Cutting of 1.2 mm thick austenitic stainless steel sheet using pulsed and CW Nd:YAG laser

Abstract

Stainless steel is an important engineering material that is difficult to be cut by oxy-fuel methods because of the high melting point and low viscosity of the formed oxides. However, it is suitable to be cut by laser. This work aims to evaluate the optimum laser cutting parameters for 1.2 mm austenitic stainless steel sheets by using pulsed and CW Nd:YAG laser beam and nitrogen or oxygen as assistant gases, each one separately. It was shown that the laser cutting quality depends mainly on the laser power, pulse frequency, cutting speed and focus position. The optimum cutting was achieved during pulsed mode at applied power 210 W, frequency 200–250 Hz, speed 1–1.5 m/min, focus position −0.5 to −1 mm, nitrogen pressure 9–11 bar and oxygen pressure 2–4 bar. Increasing the frequency and cutting speed decreased the kerf width and the roughness of cut surface, while increasing the power and gas pressure increased the kerf width and roughness. Comparing with oxygen, nitrogen produced brighter and smoother cut surface with smaller kerf, although it did not prove to be economical. In CW mode, the speed can be increased to more than 8 m/min with equivalent power and gas pressure (limited by the laser system). Pulsed mode was also not economical, especially in limited frequency laser systems, where the pulse overlap should be controlled by both frequency and speed. In CW, the speed can be increased to the maximum system limit. The results should be included in computerized database for the automatic implementation of laser process.

Introduction

Laser cutting of metals has become a reliable technology for industrial production. Currently, it is considered as a feasible alternative to mechanical cutting and blanking due to its flexibility and ability to process variable quantities of sheet metal parts in a very short time with very high programmability and minimum amount of waste. Laser cutting does not need special fixtures or jigs for the work piece because it is a non-contact operation. Additionally, it does not need expensive or replaceable tools and does not produce mechanical force that can damage thin or delicate work pieces. References [1], [2] give detailed descriptions of the laser phenomena and the mechanism of laser cutting of materials and Fig. 1 shows the typical laser cutting process. Laser cutting is classified as a typical thermal process that has special advantages over other known thermal processes due to the high quality and very smooth cut surface, narrow kerf width, small heat affected zone (HAZ), small metal deformation, perpendicular and sharp cut sides, square corners of cut edges and little or no oxide layer.

Nowadays, two types of laser—emitting infrared spectral radiation—are used in the processing of metals: CO₂ and Nd:YAG lasers. CO₂ lasers (10.6 μm wavelength) have higher efficiency of about 30%, higher beam quality (near to the optimum Gaussian mode [2]), higher depth of focus, smaller beam diameter and able to cut 16 mm of mild steel and 12 mm of stainless steel using 2000 W continuous laser beam. On the other hand, solid state Nd:YAG lasers have the advantages of less floor space, simple maintenance requirements, easy beam alignment, easy sharing, smaller wave length (1.06 μm) that can cut materials having greater reflectivity to CO₂ lasers. Using diode pumped Nd:YAG lasers increase their efficiency up to 20–30% [3]. Moreover, Nd:YAG laser beam can be delivered and shared by using optical fibers to multiple and far work stations. This also allows the spread implementations of robot manipulators that can deliver the beam through complicated and curved 3D paths, for example, automobile and aircrafts bodies. Nevertheless, using multi-axes robots lacks the high accuracy of CNC tables because of the effect of robot motors dynamic accuracy and the vibration induced during arms motion especially when high gas pressure is supplied [4].

Laser can be emitted in continuous, constant and steady power (CW mode) or in the form of very short and intense burst of energy (pulsed mode). There are specific areas where either a pulsed laser or CW laser is more suitable. Peak pulse powers for pulsing Nd:YAG lasers can reach values of over 30 times greater than the maximum average power levels. This allows low-to-medium power lasers to achieve enough energy to reach vaporization temperatures for most materials.

Almost all materials can be cut by laser. However, material properties such as absorption to electromagnetic wave length, thermal and electrical conductivity, melting temperature and surface condition govern the selection of laser and optics systems [2]. Very thin sheet metals and other non-metals such as wood and polymers can be cut only by laser energy, where the focused laser beam elevates the material temperature up to the melting and evaporation temperature then the vapor is removed by the assistant gas. This is called “sublimation laser cutting”. For thicker sheet metals, “laser fusion cutting” is dominating where metals are only melted and the molten materials are removed by the assistant gas jet. If the applied laser energy, cutting speed, focus position and assistant gas pressure are not controlled properly, incomplete melting occurs or traces of molten metal re-solidify over the cut sides forming undesired dross. For thick metal plates—especially mild steel—or high speed cutting the “exothermal laser cutting” should take place and oxygen should be used as an assistant gas to (i) add additional exothermal energy to the cutting laser energy by its reaction with the material when heated by laser to the ignition temperature and (ii) generate a low viscosity, low surface tension molten metal particles which does not adhere to the parent metal and can be removed easily from the cut zone [5], [6].

Stainless steel is a distinguishable engineering material that has high potentials in many important industries especially automobiles and house appliances. In high humidity weather countries in Asia, stainless is replacing the mild steel and it is used as a common building and decorative material. Stainless steel cannot be cut using traditional oxy-fuel cutting equipment because of the high melting point and viscosity of the oxide formed. Even oxy-acetylene cannot provide sufficient heat intensity to avoid the molten oxide to solidify. However, the oxy-fuel process can be used with powder cutting in which additional heat is generated from the injection of iron powder into the cutting zone.

Laser cutting process is ideally suitable for stainless steel and achieves accuracy and productivity. One of the key factors for the optimum cutting of stainless steel is the selection of the assistant gas which could be oxygen (active gas) or nitrogen (inner gas). Using oxygen, thicker sheets can be cut and more cutting speeds can be used. This is due to the fact that oxidation of iron releases a certain degree of heat (100–150 J/molecular weight × degree), which is an added energy to the energy of the focused laser beam. As an average, the amount of energy supplied by the burning reaction is about 60% for mild and stainless steels and up to 90% for reactive metals such as titanium [5]. The balanced chemical equations for the exothermal reaction of the austenitic stainless steel are:

Fe + O → FeO + heat (267 kJ)

3Fe + 2O₂ → Fe₃O₄ + heat (1120 kJ)

2Fe + 1.5O₂ → Fe₂O₃ + heat (825 kJ)

Cr + 3O₂ → 2Cr₂O₃ + heat (1163.67 kJ)

Ni + 1.5O₂ → NiO + heat (248.23 kJ)

However, using oxygen is not recommended for stainless steel sheet with thickness more than 1 mm because it leads to oxidized (blue) cut edges and forms high-melting-point oxides with a rising alloying element contents that affect the cutting quality and causes the formation of low viscosity dross that adhere to the lower cut surface. Oxidation of cutting edges could lead to corrosion cracking because of the increasing of nickel contents and the decreasing of chromium and manganese contents.

Using nitrogen as an inner gas produces very bright and clean cut surface and does not affect the corrosion resistance of the cut edge. But it usually requires very high gas pressure (more than 10 bar to cut 1 mm stainless steel [3]) and low cutting speed to achieve good cutting quality and dross-free lower surface. Inert gases do not contribute to the reaction energy that comes only from the focused laser beam. They act as a mechanical force to remove dross from the lower cutting edges and protect laser optics from being damaged by the resulting ejected spatters and also remove the formed plasma that reduces material absorption to laser energy. Nitrogen is also used to cut mild steel and stainless steel at very high speeds such as 20 m/min, where oxygen cannot be used because of the slower rate of the exothermal reaction under laser–oxygen conditions. Gabzdyl [7] showed that the purity of the supplied gas affects cutting quality and the cutting speed. He proved that increasing the oxygen purity from 99.7 to 99.95% gives a 10% increase in cutting speed. He also showed that the presence of oxygen at a level as low as 50 vmp in the supplied nitrogen gas could produce detrimental effect and give straw colored cut edges.

As the cost of the supplied gas constitutes the largest portion of the operation cost, it is very important to optimize the cutting speed and other parameters in order to minimize the amount of supplied gas, thus the total production cost.

Grum and Zuljan [8] monitored the intensity of infrared radiation emitting from the laser cut front of austenitic stainless steel and used this to measure the temperature signals released during laser cutting at different speeds. They showed that the amount of energy input into the cutting front varies due to the oscillation in laser source power and leads to changes in the heat released in exothermal reactions and heat losses. The temperature in the cutting front ranged from 1950 to 2250 °C, which for the chromium–nickel steel is estimated as too low for narrowest cut ensuring high surface quality and considering the melting temperature of iron and chromium oxides, the lowest temperature achieved in the cutting front according to Grum and Zuljan [8] prediction is around 2200 °C. They also showed that the exothermal reactions are induced in the austenitic stainless steel and a greater quantity of energy compared to low carbon steel is released. Also, the thermal conductivity of low carbon steel is much higher than that of high alloy austenitic steel, and the result of lower conductivity and higher energy in the cutting front is that a wider cut is made than that in low carbon steel.

For CW or pulsed Nd:YAG laser systems (also to CO₂ systems), laser cutting quality is governed by many parameters; some of them are related to the laser equipment such as (1) maximum laser power, (2) wave length, (3) efficiency and (4) emerging beam waist diameter. Others are related to the delivery optics such as (5) focused beam diameter, (6) fiber diameter (in case of optical fiber delivery), (7) focal length of the focusing lens, (8) focus depth and (9) amount of power loss in the delivery optics and fiber. The remaining are related to the operation such as the (10) cutting speed, (11) focus position related to the upper material surface, (12) nozzle tip diameter, (13) distance between the tip and the material (stand-off distance), (14) assistant gas type and (15) assistant gas pressure. In the case of pulsed laser, (16) pulse frequency, (17) duty percentage (time for laser on) and (18) peak should be also considered. (1)–(9) are system specifications that determine the type and thickness of material to cut that system and cannot be modified by the operator. For example, Nd:YAG laser with smaller wave length can cut metals with higher reflective surface such as aluminum and titanium. On the other hand, (10)–(18) are the variable parameters that should be optimized in order to get the desired laser quality. Every material and thickness have their optimum laser cutting parameters. Moreover, laser cutting power should be optimized while considering the thermal losses that occur due to material evaporation that is too strong, heat transfer into the surroundings of the work piece material, heat radiation and energy loss due to acoustic emission.

An additional cutting variable that affects the cutting quality is the initial work piece temperature or preheating. In general, preheating can improve the cutting quality by an increased cutting speed and reduced temperature gradient in the work piece. However, when many contours with complicated shapes are nested and cut with a constant cutting speed and laser power, work piece preheating may cause deterioration of cutting quality due to the excessive heating by the heat accumulation from the previous cutting contours.

Sharp [9] mentioned that the cutting speed and the minimum pulse frequency have to be calculated to ensure that two neighbor laser spots on material surface will overlap 60–80%. In addition, Chmelickova and Polak [10] calculated the approximate optimum cutting speed for stainless steel by using the equation for heat and energy: laser energy is equal to sum of energy necessary to vaporize materials in cutting path:

$$\rho V(c\Delta T+L_{\text{v}})+Q_1=P(1-R)$$

where ρ is the material density, V the product of beam diameter D, cut depth h and cut length x, c the specific heat of vaporized materials, ΔT the difference between vaporization and normal temperature, L_v the latent heat of vaporization, Q₁ the heat losses, R the surface reflectivity and P is the laser energy. They showed that for 1 mm stainless steel cut by pulse frequency within the range of 50–200 Hz and pulse length within the range of 0.2–0.3 ms, the optimum speed lays within the range of 0.9–1.5 m/min. This result is close to the experimental data in Ref. [3].

Hamoudi [5] showed that, during cutting by 2 kW CO₂ laser assisted by 10 bar of nitrogen, increasing cutting speed leads to increasing the kerf width, improved surface roughness and narrow HAZ. Little dross was formed at low speed less than 1 m/min and the best cutting quality was achieved at 2 m/min. The same results were achieved under exothermal conditions (using oxygen as an assistant gas) except that the kerf width decreased with increasing the speed. Rajaram et al. [11] showed that power had a major effect on the kerf width and HAZ while cutting speed played a minor role. They also showed that the cutting speed has a major effect on surface roughness and striation frequency and that power has a small effect on surface roughness and no effect on striation frequency. Yilbas [12] also showed almost similar results.

The efficiency of cutting a material by laser depends on the physical properties of the material including heat conduction, phase change, plasma formation, surface absorption and molten-layer flow. The heat conduction into the work piece influences grain refinement, carbide formation and other sulfide and phosphide impurities that might exist due to the alloying elements in stainless steel. These phenomena result in the formation of small HAZ within the thickness range of 10–50 μm. Masumoto et al. [13] investigated the corrosion zone formed in the welded joints of austenitic stainless steel. They used surface treatment by laser to achieve very high heating and quenching rates that prevent the re-precipitation of the carbides. However, they did not take into account the region lying a little further away from the laser beam center which have lower heating and cooling rates and is hence once again susceptible to carbides precipitation. Nakao and Nishimoto [14] studied austenitic stainless steels that were sensitized in the HAZ due to the depletion of chromium. However, they did not discuss the extent of the HAZ due to limitation in the laser process itself.

Sheng and Joshi [15] showed that the HAZ increases with increasing the laser power. This can be attributed to the fact that materials see more laser heat with increasing laser power. On the other hand, the HAZ decreases with increasing the speed. As the laser beam moves faster, any point in the material receives less heat.

Generally, laser cutting of metal sheet develops a periodic striation patterns (see Fig. 1) which affect the quality of the finished cut surface, especially when using oxygen as an assistant gas. Many studies have been done to investigate striation formation mechanism. Miyamoto and Maruo [16] had firstly explained these striation lines by the model of ignition–extinction cycle. Later, Ivarson et al. [17] explained that the formation mechanism of striations is not one of the following: (i) a resolidification process; (2) gas dynamics; (iii) melt boiling; (iv) optical effects; (v) time-based fluctuations of power input, etc. They explained that the most likely sources of formation mechanism are: (i) a cyclic variation in the driving force of the oxidation reaction, the variation being brought about by changes in the oxygen partial pressure in the cut zone melt, and (ii) viscosity and surface tension effects associated with melt removal. The curvature of the striation lines match the curvature of the laser beam cut front and so the inclination occurs when the energy flux velocity equal to the cutting rate. Using numerical simulation and experimental work, Kim et al. [18] showed that the laser power intensity is the most important factor in the formation of striation patterns, since the laser power density is the most influential in the heating of metal and striation formation is caused by the ejection of molten metal and evaporation during laser cutting process. They showed that high laser power resulted in clear regular striation pattern and relatively low power density caused the formation of a hot spot, which hindered the formation of regular striation patterns and caused less striation.

Powell et al. [19] and Lobo et al. [20] examined the particles ejected during laser cutting. Powell et al. showed that the particles have size ranging from 50 to 500 μm and they are mainly spheres of iron surrounded by a shell of iron oxide or sphere of iron and iron oxide containing a suspension of iron and iron oxide and surrounded by a shell of iron oxide. Stainless steel particles are mainly spheres of metals surrounded by Fe₂O₃ and Cr₂O₃.