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A review of the use of high power diode lasers in surface hardening
Journal of Materials Processing Technology 155–156 (2004) 1855–1860
A review of the use of high power diode lasers in surface hardening E. Kennedy a,∗ , G. Byrne a , D.N. Collins b a
Advanced Manufacturing Science Research Group, Department of Mechanical Engineering, University College Dublin, Dublin, Ireland b Department of Mechanical Engineering, University College Dublin, Dublin, Ireland
Abstract Laser surface hardening, although possible for a number of years, is a technology which is still in its infancy. The process involves the use of high intensity laser radiation to rapidly heat the surface of a steel into the austenitic region. Due to high rates of heat transfer, steep temperature gradients are set up which result in rapid cooling by conduction. This causes the transformation from austenite to martensite without the need for external quenching. Other mechanisms exist for the thermal hardening of some non-ferrous alloys. Until recently, the widespread use of lasers for materials processing has been hindered by the size, complexity and high investment cost of the laser systems. These molecular and solid-state laser systems are now beginning to give way to a new generation of rapidly evolving lasers called high power diode lasers or HPDLs. The wavelength of the emitted radiation allows high metallic absorption, which when coupled with favourable spatial and temporal beam profiles allows the HPDL to achieve a high efficiency. The following review paper is a synopsis of the fundamentals of laser hardening, outlining some of its benefits compared with conventional hardening techniques. A selective review of the experimental research carried out in this area is presented. Particular reference is made to hardening using HPDLs. A description of the construction and operation of HPDLs is also presented with emphasis on the technical and economic factors which make them advantageous for surface hardening applications. © 2004 Elsevier B.V. All rights reserved. Keywords: Laser hardening; High power diode laser
1. Introduction Diode lasers with a continuous wave output power in excess of 0.5 W are referred to as high power diode lasers or HPDLs. Although semiconductor lasers were developed only a short time after the first ruby laser, their output powers were very low and their operation restricted to cryogenic temperatures. Since then, considerable research has been conducted into the crystalline structure of the diodes themselves and improvement of the manufacturing processes so that by the year 2002, the total diode laser market had reached $ 366 billion. However, only a small percentage (0.3%) of this market is held by direct application diode lasers for materials processing [1]. Semiconductor lasers have seen wide applications in the telecommunications and electronics industry for some time but their application in materials processing has been limited to low power applications such as plastics welding, due to their sub-kilowatt power output and low irradiance. Recent advances in HPDL technology has seen the introduction of ∗ Corresponding author. E-mail address: [email protected] (E. Kennedy).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.276
multi-kilowatt HPDLs on the market which greatly broadens their range of applications in materials processing. The output power of many individual, lower power diode lasers is combined using state-of-the-art focusing techniques to produce output powers of up to 6 kW. Extensive work has been carried out in the field of laser hardening to date with CO2 and Nd:YAG lasers, as up to recently these were the only practical laser types which could produce the required continuous wave power. Laser hardening with CO2 and Nd:YAG lasers is relatively expensive due to their low efficiency, high capital costs (especially for Nd:YAG lasers) and the frequent need for coatings to increase absorption. Their approximately Gaussian beam profile is also not ideal for heat treatment applications, often requiring complex and expensive focusing techniques to create a beam profile which is similar to that characteristic of the HPDL. The advent of HPDLs with powers in the kilowatt range has presented a new and powerful tool, not just for laser hardening, but for laser materials processing in general. Laser hardening can be performed using a diode laser with a power of just 15 W [2] but its applications are severely limited. Modern HPDLs can provide sufficient power density to cover a wide range of transformation hardening applications.
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2. High power diode lasers (HPDLs) The diode laser operates on the principle that under the correct excitation conditions, coherent laser light can be produced from the p–n junction of a diode when electrons recombine with holes in its depletion region. The basic construction of a single diode laser is one of a p–n junction formed from two slabs of semiconductor material, generally from InGaAs/GaAs which emits light at 808 nm or from InGaAlAs/GaAs which emits light at 940 nm [3]. Two opposite cleaved facets of the semiconductor material form the mirrors for the so-called Fabry–Perot lasing cavity and the sides of the optical cavity may be purposely roughened to reduce reflectivity. Wave-guiding techniques such as the use of a single or double heterostructure and the incorporation of a ‘stripe geometry’ through index or gain guiding are used to confine the light in the active region and reduce losses. The dimensions of a single diode laser are of the order of a few hundred microns and the output power is generally restricted to a few milliwatts [4]. To increase the output power, several individual emitters are arranged in a line on a one-dimensional array approximately 10 mm long referred to as a HPDL bar. Output powers of 30–50 W are typical from a single diode laser bar although in laboratory assemblies, more than 120 W have been achieved [5]. Although diode lasers exhibit a far greater optical to electrical efficiency (20–35%) [6,7] than their molecular and solid state counterparts, considerable amounts of heat are generated from the lasing process which must be dissipated in order to increase the output power. Modern diode laser bars dissipate heat through the use of an active cooling medium such as de-ionised water, which flows through micro-cooling channels on which the diode lasers are mounted. Much research has been conducted in recent years into reducing the temperature rise across the diode by improving the diode heat sink performance [5]. Since emission from a single diode laser is confined to the narrow junction region (typically 1 m), diffraction of the light results in a large beam divergence of up to 45◦ half angle in the direction perpendicular to the emission line (‘Gaussian’ or ‘fast’ axis) [7] and up to 10◦ half angle in the direction parallel to the emission line (‘slow’axis) [8]. This is shown schematically in Fig. 1. The fast axis is collimated by cylindrical micro-lenses to produce parallel light and these bars can then be stacked on top of one another so that output power of up to 1 kW can be achieved with this unit. Two or more of these stacks can be further combined using stripe mirrors and can be directed onto the same optical path using either wavelength or polarisation coupling [4]. These coupling methods arise from the fact that to combine two beams from different stacks onto the same area, the wavelength or the polarisation direction of the beams must be different. To combine two stacks, polarisation or wavelength coupling can be used, however to combine four stacks, both wavelength and polarisation coupling must be employed. The slow axis is collimated by a
Fig. 1. Output of a typical diode laser. Source: [8].
Fig. 2. Visualisation of focusing of light from a diode laser stack. Source: [5].
cylindrical lens, the output from which is an approximately parallel beam of light in both the top-hat and Gaussian directions. A spherical lens then focuses the beam to a spot [4] as shown in Fig. 2. Many different focusing apparatuses are commercially available for HPDLs including high speed rasterizers, ‘zoom’ optics for continually varying the beam spot size in one direction [9] and attachments to allow the beam to be fibre coupled.
3. Characteristics of HPDLs HPDLs have many inherent properties which make them particularly suitable for heat treating applications. Some of their main characteristics are as follows: 3.1. Wavelength The wavelength of the radiation emitted from most practical HPDLs is between 800 and 940 nm. A major concern in laser materials processing is the fraction of incident laser radiation which is reflected from the surface of the workpiece. This is characterised by the workpiece reflectivity, defined as the ratio of the radiant power reflected from the surface to the radiant power incident on the surface. The relationship between the reflectivity of various metals and the wavelength of the incident light is shown in Fig. 3. It can be seen that due to its shorter wavelength, HPDL radiation has a considerably higher degree of absorption into metallic surfaces than
E. Kennedy et al. / Journal of Materials Processing Technology 155–156 (2004) 1855–1860
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of equivalent power, and considerably less expensive than Nd:YAG lasers of equivalent power. At present, the cost of a typical 3 kW direct application diode laser ranges from approximately 155,000–200,000. 3.3. Compact size
Fig. 3. Reflectivity as a function of wavelength for various materials. Data from [8].
CO2 lasers which generally emit radiation at 10.6 m and a slightly higher degree of absorption than Nd:YAG lasers which generally emit radiation at 1.06 m. Ehlers et al. state that the use of an 800 nm HPDL over a CO2 laser increases the absorption from approximately 1.2–13% when processing aluminium [10]. 3.2. Lifetime, efficiency and costs The expected lifetime of diode lasers is lower than other materials processing lasers and represents one of their major drawbacks. The typical lifetime of the diodes is 10,000 operational hours, with most manufacturers only guaranteeing the laser for a fraction of this time. Maintenance and running costs of HPDLs, however, are less than those of competing laser types. Loosen et al. states that CO2 and Nd:YAG lasers have a maintenance period of 1000 and 200 h, respectively, whereas HPDLs are practically maintenance free [5]. Diode pumped Nd:YAG lasers, however, have a longer maintenance period than lamp-pumped Nd:YAG lasers as their diodes do not need to be replaced on a regular basis. The diode laser is the most efficient of all laser types with a typical electrical to optical efficiency of 20–35% compared with CO2 and Nd:YAG (arc lamp pumped) lasers which have typical efficiencies of 10–15 and 1–5%, respectively [6,7]. Some diode lasers can reach a max. efficiency of 50%, although theoretical values can be up to 90% [7]. Since 1999, the commercial availability of diode pumped Nd:YAG lasers has narrowed the gap between the efficiency of diode and Nd:YAG lasers. Hugel (2000) states that efficiencies of up to 13% have been reported for diode pumped Nd:YAG lasers [11], and the LIA Handbook of Laser Materials Processing quotes a typical electrical to optical efficiency of 15% for diode pumped Nd:YAG lasers [6]. Despite this, the HPDL still remains the market leader in terms of efficiency. The capital costs of HPDLs are also continually decreasing, with prices from some manufacturers now less than CO2 lasers
Because of the nature of the HPDL construction and the reduced cooling requirements of diode lasers compared with other laser types, HPDLs are considerably more compact than both CO2 and Nd:YAG lasers. The size of the laser optical head is very much dependant on the power output and the manufacturer, with a volume of approximately 5–12 litres being typical for a 3 kW direct application HPDL. The HPDL optical head volume can be up to three orders of magnitude lower than competing laser types [5] and this allows them to be relatively easily integrated into conventional machining centres or onto robot arms. 3.4. Beam profile, quality and stability The beam profile of a HPDL is generally top-hat in the slow axis direction and Gaussian in the fast axis direction with a rectangular shaped spot due to the nature of the beam formation process. This profile is beneficial for many applications where uniform heating of a surface is required, as in laser surface transformation hardening, laser alloying and laser cladding. The beam quality, however, is poor with typical values of 85 mrad × 200 mrad compared with 1–5 mrad and 1–10 mrad for CO2 and Nd:YAG lasers, respectively [8]. This severely limits their applications in cutting and welding of metals. One of the major differences between HPDL systems and other laser sources is the number of laser beams generated in the system (CO2 and Nd:YAG lasers generating just a single beam). With conventional lasers, small variations during the excitement of the laser active medium results in intensity fluctuations, apparent as irregular spikes in the beam profile. The great number of superimposed laser beams of a HPDL system compensates fluctuations within individual beams, giving the combined beam an outstanding modal stability [3].
4. Laser transformation hardening Transformation hardening is a well established technique for the thermal hardening of steel. Like all thermal hardening processes, the principle is to attempt to trap a structure as it exists at an elevated temperature and so preserve this structure at room temperature. At room temperature, carbon steel is composed of a mixture of cementite (Fe3 C) and BCC ferrite, which can dissolve a maximum of up to 0.006% carbon. Upon heating to above the Ac3 temperature, a FCC austentic structure is formed which can dissolve carbon in amounts of up to 2%. When the metal is
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cooled slowly to room temperature, the FCC structure reverts back to a BCC structure and the carbon is precipitated out as cementite. However, if cooled quickly to below the Ms temperature (approximately 250 ◦ C), carbon does not have sufficient time to precipitate out and the non-equilibrium phase martensite begins to form by a process of ‘diffusionless phase transformation’ [12]. Martensite is an extremely hard, brittle material which is desirable on the surface of many components where a hard, wear resistant case is required. Surface transformation hardening involves hardening only a thin layer on the surface of the component, leaving the desirable bulk properties of the material such as toughness and ductility unchanged. Laser transformation hardening involves the use of high intensity laser radiation to rapidly heat the surface of a steel into the austenitic region. Due to high rates of heat transfer, steep temperature gradients are set up which result in rapid cooling by conduction. This causes the transformation from austenite to martensite without the need for external quenching. This self-quenching occurs as the cold interior of the workpiece constitutes a sufficiently large heat sink to quench the hot surface by heat conduction to the interior at a rate high enough to prevent pearlite or bainite formation at the surface, resulting in martensite formation instead. Other mechanisms exist for the thermal hardening of some non-ferrous alloys such as aluminium bronzes [13]. It should be noted that the laser radiation heats only the top layer of atoms or molecules on the surface of the material and all heat penetration into the material is caused by conduction from the surface. The kinetics of the rapid heating cycle are still the cause of much uncertainty, making the design of a laser hardening process more difficult than that of a conventional through hardening procedure [14]. The non-equilibrium nature of the rapid heating process causes the Ac3 temperature to be shifted upwards, as carbon diffusion rates are too sluggish at lower temperatures to allow complete transformation to austenite within the interaction time [14]. Meijer and van Sprang state that for steel with more than 0.4% carbon and heating times within 10 s, a constant Ac3 temperature of 910 ◦ C may be used, which covers almost all practical applications [15]. The results of the laser hardening process depend primarily on the beam irradiance on the surface of the workpiece, the processing rate and the thermophysical properties of the material. The processing rate in laser hardening can be more usefully expressed as the interaction time, defined for a continuous process as the length of time taken for the laser spot to travel one diameter relative to the workpiece [3]. A low interaction time and a high power results in a shallow hardening depth whereas the converse results in a deeper hardening depth. However, the higher peak temperatures and cooling rates required for short processing times may result in the formation of unacceptable quantities of retained austenite [6] in high carbon steels. The laser output power and beam spot size are important in choosing the correct processing conditions but as the
Fig. 4. Effects and possible applications of lasers under various operating conditions. Data from [7].
irradiance links these two independent variables, it often is more useful to describe processes in terms of the irradiance required. A process map for the range of irradiances used for various laser materials processing applications is shown in Fig. 4. As can be seen, the irradiance required for laser hardening lies approximately between 101 and 102.5 W/mm2 . Haag et al. suggests a slightly lower upper limit of 104 W/cm2 (102 W/mm2 ) for laser hardening applications. Shock hardening, shown in the top right portion of Fig. 4 is an entirely different method of hardening metals which is similar to shot-peening and should not be confused with laser transformation hardening. The most significant thermophysical property of a material for laser hardening is its thermal diffusivity, α, where α = K/ρc (where K is the thermal conductivity, ρ the density and c the heat capacitance). This factor is involved in all unsteady-state heat flow processes and its significance is that it determines how rapidly a material will accept and conduct thermal energy [8]. The reflectivity of the surface of the workpiece also plays an important role in laser hardening and in laser materials processing in general. The reflectivity of the surface is not only dependent on the wavelength of the incident light, but also on the angle of incidence, if the light is polarised. Gutu et al. observed a 2.2–4-fold increase in the surface absorption of the incident P-polarised light from a CO2 laser for an incident angle in the range 70–80◦ . Gutu et al. also noted that this principle can also be applied to other laser types and is not restricted to CO2 lasers [16]. A wide range of spot geometries is available with various lasers and there has been considerable research into beam shaping optics. The beam shaping procedure can be complicated and expensive for CO2 and Nd:YAG lasers and complex integrators and kaleidoscopes have been used to achieve a rectangular beam shape and profiles such as the top-hat or ‘armchair’ profiles which are optimum for heat treating applications [6]. As mentioned previously, CO2 and Nd:YAG lasers generally have a Gaussian distributed beam with a high intensity at the centre of the beam, decreasing rapidly with distance from the centre. This is generally unfavourable for hardening
E. Kennedy et al. / Journal of Materials Processing Technology 155–156 (2004) 1855–1860
applications as it leads to an uneven hardened depth and in extreme cases, melting may occur in the centre of the spot where the austenitizing temperature has not been reached towards the beam perimeter. In general, a rectangular spot with a top-hat intensity distribution such as that produced by a HPDL is favourable for laser hardening applications as it leads to a uniform hardened depth [3]. In 2000, Ehlers et al. described the construction of two cylindrical zoom optics to continually vary the spot size produced from a HPDL in one direction. One lens allowed the spot size to be varied from 6 to 22 mm in one direction and the other allowed the spot size to be varied from both 1.2 to 6 mm and from 2.4 to 12 mm in one direction [9]. These zoom optics are now available commercially. One of the major advantages of laser hardening is the very low thermal distortion of the component associated with the process, compared with induction, flame and in particular case hardening. The LIA Handbook of Laser Materials Processing [6] gives an example from industry where an expensive post-hardening honing operation was eliminated by switching from induction hardening to laser hardening. With the correct choice of processing parameters, laser hardening can create compressive stresses on the surface of a hardened component due to the volume expansion associated with the formation of martensite. Work by De la Cruz et al. showed that laser hardening with a 2.5 kW CO2 laser increased the fatigue limit of smooth and notched B–Mn steel specimens by 18 and 56%, respectively [17]. Laser hardening is also highly suited to automation and there is a high degree of process controllability [18]. As mentioned earlier, the compact size of the HPDL makes them ideal for integrating into conventional machining tools. Such integration is an application of the principles of concurrent engineering and by eliminating non-value added operations, a significant return on investment may be achieved. A drawback of laser hardening is that it may be necessary to make multiple passes with the laser over the surface of the workpiece if the beam spot size is not large enough to cover the entire area. Lateral heat flow into the previously hardened track may cause ‘back tempering’ which can reduce the hardness in the affected area considerably.
5. Laser hardening using HPDLs Klocke et al. achieved hardening of 42CrMo4 steel to a hardness of over 700 HV to a depth of approximately 0.5 mm using the first commercially available 650 W HPDL. The work showed that a high intensity of the laser beam during hardening leads to a more efficient process as the higher laser input energy over time reduces the losses attributed to thermal conduction. A slightly higher hardness was also noticed with the samples treated with higher beam intensity and higher processing rates [19]. Loosen et al. demonstrated that laser hardening of 42CrMoS4 steel could be achieved using a 200 W HPDL with a focal spot of 0.8 mm × 5 mm and a
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processing speed of 25 mm/min [5]. Work by Ehlers et al. showed that a maximum hardness in the range 670–740 HV1 could be obtained on the surface of 4140 HT steel to a hardened depth of 1.9 mm using a 2.5 kW laser with 2 kW incident power. Steel rods were also hardened to a depth of 0.2–0.4 mm and resulting hardness was 900 ± 50 HV0.1 [10]. Bachmann reported the use of a HPDL to harden automotive torsion springs to a depth of 0.2–0.4 mm [4]. Haag used a 300 W early diode laser model to harden the surface of a controller shaft of 100Cr6 steel to a depth of up to 0.45 mm. The average hardness in the tracks was measured to be 850 HV0.2 [3]. This process was previously carried out with an Nd:YAG laser and Haag stated that the tracks produced by the HPDL had a preferable hardened zone due to its rectangular beam spot in combination with the top-hat profile. The LIA Handbook of Materials Processing reported the use of a 2 kW diode laser to harden flat 20 mm thick plates of M1044 alloy steel. A zoom lens was used to create spots sizes of 11 mm × 2.8 mm and 22 mm × 2.8 mm [6]. At a power of 1.4 kW, a spot size of 11 mm × 2.8 mm and a processing speed of 300 mm/min, a 9 mm wide and 0.65 mm deep hardened track with a hardness of 750 HV0.05 was obtained. At a power of 1.8 kW, a spot size of 22 mm and a processing speed of 200 mm/min, a 19 mm wide and 0.7 mm deep hardened track with a hardness of 650 HV0.5 was obtained. A range of hardening depths was achieved for a corresponding range of processing speeds, the hardening depth decreasing with increasing processing speed. It was noted that the width of the hardened track is slightly less than the width of the laser beam due to the larger heat transfer into the base of the material at the sides the track. This leaves less energy at the limits of the track to heat the material sufficiently. 6. Conclusions The last number of years has seen the introduction of high power diode lasers with power densities in the range 104 to 105 W/cm2 . For applications where moderate power density is required without the constraint of a high beam quality, today’s HPDLs offer an economical, compact solution, ideal for integration into conventional machine systems and robotics. Heat treating applications are the forte of HPDL technology as their inherent beam stability and top-hat profile leads to uniform heating of the surface over a relatively large area. Although HPDLs have not yet reached sufficient beam quality or irradiance to be used in most drilling and cutting applications, their high efficiency, compact size and ever decreasing capital costs will undoubtedly drive research into conquering these applications.
References [1] F. Bachmann, Industrial applications of high power diode lasers in materials processing, Appl. Surf. Sci. 208/209 (2003) 125–136.
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[2] S. Bonss, B. Brenner, E. Beyer, F. Bachmann, Diode laser applications hardening and welding, in: Proceedings of the ICALEO’98, vol. 85G (1998) pp. 121–130. [3] M. Haag, T. Rudlaff, Assessment of different high power diode lasers for materials processing, Proc. SPIE 3097 (1997) 583–591. [4] F. Bachmann, High power diode laser technology and applications, Proc. SPIE 3888 (2000) 394–403. [5] P. Loosen, G. Treusch, C.R. Haas, U. Gardenier, M. Weck, V. Sinnhoff, St. Kasperowski, R. vor dem Esche, High-power diodelasers and their direct industrial applications, Proc. SPIE 2382 (1995) 79–88. [6] Laser Institute of America, Handbook of Laser Materials Processing, Magnolia Publishing Inc., 2001. [7] L. Li, The advances and characteristics of high power diode laser materials processing, Opt. Lasers Eng. 34 (2000) 231–253. [8] J.F. Ready, Industrial Applications of Lasers, 2nd ed., Academic Press, 1997. [9] B. Ehlers, U. Walz, S. Heineman, Cylindrical zoom optics for high power diode lasers, Proc. SPIE 3929 (2000) 185–192. [10] B. Ehlers, H.J. Herfurth, S. Heinemann, Hardening and welding with high power diode lasers, Proc. SPIE 3945 (2000) 63– 70.
[11] H. Hugel, New solid state laser and their application potentials, Opt. Lasers Eng. 34 (2000) 213–229. [12] R.A. Higgins, Engineering Metallurgy. Part 1: Applied Physical Metallurgy, 6th ed., Edward Arnold, London, 1993. [13] S.M. Levcovici, D.T. Levcovici, M.M. Paraschiv, Laser hardening of aluminium bronzes, Mater. Manuf. Process. 17 (1) (2002) 13–21. [14] ASM International, ASM Handbook: Heat Treating, vol. 4, ASM International, 1994. [15] J. Meijer, I. van Sprang, Optimisation of laser beam transformation hardening by one single parameter, Ann. CIRP 40 (1) (1991) 183– 186. [16] I. Gutu, C. Petre, I.N. Mihailescu, M. Taca, E. Alexandrescu, I. Ivanov, Surface treatment with linearly polarised laser beam at oblique incidence, Opt. Laser Technol. 34 (2002) 381–388. [17] P. De la Cruz, M. Oden, T. Ericsson, Effect of laser hardening on the fatigue strength and fracture of a B–Mn steel, Int. J. Fatigue 20 (5) (1998) 389–398. [18] A.I. Katsamas, G.N. Haidemenopoulos, Laser-beam carburizing of low-alloy steels, Surf. Coat. Technol. 139 (2001) 183–191. [19] F. Klocke, A. Demmer, A. Zaboklicki, Investigation into the use of high power diode lasers for hardening and thermal conduction welding, Proc. SPIE 3097 (1997) 592–598.
A review of the use of high power diode lasers in surface hardening E. Kennedy a,∗ , G. Byrne a , D.N. Collins b a
Advanced Manufacturing Science Research Group, Department of Mechanical Engineering, University College Dublin, Dublin, Ireland b Department of Mechanical Engineering, University College Dublin, Dublin, Ireland
Abstract Laser surface hardening, although possible for a number of years, is a technology which is still in its infancy. The process involves the use of high intensity laser radiation to rapidly heat the surface of a steel into the austenitic region. Due to high rates of heat transfer, steep temperature gradients are set up which result in rapid cooling by conduction. This causes the transformation from austenite to martensite without the need for external quenching. Other mechanisms exist for the thermal hardening of some non-ferrous alloys. Until recently, the widespread use of lasers for materials processing has been hindered by the size, complexity and high investment cost of the laser systems. These molecular and solid-state laser systems are now beginning to give way to a new generation of rapidly evolving lasers called high power diode lasers or HPDLs. The wavelength of the emitted radiation allows high metallic absorption, which when coupled with favourable spatial and temporal beam profiles allows the HPDL to achieve a high efficiency. The following review paper is a synopsis of the fundamentals of laser hardening, outlining some of its benefits compared with conventional hardening techniques. A selective review of the experimental research carried out in this area is presented. Particular reference is made to hardening using HPDLs. A description of the construction and operation of HPDLs is also presented with emphasis on the technical and economic factors which make them advantageous for surface hardening applications. © 2004 Elsevier B.V. All rights reserved. Keywords: Laser hardening; High power diode laser
1. Introduction Diode lasers with a continuous wave output power in excess of 0.5 W are referred to as high power diode lasers or HPDLs. Although semiconductor lasers were developed only a short time after the first ruby laser, their output powers were very low and their operation restricted to cryogenic temperatures. Since then, considerable research has been conducted into the crystalline structure of the diodes themselves and improvement of the manufacturing processes so that by the year 2002, the total diode laser market had reached $ 366 billion. However, only a small percentage (0.3%) of this market is held by direct application diode lasers for materials processing [1]. Semiconductor lasers have seen wide applications in the telecommunications and electronics industry for some time but their application in materials processing has been limited to low power applications such as plastics welding, due to their sub-kilowatt power output and low irradiance. Recent advances in HPDL technology has seen the introduction of ∗ Corresponding author. E-mail address: [email protected] (E. Kennedy).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.276
multi-kilowatt HPDLs on the market which greatly broadens their range of applications in materials processing. The output power of many individual, lower power diode lasers is combined using state-of-the-art focusing techniques to produce output powers of up to 6 kW. Extensive work has been carried out in the field of laser hardening to date with CO2 and Nd:YAG lasers, as up to recently these were the only practical laser types which could produce the required continuous wave power. Laser hardening with CO2 and Nd:YAG lasers is relatively expensive due to their low efficiency, high capital costs (especially for Nd:YAG lasers) and the frequent need for coatings to increase absorption. Their approximately Gaussian beam profile is also not ideal for heat treatment applications, often requiring complex and expensive focusing techniques to create a beam profile which is similar to that characteristic of the HPDL. The advent of HPDLs with powers in the kilowatt range has presented a new and powerful tool, not just for laser hardening, but for laser materials processing in general. Laser hardening can be performed using a diode laser with a power of just 15 W [2] but its applications are severely limited. Modern HPDLs can provide sufficient power density to cover a wide range of transformation hardening applications.
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2. High power diode lasers (HPDLs) The diode laser operates on the principle that under the correct excitation conditions, coherent laser light can be produced from the p–n junction of a diode when electrons recombine with holes in its depletion region. The basic construction of a single diode laser is one of a p–n junction formed from two slabs of semiconductor material, generally from InGaAs/GaAs which emits light at 808 nm or from InGaAlAs/GaAs which emits light at 940 nm [3]. Two opposite cleaved facets of the semiconductor material form the mirrors for the so-called Fabry–Perot lasing cavity and the sides of the optical cavity may be purposely roughened to reduce reflectivity. Wave-guiding techniques such as the use of a single or double heterostructure and the incorporation of a ‘stripe geometry’ through index or gain guiding are used to confine the light in the active region and reduce losses. The dimensions of a single diode laser are of the order of a few hundred microns and the output power is generally restricted to a few milliwatts [4]. To increase the output power, several individual emitters are arranged in a line on a one-dimensional array approximately 10 mm long referred to as a HPDL bar. Output powers of 30–50 W are typical from a single diode laser bar although in laboratory assemblies, more than 120 W have been achieved [5]. Although diode lasers exhibit a far greater optical to electrical efficiency (20–35%) [6,7] than their molecular and solid state counterparts, considerable amounts of heat are generated from the lasing process which must be dissipated in order to increase the output power. Modern diode laser bars dissipate heat through the use of an active cooling medium such as de-ionised water, which flows through micro-cooling channels on which the diode lasers are mounted. Much research has been conducted in recent years into reducing the temperature rise across the diode by improving the diode heat sink performance [5]. Since emission from a single diode laser is confined to the narrow junction region (typically 1 m), diffraction of the light results in a large beam divergence of up to 45◦ half angle in the direction perpendicular to the emission line (‘Gaussian’ or ‘fast’ axis) [7] and up to 10◦ half angle in the direction parallel to the emission line (‘slow’axis) [8]. This is shown schematically in Fig. 1. The fast axis is collimated by cylindrical micro-lenses to produce parallel light and these bars can then be stacked on top of one another so that output power of up to 1 kW can be achieved with this unit. Two or more of these stacks can be further combined using stripe mirrors and can be directed onto the same optical path using either wavelength or polarisation coupling [4]. These coupling methods arise from the fact that to combine two beams from different stacks onto the same area, the wavelength or the polarisation direction of the beams must be different. To combine two stacks, polarisation or wavelength coupling can be used, however to combine four stacks, both wavelength and polarisation coupling must be employed. The slow axis is collimated by a
Fig. 1. Output of a typical diode laser. Source: [8].
Fig. 2. Visualisation of focusing of light from a diode laser stack. Source: [5].
cylindrical lens, the output from which is an approximately parallel beam of light in both the top-hat and Gaussian directions. A spherical lens then focuses the beam to a spot [4] as shown in Fig. 2. Many different focusing apparatuses are commercially available for HPDLs including high speed rasterizers, ‘zoom’ optics for continually varying the beam spot size in one direction [9] and attachments to allow the beam to be fibre coupled.
3. Characteristics of HPDLs HPDLs have many inherent properties which make them particularly suitable for heat treating applications. Some of their main characteristics are as follows: 3.1. Wavelength The wavelength of the radiation emitted from most practical HPDLs is between 800 and 940 nm. A major concern in laser materials processing is the fraction of incident laser radiation which is reflected from the surface of the workpiece. This is characterised by the workpiece reflectivity, defined as the ratio of the radiant power reflected from the surface to the radiant power incident on the surface. The relationship between the reflectivity of various metals and the wavelength of the incident light is shown in Fig. 3. It can be seen that due to its shorter wavelength, HPDL radiation has a considerably higher degree of absorption into metallic surfaces than
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of equivalent power, and considerably less expensive than Nd:YAG lasers of equivalent power. At present, the cost of a typical 3 kW direct application diode laser ranges from approximately 155,000–200,000. 3.3. Compact size
Fig. 3. Reflectivity as a function of wavelength for various materials. Data from [8].
CO2 lasers which generally emit radiation at 10.6 m and a slightly higher degree of absorption than Nd:YAG lasers which generally emit radiation at 1.06 m. Ehlers et al. state that the use of an 800 nm HPDL over a CO2 laser increases the absorption from approximately 1.2–13% when processing aluminium [10]. 3.2. Lifetime, efficiency and costs The expected lifetime of diode lasers is lower than other materials processing lasers and represents one of their major drawbacks. The typical lifetime of the diodes is 10,000 operational hours, with most manufacturers only guaranteeing the laser for a fraction of this time. Maintenance and running costs of HPDLs, however, are less than those of competing laser types. Loosen et al. states that CO2 and Nd:YAG lasers have a maintenance period of 1000 and 200 h, respectively, whereas HPDLs are practically maintenance free [5]. Diode pumped Nd:YAG lasers, however, have a longer maintenance period than lamp-pumped Nd:YAG lasers as their diodes do not need to be replaced on a regular basis. The diode laser is the most efficient of all laser types with a typical electrical to optical efficiency of 20–35% compared with CO2 and Nd:YAG (arc lamp pumped) lasers which have typical efficiencies of 10–15 and 1–5%, respectively [6,7]. Some diode lasers can reach a max. efficiency of 50%, although theoretical values can be up to 90% [7]. Since 1999, the commercial availability of diode pumped Nd:YAG lasers has narrowed the gap between the efficiency of diode and Nd:YAG lasers. Hugel (2000) states that efficiencies of up to 13% have been reported for diode pumped Nd:YAG lasers [11], and the LIA Handbook of Laser Materials Processing quotes a typical electrical to optical efficiency of 15% for diode pumped Nd:YAG lasers [6]. Despite this, the HPDL still remains the market leader in terms of efficiency. The capital costs of HPDLs are also continually decreasing, with prices from some manufacturers now less than CO2 lasers
Because of the nature of the HPDL construction and the reduced cooling requirements of diode lasers compared with other laser types, HPDLs are considerably more compact than both CO2 and Nd:YAG lasers. The size of the laser optical head is very much dependant on the power output and the manufacturer, with a volume of approximately 5–12 litres being typical for a 3 kW direct application HPDL. The HPDL optical head volume can be up to three orders of magnitude lower than competing laser types [5] and this allows them to be relatively easily integrated into conventional machining centres or onto robot arms. 3.4. Beam profile, quality and stability The beam profile of a HPDL is generally top-hat in the slow axis direction and Gaussian in the fast axis direction with a rectangular shaped spot due to the nature of the beam formation process. This profile is beneficial for many applications where uniform heating of a surface is required, as in laser surface transformation hardening, laser alloying and laser cladding. The beam quality, however, is poor with typical values of 85 mrad × 200 mrad compared with 1–5 mrad and 1–10 mrad for CO2 and Nd:YAG lasers, respectively [8]. This severely limits their applications in cutting and welding of metals. One of the major differences between HPDL systems and other laser sources is the number of laser beams generated in the system (CO2 and Nd:YAG lasers generating just a single beam). With conventional lasers, small variations during the excitement of the laser active medium results in intensity fluctuations, apparent as irregular spikes in the beam profile. The great number of superimposed laser beams of a HPDL system compensates fluctuations within individual beams, giving the combined beam an outstanding modal stability [3].
4. Laser transformation hardening Transformation hardening is a well established technique for the thermal hardening of steel. Like all thermal hardening processes, the principle is to attempt to trap a structure as it exists at an elevated temperature and so preserve this structure at room temperature. At room temperature, carbon steel is composed of a mixture of cementite (Fe3 C) and BCC ferrite, which can dissolve a maximum of up to 0.006% carbon. Upon heating to above the Ac3 temperature, a FCC austentic structure is formed which can dissolve carbon in amounts of up to 2%. When the metal is
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cooled slowly to room temperature, the FCC structure reverts back to a BCC structure and the carbon is precipitated out as cementite. However, if cooled quickly to below the Ms temperature (approximately 250 ◦ C), carbon does not have sufficient time to precipitate out and the non-equilibrium phase martensite begins to form by a process of ‘diffusionless phase transformation’ [12]. Martensite is an extremely hard, brittle material which is desirable on the surface of many components where a hard, wear resistant case is required. Surface transformation hardening involves hardening only a thin layer on the surface of the component, leaving the desirable bulk properties of the material such as toughness and ductility unchanged. Laser transformation hardening involves the use of high intensity laser radiation to rapidly heat the surface of a steel into the austenitic region. Due to high rates of heat transfer, steep temperature gradients are set up which result in rapid cooling by conduction. This causes the transformation from austenite to martensite without the need for external quenching. This self-quenching occurs as the cold interior of the workpiece constitutes a sufficiently large heat sink to quench the hot surface by heat conduction to the interior at a rate high enough to prevent pearlite or bainite formation at the surface, resulting in martensite formation instead. Other mechanisms exist for the thermal hardening of some non-ferrous alloys such as aluminium bronzes [13]. It should be noted that the laser radiation heats only the top layer of atoms or molecules on the surface of the material and all heat penetration into the material is caused by conduction from the surface. The kinetics of the rapid heating cycle are still the cause of much uncertainty, making the design of a laser hardening process more difficult than that of a conventional through hardening procedure [14]. The non-equilibrium nature of the rapid heating process causes the Ac3 temperature to be shifted upwards, as carbon diffusion rates are too sluggish at lower temperatures to allow complete transformation to austenite within the interaction time [14]. Meijer and van Sprang state that for steel with more than 0.4% carbon and heating times within 10 s, a constant Ac3 temperature of 910 ◦ C may be used, which covers almost all practical applications [15]. The results of the laser hardening process depend primarily on the beam irradiance on the surface of the workpiece, the processing rate and the thermophysical properties of the material. The processing rate in laser hardening can be more usefully expressed as the interaction time, defined for a continuous process as the length of time taken for the laser spot to travel one diameter relative to the workpiece [3]. A low interaction time and a high power results in a shallow hardening depth whereas the converse results in a deeper hardening depth. However, the higher peak temperatures and cooling rates required for short processing times may result in the formation of unacceptable quantities of retained austenite [6] in high carbon steels. The laser output power and beam spot size are important in choosing the correct processing conditions but as the
Fig. 4. Effects and possible applications of lasers under various operating conditions. Data from [7].
irradiance links these two independent variables, it often is more useful to describe processes in terms of the irradiance required. A process map for the range of irradiances used for various laser materials processing applications is shown in Fig. 4. As can be seen, the irradiance required for laser hardening lies approximately between 101 and 102.5 W/mm2 . Haag et al. suggests a slightly lower upper limit of 104 W/cm2 (102 W/mm2 ) for laser hardening applications. Shock hardening, shown in the top right portion of Fig. 4 is an entirely different method of hardening metals which is similar to shot-peening and should not be confused with laser transformation hardening. The most significant thermophysical property of a material for laser hardening is its thermal diffusivity, α, where α = K/ρc (where K is the thermal conductivity, ρ the density and c the heat capacitance). This factor is involved in all unsteady-state heat flow processes and its significance is that it determines how rapidly a material will accept and conduct thermal energy [8]. The reflectivity of the surface of the workpiece also plays an important role in laser hardening and in laser materials processing in general. The reflectivity of the surface is not only dependent on the wavelength of the incident light, but also on the angle of incidence, if the light is polarised. Gutu et al. observed a 2.2–4-fold increase in the surface absorption of the incident P-polarised light from a CO2 laser for an incident angle in the range 70–80◦ . Gutu et al. also noted that this principle can also be applied to other laser types and is not restricted to CO2 lasers [16]. A wide range of spot geometries is available with various lasers and there has been considerable research into beam shaping optics. The beam shaping procedure can be complicated and expensive for CO2 and Nd:YAG lasers and complex integrators and kaleidoscopes have been used to achieve a rectangular beam shape and profiles such as the top-hat or ‘armchair’ profiles which are optimum for heat treating applications [6]. As mentioned previously, CO2 and Nd:YAG lasers generally have a Gaussian distributed beam with a high intensity at the centre of the beam, decreasing rapidly with distance from the centre. This is generally unfavourable for hardening
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applications as it leads to an uneven hardened depth and in extreme cases, melting may occur in the centre of the spot where the austenitizing temperature has not been reached towards the beam perimeter. In general, a rectangular spot with a top-hat intensity distribution such as that produced by a HPDL is favourable for laser hardening applications as it leads to a uniform hardened depth [3]. In 2000, Ehlers et al. described the construction of two cylindrical zoom optics to continually vary the spot size produced from a HPDL in one direction. One lens allowed the spot size to be varied from 6 to 22 mm in one direction and the other allowed the spot size to be varied from both 1.2 to 6 mm and from 2.4 to 12 mm in one direction [9]. These zoom optics are now available commercially. One of the major advantages of laser hardening is the very low thermal distortion of the component associated with the process, compared with induction, flame and in particular case hardening. The LIA Handbook of Laser Materials Processing [6] gives an example from industry where an expensive post-hardening honing operation was eliminated by switching from induction hardening to laser hardening. With the correct choice of processing parameters, laser hardening can create compressive stresses on the surface of a hardened component due to the volume expansion associated with the formation of martensite. Work by De la Cruz et al. showed that laser hardening with a 2.5 kW CO2 laser increased the fatigue limit of smooth and notched B–Mn steel specimens by 18 and 56%, respectively [17]. Laser hardening is also highly suited to automation and there is a high degree of process controllability [18]. As mentioned earlier, the compact size of the HPDL makes them ideal for integrating into conventional machining tools. Such integration is an application of the principles of concurrent engineering and by eliminating non-value added operations, a significant return on investment may be achieved. A drawback of laser hardening is that it may be necessary to make multiple passes with the laser over the surface of the workpiece if the beam spot size is not large enough to cover the entire area. Lateral heat flow into the previously hardened track may cause ‘back tempering’ which can reduce the hardness in the affected area considerably.
5. Laser hardening using HPDLs Klocke et al. achieved hardening of 42CrMo4 steel to a hardness of over 700 HV to a depth of approximately 0.5 mm using the first commercially available 650 W HPDL. The work showed that a high intensity of the laser beam during hardening leads to a more efficient process as the higher laser input energy over time reduces the losses attributed to thermal conduction. A slightly higher hardness was also noticed with the samples treated with higher beam intensity and higher processing rates [19]. Loosen et al. demonstrated that laser hardening of 42CrMoS4 steel could be achieved using a 200 W HPDL with a focal spot of 0.8 mm × 5 mm and a
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processing speed of 25 mm/min [5]. Work by Ehlers et al. showed that a maximum hardness in the range 670–740 HV1 could be obtained on the surface of 4140 HT steel to a hardened depth of 1.9 mm using a 2.5 kW laser with 2 kW incident power. Steel rods were also hardened to a depth of 0.2–0.4 mm and resulting hardness was 900 ± 50 HV0.1 [10]. Bachmann reported the use of a HPDL to harden automotive torsion springs to a depth of 0.2–0.4 mm [4]. Haag used a 300 W early diode laser model to harden the surface of a controller shaft of 100Cr6 steel to a depth of up to 0.45 mm. The average hardness in the tracks was measured to be 850 HV0.2 [3]. This process was previously carried out with an Nd:YAG laser and Haag stated that the tracks produced by the HPDL had a preferable hardened zone due to its rectangular beam spot in combination with the top-hat profile. The LIA Handbook of Materials Processing reported the use of a 2 kW diode laser to harden flat 20 mm thick plates of M1044 alloy steel. A zoom lens was used to create spots sizes of 11 mm × 2.8 mm and 22 mm × 2.8 mm [6]. At a power of 1.4 kW, a spot size of 11 mm × 2.8 mm and a processing speed of 300 mm/min, a 9 mm wide and 0.65 mm deep hardened track with a hardness of 750 HV0.05 was obtained. At a power of 1.8 kW, a spot size of 22 mm and a processing speed of 200 mm/min, a 19 mm wide and 0.7 mm deep hardened track with a hardness of 650 HV0.5 was obtained. A range of hardening depths was achieved for a corresponding range of processing speeds, the hardening depth decreasing with increasing processing speed. It was noted that the width of the hardened track is slightly less than the width of the laser beam due to the larger heat transfer into the base of the material at the sides the track. This leaves less energy at the limits of the track to heat the material sufficiently. 6. Conclusions The last number of years has seen the introduction of high power diode lasers with power densities in the range 104 to 105 W/cm2 . For applications where moderate power density is required without the constraint of a high beam quality, today’s HPDLs offer an economical, compact solution, ideal for integration into conventional machine systems and robotics. Heat treating applications are the forte of HPDL technology as their inherent beam stability and top-hat profile leads to uniform heating of the surface over a relatively large area. Although HPDLs have not yet reached sufficient beam quality or irradiance to be used in most drilling and cutting applications, their high efficiency, compact size and ever decreasing capital costs will undoubtedly drive research into conquering these applications.
References [1] F. Bachmann, Industrial applications of high power diode lasers in materials processing, Appl. Surf. Sci. 208/209 (2003) 125–136.
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[2] S. Bonss, B. Brenner, E. Beyer, F. Bachmann, Diode laser applications hardening and welding, in: Proceedings of the ICALEO’98, vol. 85G (1998) pp. 121–130. [3] M. Haag, T. Rudlaff, Assessment of different high power diode lasers for materials processing, Proc. SPIE 3097 (1997) 583–591. [4] F. Bachmann, High power diode laser technology and applications, Proc. SPIE 3888 (2000) 394–403. [5] P. Loosen, G. Treusch, C.R. Haas, U. Gardenier, M. Weck, V. Sinnhoff, St. Kasperowski, R. vor dem Esche, High-power diodelasers and their direct industrial applications, Proc. SPIE 2382 (1995) 79–88. [6] Laser Institute of America, Handbook of Laser Materials Processing, Magnolia Publishing Inc., 2001. [7] L. Li, The advances and characteristics of high power diode laser materials processing, Opt. Lasers Eng. 34 (2000) 231–253. [8] J.F. Ready, Industrial Applications of Lasers, 2nd ed., Academic Press, 1997. [9] B. Ehlers, U. Walz, S. Heineman, Cylindrical zoom optics for high power diode lasers, Proc. SPIE 3929 (2000) 185–192. [10] B. Ehlers, H.J. Herfurth, S. Heinemann, Hardening and welding with high power diode lasers, Proc. SPIE 3945 (2000) 63– 70.
[11] H. Hugel, New solid state laser and their application potentials, Opt. Lasers Eng. 34 (2000) 213–229. [12] R.A. Higgins, Engineering Metallurgy. Part 1: Applied Physical Metallurgy, 6th ed., Edward Arnold, London, 1993. [13] S.M. Levcovici, D.T. Levcovici, M.M. Paraschiv, Laser hardening of aluminium bronzes, Mater. Manuf. Process. 17 (1) (2002) 13–21. [14] ASM International, ASM Handbook: Heat Treating, vol. 4, ASM International, 1994. [15] J. Meijer, I. van Sprang, Optimisation of laser beam transformation hardening by one single parameter, Ann. CIRP 40 (1) (1991) 183– 186. [16] I. Gutu, C. Petre, I.N. Mihailescu, M. Taca, E. Alexandrescu, I. Ivanov, Surface treatment with linearly polarised laser beam at oblique incidence, Opt. Laser Technol. 34 (2002) 381–388. [17] P. De la Cruz, M. Oden, T. Ericsson, Effect of laser hardening on the fatigue strength and fracture of a B–Mn steel, Int. J. Fatigue 20 (5) (1998) 389–398. [18] A.I. Katsamas, G.N. Haidemenopoulos, Laser-beam carburizing of low-alloy steels, Surf. Coat. Technol. 139 (2001) 183–191. [19] F. Klocke, A. Demmer, A. Zaboklicki, Investigation into the use of high power diode lasers for hardening and thermal conduction welding, Proc. SPIE 3097 (1997) 592–598.