigate their effect on the roughness of the cut surface by cutting the Al 6061-T6 alloy, it has been found that the minimum surface roughness happened at high speed and low power, as it leads to low temperature [120]. Also, when a set of the power of laser beam, the speed of cut, the pressure of the assist gas, and the standoff distance are presented as an input parameters to investigate their effect on the roughness of the cut surface and the width of the kerf of cut by cutting the aluminum alloy, it has been found that the surface roughness and kerf width are mostly influenced by the laser power, cutting speed, and the standoff distance, whereas assist gas pressure has little bearing on either. The surface roughness and kerf width are inversely proportional with cutting speed and standoff distance and proportional with power [125].
Table 6 also shows when a set of the frequency, cutting speed, assist gas pressure, and pulse width are set as input parameters, the optimal cutting parameters are 1.2 ms, 1500 mm/min, 28 Hz, and 8 bar. The assist gas pressure and pulse frequency have the dominant effects on the kerf quality [122]. Also, it shows that when a study has been done on AA2B06 with different sheet thicknesses by using a set of parameters to see their effect on the roughness of the cut surface and the energy efficiency, it was found that for sample group 1, 2000W, 6000 mm/min, and 15 bar are the ideal values for sheet thickness of 1.5 mm and 2 mm. For sample group 2, the ideal values are 2000W and 9000 mm/min for sheets that are 1.5 mm and 2 mm thick. The ideal pressures are 5 bar for 1.5 mm and 10 bar for 2 mm. Surface roughness and energy efficiency have been improved with significant values [116].
Table 6 also shows that when a study has been done on AL6061T6 with different sheet thicknesses by using a set of parameters to see their effect on the roughness of the cut surface, the results are barely impacted by the cutting speed and gas pressure. With increasing laser power, anticipated values for cutting temperature increase while those for surface roughness drop. Higher values of sheet thickness result in higher surface roughness and lower cutting temperature values [121]. But for Al 6061-T6 alloy, with different thicknesses, it has been found that both the power of laser beam and the speed of cut are the effective parameters [120].
Table 6 also shows, when comparing the types of laser sources such as comparing the fiber laser with CO2 when cutting a sheet of Al2O3 alloy by using the same set of input parameters for both types, that the optimal cutting parameters for fiber laser are 250 W, 6000 mm/min, 0.3 mm for sod, 0.3 mm for focus, and 12 bar. The optimal cutting parameters for CO2 laser are 180 W, 7200 mm/min, 0.4 mm for sod, − 0.25 mm for focus, and 12 bar [123]. It also shows, when using four different types of assist gas which are argon, air, nitrogen, and oxygen to compare the effect of them on the surface roughness and the HAZ width, that O2, N2, and air produced a different value of dross, width of HAZ, and a change in microstructure of the AL-CU alloy. Argon is the best assist gas for best quality of cutting with excluding the cost [124].
This review can be concluded in these points: