- Email: [email protected]
High Power Diode Laser (HPDL) surface hardening of low carbon steel: Fatigue life improvement analysis
Journal of Manufacturing Processes 28 (2017) 266–271
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
High Power Diode Laser (HPDL) surface hardening of low carbon steel: Fatigue life improvement analysis S. Guarino a,∗ , M. Barletta b , Abdelkarim Afilal c a b c
Università degli Studi Niccolò Cusano, Dipartimento di Ingegneria, Via don Carlo Gnocchi 3,00166 Rome, Italy Università degli Studi di Roma “Tre”, Dipartimento di Ingegneria, Via Vito Volterra 62, 00146 Rome, Italy Uuniversità Chouaib Doukkali, Faculté des Sciences, El Jadida, Morocco
a r t i c l e
i n f o
Article history: Received 8 February 2016 Received in revised form 2 April 2017 Accepted 24 June 2017 Keywords: Diode laser hardening Fatigue life Steel
a b s t r a c t In the present investigation, a high-power diode laser was applied to improve the fatigue life of AISI 1040 steel components. The interaction between the laser radiation and the steel surface was studied, and the effectiveness of the laser treatment was analysed by fatigue testing. First, the importance of each of the following laser operational parameters was assessed: power, scan speed, focus and number of passes. Second, rotating bending fatigue tests were performed to investigate the effects of laser treatment and the influences of the laser operating parameters on the fatigue endurance of the components. Wohler curves, determined from an analysis of the experimental results, showed that laser treatment can significantly increase the fatigue life of irradiated components, thus revealing the suitability of this method for industrial applications. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Customers today impose stringent requirements on designers and manufacturers in the development of structural components [1]. Extension of the mechanical strength and fatigue life of structural components is often pivotal in several manufacturing environments [2,3]. Designing, monitoring and providing innovative technological solutions aimed at improving the fatigue life of structural components is therefore of crucial importance. Many different approaches to improving the fatigue life of steel components have been proposed in academia in recent decades and are used in industrial practise (heat and mechanical treatments, surface alloying, overlying coatings, etc.). However, these technological solutions, while having many advantages, are often challenging and time consuming. In addition, these solutions do not allow selective treatment of small portions of the components, being intrinsically a bulk technology, thus limiting their fields of application. In fact, thermal treatment (hardening, tempering, annealing, etc.) can dramatically change the properties of metal alloys and affect the fatigue limit of the materials. Nevertheless, they are not selective and act on the whole material, zeroing the gradient of the physical, chemical and mechanical properties of the materials
∗ Corresponding author. E-mail address: [email protected] (S. Guarino).
that can be useful in improving the fatigue response. In contrast, laser surface treatment is extremely promising [4–8] as it can be focused on selected zones of the components, leaving areas not directly exposed to laser radiation unaltered. As a result, laser systems have been considered very attractive for surface-hardening heat treatment as they offer selectivity, make process control very easy, and facilitate manipulation [8]. When compared to other processes, laser hardening causes little deformation of the part; thus, post machining is virtually eliminated. In addition, a broader variety of materials can be hardened; even low carbon steels can be hardened because of the rapid heating and cooling rates generated by the laser radiation. By using a laser, it is possible to perform versatile surface treatment with great precision, controlled heating, low heat input (and therefore low distortion) and fast cycle time. On the other hand, the main disadvantage compared to conventional technologies is found with the effects of back tempering induced by multiple passes [2]. Among laser sources, the High-Power Diode Laser (HPDL) is particularly well suited to surface hardening. Industrial scale kilowatt HPDLs have recently become available with uniformly distributed beams with a rectangular or elliptically shaped spot, thus ensuring uniform heating of the treated surface [8,9]. Furthermore, the relatively short wavelength (800–940 nm) of HPDLs ensures good energy transfer to most substrates, without the need for pretreatment to improve surface absorption [8]. Finally, HPDL’s benefits include compactness, energy efficiency, low lifetime and running
http://dx.doi.org/10.1016/j.jmapro.2017.06.015 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
costs, thus often representing the ideal candidate for surface hardening. Various applications of diode lasers for material hardening have been reported over the past decades. Klocke et al. hardened 42CrMo4 steel to over 700 HV and a depth of approximately 0.5 mm, using the first commercially available 650 W HPDL [9]. Ehlers et al. reported the usage of an HPDL for surface-hardening of M1044 steel components, achieving a consistently hardened depth of approximately 1 mm with a width of approximately 20 mm [10]. Hyungson et al. studied the laser hardening process by developing an intensity–interaction time diagram to define which part of the heat treatable region was successfully hardened [11]. Gupta et al. analysed and implemented a dual weighted residual (DWR) approach to solve an optimal control problem with ˇ y´ et al. proposed the laser surface hardening of steel [12]. Cern an experimental evaluation of fatigue resistance and initiation mechanisms of heat treated and laser hardened 42CrMo4 steel. The results confirm the significant beneficial effects of compressive stresses induced by laser hardening on fatigue resistance caused by the retardation or arrest of short fatigue cracks emanating from microstructure defects [13]. Wang et al. studied the effect of the shot peening and dual shot peening on laser hardened 17-4PH steel. The work shows that the fatigue life of laser hardened steel increases dramatically after shot peening treatment [14]. Ren et al. studied laser shock peening treatment on ASTM: 410 L 00Cr12 microstructures and fatigue resistance. Fatigue life was enhanced by approximately 58% at elevated temperatures (up to 600 ◦ C) [15]. Mahmoudi et al. studied AISI 420 martensitic stainless steel surface-hardening by a pulsed Nd:YAG laser and the influence of process parameters (laser pulse energy, duration time and travel speed) on the depth and hardness of the laser treated area and its corrosion behaviour. The beneficial effects of laser surface hardening by refining the microstructure and enhancing the pitting corrosion resistance of the martensitic stainless steel were observed [16]. Other experimental and numerical studies involving high power diode lasers are also reported in the literature [17–19]. Apart from the specificity of each work reported in the literature, defining and controlling the absorption of laser radiation during surface treatment of metallic alloys by HPDL were always found to be crucial to ensure process effectiveness, whatever was the metal workpiece involved and whatever was the objective of the study. Indeed, small variation in metal absorption can have a heavy effect on the setting of laser parameters and, consequently, on the amount of energy that must be delivered and, therefore, consumed during process. Pantsar et al. found absorptivity of machined steel surface during HPDL transformation hardening to vary in a very broad range (46%–72%) [6], with metal surface reprocessing being the only possibility to increase further and, above all, stabilize the absorption coefficient (66%–81% by shot peening or 85% and over by the application of graphite coatings to the workpiece surface). Reprocessing of the workpiece surface before laser treatment can, however, make the difference between competitive and economically sustainable processes or not. Therefore, in the present investigation, a high-power diode laser was used to improve the fatigue life of cylindrical substrates comprised of AISI 1040 low carbon steel without any further pretreatments that could increase the overall cost of the operation. In this specific framework, the interactions between the laser source and steel surface were investigated by varying the laser operating parameters (that is, laser power, scan speed and number of passes). Laser treatment was performed by simultaneously rotating and translating the cylindrical substrate under a stationary laser head to generate a helical scanning pattern. The effectiveness of the laser treatment was studied by rotating bending fatigue tests and microscopic analysis of the as-is and hardened substrates.
267
Fig. 1. Substrate geometry. Table 1 (a)Chemical composition (wt.%). (b) Mechanical properties. (a) C
Mn
Si
Altre
0.37–0.44
0.50–0.80
0.15–0.40
P and S ≤0.035
Rm (MPa)
E (GPa)
Hardness [HRC]
670
200
0.3
13
(b)
2. Materials and methods 2.1. Materials A low-medium carbon content steel (AISI 1040) suitable for surface-hardening heat treatment [17] was chosen as the starting material. Seventy-five cylindrical substrates, 116 mm long and 15 mm in diameter, were cut from 6 m long rods. The top surface of a small section (6 mm in diameter) of each sample was milled using the same cutting parameters (feed, cutting depth and cutting speed) and the same insert geometry on an instrumented universal milling machine. The substrates featured a similar average roughness, Ra , of 1.2 m. The surfaces morphology was measured before heat treatment using a profilometer (Taylor-Hobson Talysurf CLI 2000). Fig. 1 shows the geometry of the substrate. The chemical composition and mechanical properties of the steel are reported in Table 1. 2.2. Laser hardening equipment and methods Surface-hardening heat treatment was performed using a HighPower Diode Laser (Rofin-Sinar DL015) with a maximum power of 1500 W, a wavelength of 940 nm, and an elliptic spot shape of 0.6 mm along the minor axis and 1.9 mm along the major axis. A 63 mm focal length lens was chosen to ensure improved penetration of the laser beam in the material and, consequently, a deeper expected thickness of the hardened zone. During hardening tests, the substrates were held on a CNC motion system and laser treated by rotating and translating them under the stationary laser source. The resulting scanned paths were helical. Each scanned path was 36 mm long. For protection and insulation, an inert gas (Argon) flux was also directed on the substrate surface when it was being irradiated by the laser source. Fig. 2 shows a schematic of
268
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271 Table 3 Experimental conditions of varying diameter (rotating speed and feed rate).
D0 D1 D2 D3 D4
Diameter, mm
Circumference, mm
Rotating speed, cycles/s
Feed rate, mm/s
15 12.8 10.6 8.4 6
47.12 40.21 33.30 26.39 18.85
0.38 0.45 0.54 0.68 0.95
0.46 0.54 0.65 0.82 1.15
Fig. 2. HPDL system. Table 2 Experimental factors. Factor levels
Laser Power, W
Scan speed, mm/s
I II III IV V
100 150 200 250 300
12 14 16 18 20
Fig. 4. Fatigue testing equipment.
to maintain a constant peripheral speed during a laser treatment. In addition, to ensure the same feed rate, the translation speed of the CNC motion was varied accordingly. All the parameters were chosen to avoid or at least limit the overlapping phenomenon. 2.3. Characterization test Fatigue tests were performed to investigate the influences of laser power and scan speed on HPDL treatment. A four point rotating bending machine was used to perform the tests (Fig. 4). The stress on an arbitrary point of the surface of the rotating bending substrate varies in a sinusoidal manner, approaching maximum tensile and compressive stress every cycle. In the elastic field, Eq. 1 provides an estimate of the maximum stress acting on a substrate:
Fig. 3. Laser path.
the laser system employed. Table 2 summarizes the preliminary experimental results. Surface laser treatments followed the experimental schedule reported in Table 2. The experimental variables were the laser power that was incremented from 100 to 300 W with a step of 50 W and the scan speed that was incremented from 12 to 20 mm/s with a step of 2 mm/s. All the tests were replicated three times (75 experimental tests). The scan speed was the surface speed calculated as the product of the rotational speed of the substrate by the local radius of the specimen. The treatment was performed starting from the zone of the substrate with the largest section going towards the zone with the smallest section. To maintain a constant laser scan speed, the rotating speed of the substrate was varied accordingly. The details of the scanning patterns and speeds are hereby reported. Referring to Fig. 3, laser treatment started at point A, where the diameter of the specimen is 15 mm, and continued along the surface of the substrate passing through points B, C, and D. For each experimental condition, the rotation and translation speeds of the substrates were chosen to ensure no overlap of the resulting laser treatment. After each step, the treated substrates were characterized to evaluate the extent of the thermally altered zones. Because of the decrease and increase in the diameter of the components with the feed, the rotating speed of the substrates was continuously increased or decreased according to the schedule summarized in Table 3. In this way, it was possible
=
32M d3
(1)
where is the maximum surface stress, M is the bending moment and d is the critical diameter of the sample. The substrates were submitted to alternate cycles of tensile and compressive stresses as they were simultaneously bent and rotated. During fatigue tests, variable loads from 10 to 16 kg were applied, corresponding to variable maximum amplitudes of alternating stress (max ) in the range of approximately 320–460 MPa. After each rupture of the fatigue sample, their cross-sections were examined by a stereoscope (Nikon SMZ-U) and a metallographic microscope (Leica Cambridge 360). Knoop hardness testing of the cross section of the laser hardened specimens was performed to measure the effectiveness of the treatment. To determine the Knoop hardness, HK, (ISO 4545) the pyramid-shaped rhombic indenter (longitudinal edge angle 172.5◦ , transverse edge angle 130◦ ) was pressed into the specimen with a test load of 0.5 N and dwell time of 15 s. Finally, the results were converted to the Rockwell C hardness scale. 3. Results and discussions The tests were carried out according to the experimental plan in Table 2. The results show that the treatment conditions at 100 and 150 W did not lead to significant changes in the substrates. Surface melting of the substrate was triggered by increasing the
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
269
Table 4 Laser hardening power conditions. Scan speed, mm/s
Laser Power, W
Results
12 14 16 18 20
100 150 200 250 300
No significant change in substrate properties No significant change in substrate properties Change in substrate properties. No melting phenomena Change in substrate properties, but trace of melting phenomena Change in substrate properties, but melting phenomena
Table 5 Laser hardening test conditions. Test Run
Test part diameter, mm
Laser Power, W
Scan Speed, mm/s
Results
1 2 3 4 5
6
200
12 14 16 18 20
Melted material trace Melted material trace No melting phenomena No melting phenomena No melting phenomena
Fig. 5. Number of cycles vs scan speed (Laser Power 200 W).
power to 250 or 300 W, independent of the scanning speed. Table 4 summarizes the results achieved by varying the laser power. Attention was focused on the experimental treatments carried out at 200 W. The treatments at 200 W highlighted the process conditions, which impaired the substrate surface less, showing the onset of local melting phenomenon for the lowest speeds (12 and 14 mm/s). The results of the test at 200 W while varying the scan speed are reported in Table 5. These results are in good agreement with literature results. Lusquinosa et al. studied the influence of scanning speed on the superficial hardness while varying the radiated power of a diode laser. The scan speeds were set at 5, 10, 15, 20 and 25 mm/sand indicated that the highest hardness was achieved at a depth of 200 m when the scanning speed was 20 mm/min. Higher scan speeds decreased the final hardness of the treated surface as a result of the back-tempering phenomenon [18]. Fig. 5 shows the increase in the number of cycles during fatigue testing of the steel substrates vs. the scan speed of the laser treatment. Scan speed was found to be pivotal in influencing the fatigue endurance of the components. Increasing scan speed causes an increase in the fatigue life of the substrates. This result is in agreement with data reported in the literature [9,10,17]. The amount of thermal energy the steel can absorb during treatment decreases, and laser radiation can generate the appropriate heating and cooling cycle inside the material, triggering the martensitic transformation of the steel components. Fine-tuning of the amount of energy absorbed by the steel is, therefore, crucial to enhancing the fatigue endurance of the components. This is especially true as the laser radiation emitted by the diode source is absorbed effi-
ciently by the metal surface, which is different from what occurs with most of the alternative laser sources deployed to perform thermal treatment of metallic alloys (CO2 laser source, for example [6]). The increment in fatigue life seen in Fig. 5 can be attributed to the formation of an annealed martensitic structure on the substrate surface due to heating induced by the laser treatment. Laser scanning in the subsequent steps causes an additional heat treatment of the material, which leads to a slight annealing of the steel (i.e., back tempering phenomenon) [17,19]. This phenomenon is seen in Fig. 6, which shows a cross-section of the steel substrate, where the martensitic transformation approaches a thickness of approximately 200 m, in good agreement with data reported in the literature [9,17–19]. Defining and controlling the absorption of laser radiation during thermal treatment of metallic alloys is of fundamental importance, since all of the heating energy is delivered to the material by that route. Even the smallest change in absorption can significantly affect the amount of laser power required to complete the treatment. Pantsar et al. found that, depending on the cutting parameters, the absorptivity of a machined clean steel surface during HPDL transformation hardening can vary from 46% to 72% [6]. Shot peening of the surface can increase the amount of energy absorbed by the workpiece (66% to 81%), while also reducing its variability. However, the highest absorption coefficient for HPDL transformation hardening can be achieved by applying graphite coatings to the workpiece surface, with an absorption coefficient in excess of 85% being measured. Although machined steel surfaces might not have best morphology to favour absorption of laser radiation, the experimental findings show the high effectiveness of the diode laser source in the present experimental conditions and with the specific alloys (AISI 1040) involved. This result is also of practical relevance as it avoids further processing of the workpiece by either surface treatment (i.e., shot peening) or coating applications (i.e., spraying of graphite-based lacquer), which are necessary steps with different laser sources to achieve a satisfactory absorption rate of the laser radiation. Fig. 7 shows Wohler curves for the treated and untreated steel substrates. The curves show there is a significant increase in the fatigue life of the substrates treated by the diode laser. The curve asymptote is reached at 325 MPa in the as-received material and at 360 MPa after HPDL treatment, an increase of more than 10%. More importantly, the nose of the Wohler curve for the laser treated substrates is reached at a number of cycles, which is almost twice that of the as-received substrates. Similarly, laser treatment creates a Heat Altered Zone (HAZ) of 150–200 m in thickness (180 m in Fig. 6a). The micro-hardness measurement results shown in Fig. 8 indicate that this area has a harder and more brittle structure due
270
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
Fig. 6. (a) Micrograph of a laser hardened cross-section, (b) Martensite structure and, (c) perlite and ferrite structure.
Fig. 7. Wohler curves of the untreated specimen and the specimen treated at 200 W and 20 mm/s.
to significant hardening of the treated layer. Micro-melting and evaporation of the material results in a HAZ that is not perfectly homogeneous because of the initial structure of the material itself. The resulting properties of the HAZ are, however, very close to those typically achieved after similar laser treatment [20,21]. 4. Conclusions The present work investigated the effectiveness of surfacehardening by High Power Diode Lasers (HPDL) to increase the fatigue life of AISI 1040 steel. The extent of HAZs was consistently found to depend on how much fluence was delivered to the substrate during laser treatment. The best thermal treatment conditions are reached (single scan test) using a laser power of 200 W and a scan speed of 20 mm/s. Lower laser power had no effect, higher laser power causes surface melting.
Fig. 8. Hardness in the HAZ and surface zones.
Analysis of the microstructure of the laser transformation hardened substrates reveals three different areas: (i) a topmost area characterized by a homogenous distribution of annealed martensite; (ii) an intermediate area characterized by a heterogeneous distribution of annealed martensite; (iii) an unaltered underlying area. HPDL surface treatment increases the fatigue life of AISI 1040 steel. The Wohler curve asymptote is reached at 325 MPa in untreated material and at 360 MPa in HPDL treated material. Moreover, the nose of the curve for laser treated substrates is reached after a number of cycles, which is almost twice that of the as-
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
received substrates. The experimental results therefore show the great potential of HPDL for surface hardening of steel substrates. References [1] Fattorini F, Marchi Ricci FM, Senin A. Metal behaviour & surface engineering technology transfer series. IITT-International; 1989. p. 317. [2] Liverani Erica, Lutey Adrian HA, Ascari Alessandro, Fortunato Alessandro, Tomesani Luca. A complete residual stress model for laser surface hardening of complex medium carbon steel components. Surf Coat Technol 2016;302:100–6, http://dx.doi.org/10.1016/j.surfcoat.2016.05.066. [3] Sakai T, Sato Y, Oguma N. Characteristic S–N properties of highcarbon–chromium-bearing steel under axial loading in long-life fatigue. Fatigue Fract Eng Mater Struct 2002;25(8/9):765–73, http://dx.doi.org/10.1046/j.1460-2695.2002.00574.x. [4] Bachmann F. Industrial application of high power diode lasers materials processing. Appl Surf Sci 2003;208–209:p125, in http://dx.doi.org/10.1016/S0169-4332(02)01349-1. [5] Li L. Advances and characteristics of high-power diode laser materials processing. Opt Lasers Eng 2000;34:p231, http://dx.doi.org/10.1016/S0143-8166(00)00066-X. [6] Pantsar H, Kujanpää V. Diode laser beam absorption in laser transformation hardening of low alloy steel. J Laser Appl 2004;16(3):p147, http://dx.doi.org/10.2351/1.1710879. [7] Ganesh P, Sundar R, Kumar H, Kaul R, Ranganathan K, Hedaoo P, et al. Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening. Mater Design 2014;54:734–41, http://dx.doi.org/10.1016/j.matdes.2013.08.104. [8] Haag M, Rudlaff T. Assesment of different HPDL for materials processing. Laser Mater Proces 1997;3097:p717, http://dx.doi.org/10.1117/12.281120. [9] Klocke F, Demmer A, Zaboklicki A. Investigation into the use of high power diode lasers for hardening and thermal conduction welding of metals. Proc SPIE 1997;3097:592. [10] Ehlers B, Herfurth HJ, Heinmann S. Surface hardening with high power diode laser, proceedings of ICALEO’98 85G. Laser Inst Am 1998:p75.
271
[11] Ki Hyungson, So Sangwoo, Kim Sanseo. Laser transformation hardening of carbon steel sheets using a heat sink. J Mater Process Technol 2014;214(November (11)):2693–705, http://dx.doi.org/10.1016/j.jmatprotec.2014.06.013. [12] Gupta Nupur, Nataraj Neela. A dual weighted residual method for an optimal control problem of laser surface hardening of steel. Math Comput Simul 2014;103:12–32, http://dx.doi.org/10.1016/j.matcom.2013.12.007. ˇ [13] Cern y´ Ivo, Sís Jiˇrí. Fatigue strength of laser hardened 42CrMo4 steel considering effects of compressive residual stresses on short crack growth. Proc Eng 2014;74:417–20, http://dx.doi.org/10.1016/j.proeng.2014.06.292. [14] Wang Zhou, Jiang Chuanhai, Gan Xiaoyan, Chen Yanhua, Ji Vincent. Influence of shot peening on the fatigue life of laser hardened 17-4PH steel. Int J Fatigue 2011;33(4):549–56, http://dx.doi.org/10.1016/j.ijfatigue.2010.10.010. [15] Ren NF, Yang HM, Yuan SQ, Wang Y, Tang SX, Zheng LM, et al. High temperature mechanical properties and surface fatigue behavior improving of steel alloy via laser shock peening. Mater Design 2014;53(January):452–6, http://dx.doi.org/10.1016/j.matdes.2013.07.009. [16] Mahmoudi B, Torkamany MJ, Sabour Rouh Aghdam AR, Sabbaghzade J. Laser surface hardening of AISI 420 stainless steel treated by pulsed Nd:YAG laser. Mater Design 2010;31(May (5)):2553–60, http://dx.doi.org/10.1016/j.matdes.2009.11.034. [17] Gisario A, Barletta M, Boschetto A. Characterization of laser treated steels using instrumented indentation by cylindrical flat punch. Surf Coat Technol 2008;202(12):2557–69, http://dx.doi.org/10.1016/j.surfcoat.2007.09.024. ˜ ˜ [18] Lusquinosa F, Condea JC, Bonssb S, Riveiroa A, Quinteroa F, Comesanaa R, et al. Theoretical and experimental analysis of high power diode laser (HPDL) hardening of AISI 1045 steel. Appl Surf Sci 2007;254(4):948–54, http://dx.doi.org/10.1016/j.apsusc.2007.07.200. [19] Guarino S, Barletta M, Afilal A. Fatigue life improvement of steel components by surface diode laser treatments. In: International Conference on Innovative Technologies IN-TECH. 2011. [20] Incropera FP, Dewitt DP, Bergman TL, Lavine AS. Fundamentals of Heat and Mass Transfer. sixth ed. New York: John Wiley & Sons; 2007. [21] Gisario A, Barletta M, Conti C, Guarino S. Springback control in sheet metal bending by laser-assisted bending: experimental analysis, empirical and neural network modelling. Opt Laser Eng 2011;49(12):1372–83, http://dx.doi.org/10.1016/j.optlaseng.2011.07.010.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
High Power Diode Laser (HPDL) surface hardening of low carbon steel: Fatigue life improvement analysis S. Guarino a,∗ , M. Barletta b , Abdelkarim Afilal c a b c
Università degli Studi Niccolò Cusano, Dipartimento di Ingegneria, Via don Carlo Gnocchi 3,00166 Rome, Italy Università degli Studi di Roma “Tre”, Dipartimento di Ingegneria, Via Vito Volterra 62, 00146 Rome, Italy Uuniversità Chouaib Doukkali, Faculté des Sciences, El Jadida, Morocco
a r t i c l e
i n f o
Article history: Received 8 February 2016 Received in revised form 2 April 2017 Accepted 24 June 2017 Keywords: Diode laser hardening Fatigue life Steel
a b s t r a c t In the present investigation, a high-power diode laser was applied to improve the fatigue life of AISI 1040 steel components. The interaction between the laser radiation and the steel surface was studied, and the effectiveness of the laser treatment was analysed by fatigue testing. First, the importance of each of the following laser operational parameters was assessed: power, scan speed, focus and number of passes. Second, rotating bending fatigue tests were performed to investigate the effects of laser treatment and the influences of the laser operating parameters on the fatigue endurance of the components. Wohler curves, determined from an analysis of the experimental results, showed that laser treatment can significantly increase the fatigue life of irradiated components, thus revealing the suitability of this method for industrial applications. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Customers today impose stringent requirements on designers and manufacturers in the development of structural components [1]. Extension of the mechanical strength and fatigue life of structural components is often pivotal in several manufacturing environments [2,3]. Designing, monitoring and providing innovative technological solutions aimed at improving the fatigue life of structural components is therefore of crucial importance. Many different approaches to improving the fatigue life of steel components have been proposed in academia in recent decades and are used in industrial practise (heat and mechanical treatments, surface alloying, overlying coatings, etc.). However, these technological solutions, while having many advantages, are often challenging and time consuming. In addition, these solutions do not allow selective treatment of small portions of the components, being intrinsically a bulk technology, thus limiting their fields of application. In fact, thermal treatment (hardening, tempering, annealing, etc.) can dramatically change the properties of metal alloys and affect the fatigue limit of the materials. Nevertheless, they are not selective and act on the whole material, zeroing the gradient of the physical, chemical and mechanical properties of the materials
∗ Corresponding author. E-mail address: [email protected] (S. Guarino).
that can be useful in improving the fatigue response. In contrast, laser surface treatment is extremely promising [4–8] as it can be focused on selected zones of the components, leaving areas not directly exposed to laser radiation unaltered. As a result, laser systems have been considered very attractive for surface-hardening heat treatment as they offer selectivity, make process control very easy, and facilitate manipulation [8]. When compared to other processes, laser hardening causes little deformation of the part; thus, post machining is virtually eliminated. In addition, a broader variety of materials can be hardened; even low carbon steels can be hardened because of the rapid heating and cooling rates generated by the laser radiation. By using a laser, it is possible to perform versatile surface treatment with great precision, controlled heating, low heat input (and therefore low distortion) and fast cycle time. On the other hand, the main disadvantage compared to conventional technologies is found with the effects of back tempering induced by multiple passes [2]. Among laser sources, the High-Power Diode Laser (HPDL) is particularly well suited to surface hardening. Industrial scale kilowatt HPDLs have recently become available with uniformly distributed beams with a rectangular or elliptically shaped spot, thus ensuring uniform heating of the treated surface [8,9]. Furthermore, the relatively short wavelength (800–940 nm) of HPDLs ensures good energy transfer to most substrates, without the need for pretreatment to improve surface absorption [8]. Finally, HPDL’s benefits include compactness, energy efficiency, low lifetime and running
http://dx.doi.org/10.1016/j.jmapro.2017.06.015 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
costs, thus often representing the ideal candidate for surface hardening. Various applications of diode lasers for material hardening have been reported over the past decades. Klocke et al. hardened 42CrMo4 steel to over 700 HV and a depth of approximately 0.5 mm, using the first commercially available 650 W HPDL [9]. Ehlers et al. reported the usage of an HPDL for surface-hardening of M1044 steel components, achieving a consistently hardened depth of approximately 1 mm with a width of approximately 20 mm [10]. Hyungson et al. studied the laser hardening process by developing an intensity–interaction time diagram to define which part of the heat treatable region was successfully hardened [11]. Gupta et al. analysed and implemented a dual weighted residual (DWR) approach to solve an optimal control problem with ˇ y´ et al. proposed the laser surface hardening of steel [12]. Cern an experimental evaluation of fatigue resistance and initiation mechanisms of heat treated and laser hardened 42CrMo4 steel. The results confirm the significant beneficial effects of compressive stresses induced by laser hardening on fatigue resistance caused by the retardation or arrest of short fatigue cracks emanating from microstructure defects [13]. Wang et al. studied the effect of the shot peening and dual shot peening on laser hardened 17-4PH steel. The work shows that the fatigue life of laser hardened steel increases dramatically after shot peening treatment [14]. Ren et al. studied laser shock peening treatment on ASTM: 410 L 00Cr12 microstructures and fatigue resistance. Fatigue life was enhanced by approximately 58% at elevated temperatures (up to 600 ◦ C) [15]. Mahmoudi et al. studied AISI 420 martensitic stainless steel surface-hardening by a pulsed Nd:YAG laser and the influence of process parameters (laser pulse energy, duration time and travel speed) on the depth and hardness of the laser treated area and its corrosion behaviour. The beneficial effects of laser surface hardening by refining the microstructure and enhancing the pitting corrosion resistance of the martensitic stainless steel were observed [16]. Other experimental and numerical studies involving high power diode lasers are also reported in the literature [17–19]. Apart from the specificity of each work reported in the literature, defining and controlling the absorption of laser radiation during surface treatment of metallic alloys by HPDL were always found to be crucial to ensure process effectiveness, whatever was the metal workpiece involved and whatever was the objective of the study. Indeed, small variation in metal absorption can have a heavy effect on the setting of laser parameters and, consequently, on the amount of energy that must be delivered and, therefore, consumed during process. Pantsar et al. found absorptivity of machined steel surface during HPDL transformation hardening to vary in a very broad range (46%–72%) [6], with metal surface reprocessing being the only possibility to increase further and, above all, stabilize the absorption coefficient (66%–81% by shot peening or 85% and over by the application of graphite coatings to the workpiece surface). Reprocessing of the workpiece surface before laser treatment can, however, make the difference between competitive and economically sustainable processes or not. Therefore, in the present investigation, a high-power diode laser was used to improve the fatigue life of cylindrical substrates comprised of AISI 1040 low carbon steel without any further pretreatments that could increase the overall cost of the operation. In this specific framework, the interactions between the laser source and steel surface were investigated by varying the laser operating parameters (that is, laser power, scan speed and number of passes). Laser treatment was performed by simultaneously rotating and translating the cylindrical substrate under a stationary laser head to generate a helical scanning pattern. The effectiveness of the laser treatment was studied by rotating bending fatigue tests and microscopic analysis of the as-is and hardened substrates.
267
Fig. 1. Substrate geometry. Table 1 (a)Chemical composition (wt.%). (b) Mechanical properties. (a) C
Mn
Si
Altre
0.37–0.44
0.50–0.80
0.15–0.40
P and S ≤0.035
Rm (MPa)
E (GPa)
Hardness [HRC]
670
200
0.3
13
(b)
2. Materials and methods 2.1. Materials A low-medium carbon content steel (AISI 1040) suitable for surface-hardening heat treatment [17] was chosen as the starting material. Seventy-five cylindrical substrates, 116 mm long and 15 mm in diameter, were cut from 6 m long rods. The top surface of a small section (6 mm in diameter) of each sample was milled using the same cutting parameters (feed, cutting depth and cutting speed) and the same insert geometry on an instrumented universal milling machine. The substrates featured a similar average roughness, Ra , of 1.2 m. The surfaces morphology was measured before heat treatment using a profilometer (Taylor-Hobson Talysurf CLI 2000). Fig. 1 shows the geometry of the substrate. The chemical composition and mechanical properties of the steel are reported in Table 1. 2.2. Laser hardening equipment and methods Surface-hardening heat treatment was performed using a HighPower Diode Laser (Rofin-Sinar DL015) with a maximum power of 1500 W, a wavelength of 940 nm, and an elliptic spot shape of 0.6 mm along the minor axis and 1.9 mm along the major axis. A 63 mm focal length lens was chosen to ensure improved penetration of the laser beam in the material and, consequently, a deeper expected thickness of the hardened zone. During hardening tests, the substrates were held on a CNC motion system and laser treated by rotating and translating them under the stationary laser source. The resulting scanned paths were helical. Each scanned path was 36 mm long. For protection and insulation, an inert gas (Argon) flux was also directed on the substrate surface when it was being irradiated by the laser source. Fig. 2 shows a schematic of
268
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271 Table 3 Experimental conditions of varying diameter (rotating speed and feed rate).
D0 D1 D2 D3 D4
Diameter, mm
Circumference, mm
Rotating speed, cycles/s
Feed rate, mm/s
15 12.8 10.6 8.4 6
47.12 40.21 33.30 26.39 18.85
0.38 0.45 0.54 0.68 0.95
0.46 0.54 0.65 0.82 1.15
Fig. 2. HPDL system. Table 2 Experimental factors. Factor levels
Laser Power, W
Scan speed, mm/s
I II III IV V
100 150 200 250 300
12 14 16 18 20
Fig. 4. Fatigue testing equipment.
to maintain a constant peripheral speed during a laser treatment. In addition, to ensure the same feed rate, the translation speed of the CNC motion was varied accordingly. All the parameters were chosen to avoid or at least limit the overlapping phenomenon. 2.3. Characterization test Fatigue tests were performed to investigate the influences of laser power and scan speed on HPDL treatment. A four point rotating bending machine was used to perform the tests (Fig. 4). The stress on an arbitrary point of the surface of the rotating bending substrate varies in a sinusoidal manner, approaching maximum tensile and compressive stress every cycle. In the elastic field, Eq. 1 provides an estimate of the maximum stress acting on a substrate:
Fig. 3. Laser path.
the laser system employed. Table 2 summarizes the preliminary experimental results. Surface laser treatments followed the experimental schedule reported in Table 2. The experimental variables were the laser power that was incremented from 100 to 300 W with a step of 50 W and the scan speed that was incremented from 12 to 20 mm/s with a step of 2 mm/s. All the tests were replicated three times (75 experimental tests). The scan speed was the surface speed calculated as the product of the rotational speed of the substrate by the local radius of the specimen. The treatment was performed starting from the zone of the substrate with the largest section going towards the zone with the smallest section. To maintain a constant laser scan speed, the rotating speed of the substrate was varied accordingly. The details of the scanning patterns and speeds are hereby reported. Referring to Fig. 3, laser treatment started at point A, where the diameter of the specimen is 15 mm, and continued along the surface of the substrate passing through points B, C, and D. For each experimental condition, the rotation and translation speeds of the substrates were chosen to ensure no overlap of the resulting laser treatment. After each step, the treated substrates were characterized to evaluate the extent of the thermally altered zones. Because of the decrease and increase in the diameter of the components with the feed, the rotating speed of the substrates was continuously increased or decreased according to the schedule summarized in Table 3. In this way, it was possible
=
32M d3
(1)
where is the maximum surface stress, M is the bending moment and d is the critical diameter of the sample. The substrates were submitted to alternate cycles of tensile and compressive stresses as they were simultaneously bent and rotated. During fatigue tests, variable loads from 10 to 16 kg were applied, corresponding to variable maximum amplitudes of alternating stress (max ) in the range of approximately 320–460 MPa. After each rupture of the fatigue sample, their cross-sections were examined by a stereoscope (Nikon SMZ-U) and a metallographic microscope (Leica Cambridge 360). Knoop hardness testing of the cross section of the laser hardened specimens was performed to measure the effectiveness of the treatment. To determine the Knoop hardness, HK, (ISO 4545) the pyramid-shaped rhombic indenter (longitudinal edge angle 172.5◦ , transverse edge angle 130◦ ) was pressed into the specimen with a test load of 0.5 N and dwell time of 15 s. Finally, the results were converted to the Rockwell C hardness scale. 3. Results and discussions The tests were carried out according to the experimental plan in Table 2. The results show that the treatment conditions at 100 and 150 W did not lead to significant changes in the substrates. Surface melting of the substrate was triggered by increasing the
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
269
Table 4 Laser hardening power conditions. Scan speed, mm/s
Laser Power, W
Results
12 14 16 18 20
100 150 200 250 300
No significant change in substrate properties No significant change in substrate properties Change in substrate properties. No melting phenomena Change in substrate properties, but trace of melting phenomena Change in substrate properties, but melting phenomena
Table 5 Laser hardening test conditions. Test Run
Test part diameter, mm
Laser Power, W
Scan Speed, mm/s
Results
1 2 3 4 5
6
200
12 14 16 18 20
Melted material trace Melted material trace No melting phenomena No melting phenomena No melting phenomena
Fig. 5. Number of cycles vs scan speed (Laser Power 200 W).
power to 250 or 300 W, independent of the scanning speed. Table 4 summarizes the results achieved by varying the laser power. Attention was focused on the experimental treatments carried out at 200 W. The treatments at 200 W highlighted the process conditions, which impaired the substrate surface less, showing the onset of local melting phenomenon for the lowest speeds (12 and 14 mm/s). The results of the test at 200 W while varying the scan speed are reported in Table 5. These results are in good agreement with literature results. Lusquinosa et al. studied the influence of scanning speed on the superficial hardness while varying the radiated power of a diode laser. The scan speeds were set at 5, 10, 15, 20 and 25 mm/sand indicated that the highest hardness was achieved at a depth of 200 m when the scanning speed was 20 mm/min. Higher scan speeds decreased the final hardness of the treated surface as a result of the back-tempering phenomenon [18]. Fig. 5 shows the increase in the number of cycles during fatigue testing of the steel substrates vs. the scan speed of the laser treatment. Scan speed was found to be pivotal in influencing the fatigue endurance of the components. Increasing scan speed causes an increase in the fatigue life of the substrates. This result is in agreement with data reported in the literature [9,10,17]. The amount of thermal energy the steel can absorb during treatment decreases, and laser radiation can generate the appropriate heating and cooling cycle inside the material, triggering the martensitic transformation of the steel components. Fine-tuning of the amount of energy absorbed by the steel is, therefore, crucial to enhancing the fatigue endurance of the components. This is especially true as the laser radiation emitted by the diode source is absorbed effi-
ciently by the metal surface, which is different from what occurs with most of the alternative laser sources deployed to perform thermal treatment of metallic alloys (CO2 laser source, for example [6]). The increment in fatigue life seen in Fig. 5 can be attributed to the formation of an annealed martensitic structure on the substrate surface due to heating induced by the laser treatment. Laser scanning in the subsequent steps causes an additional heat treatment of the material, which leads to a slight annealing of the steel (i.e., back tempering phenomenon) [17,19]. This phenomenon is seen in Fig. 6, which shows a cross-section of the steel substrate, where the martensitic transformation approaches a thickness of approximately 200 m, in good agreement with data reported in the literature [9,17–19]. Defining and controlling the absorption of laser radiation during thermal treatment of metallic alloys is of fundamental importance, since all of the heating energy is delivered to the material by that route. Even the smallest change in absorption can significantly affect the amount of laser power required to complete the treatment. Pantsar et al. found that, depending on the cutting parameters, the absorptivity of a machined clean steel surface during HPDL transformation hardening can vary from 46% to 72% [6]. Shot peening of the surface can increase the amount of energy absorbed by the workpiece (66% to 81%), while also reducing its variability. However, the highest absorption coefficient for HPDL transformation hardening can be achieved by applying graphite coatings to the workpiece surface, with an absorption coefficient in excess of 85% being measured. Although machined steel surfaces might not have best morphology to favour absorption of laser radiation, the experimental findings show the high effectiveness of the diode laser source in the present experimental conditions and with the specific alloys (AISI 1040) involved. This result is also of practical relevance as it avoids further processing of the workpiece by either surface treatment (i.e., shot peening) or coating applications (i.e., spraying of graphite-based lacquer), which are necessary steps with different laser sources to achieve a satisfactory absorption rate of the laser radiation. Fig. 7 shows Wohler curves for the treated and untreated steel substrates. The curves show there is a significant increase in the fatigue life of the substrates treated by the diode laser. The curve asymptote is reached at 325 MPa in the as-received material and at 360 MPa after HPDL treatment, an increase of more than 10%. More importantly, the nose of the Wohler curve for the laser treated substrates is reached at a number of cycles, which is almost twice that of the as-received substrates. Similarly, laser treatment creates a Heat Altered Zone (HAZ) of 150–200 m in thickness (180 m in Fig. 6a). The micro-hardness measurement results shown in Fig. 8 indicate that this area has a harder and more brittle structure due
270
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
Fig. 6. (a) Micrograph of a laser hardened cross-section, (b) Martensite structure and, (c) perlite and ferrite structure.
Fig. 7. Wohler curves of the untreated specimen and the specimen treated at 200 W and 20 mm/s.
to significant hardening of the treated layer. Micro-melting and evaporation of the material results in a HAZ that is not perfectly homogeneous because of the initial structure of the material itself. The resulting properties of the HAZ are, however, very close to those typically achieved after similar laser treatment [20,21]. 4. Conclusions The present work investigated the effectiveness of surfacehardening by High Power Diode Lasers (HPDL) to increase the fatigue life of AISI 1040 steel. The extent of HAZs was consistently found to depend on how much fluence was delivered to the substrate during laser treatment. The best thermal treatment conditions are reached (single scan test) using a laser power of 200 W and a scan speed of 20 mm/s. Lower laser power had no effect, higher laser power causes surface melting.
Fig. 8. Hardness in the HAZ and surface zones.
Analysis of the microstructure of the laser transformation hardened substrates reveals three different areas: (i) a topmost area characterized by a homogenous distribution of annealed martensite; (ii) an intermediate area characterized by a heterogeneous distribution of annealed martensite; (iii) an unaltered underlying area. HPDL surface treatment increases the fatigue life of AISI 1040 steel. The Wohler curve asymptote is reached at 325 MPa in untreated material and at 360 MPa in HPDL treated material. Moreover, the nose of the curve for laser treated substrates is reached after a number of cycles, which is almost twice that of the as-
S. Guarino et al. / Journal of Manufacturing Processes 28 (2017) 266–271
received substrates. The experimental results therefore show the great potential of HPDL for surface hardening of steel substrates. References [1] Fattorini F, Marchi Ricci FM, Senin A. Metal behaviour & surface engineering technology transfer series. IITT-International; 1989. p. 317. [2] Liverani Erica, Lutey Adrian HA, Ascari Alessandro, Fortunato Alessandro, Tomesani Luca. A complete residual stress model for laser surface hardening of complex medium carbon steel components. Surf Coat Technol 2016;302:100–6, http://dx.doi.org/10.1016/j.surfcoat.2016.05.066. [3] Sakai T, Sato Y, Oguma N. Characteristic S–N properties of highcarbon–chromium-bearing steel under axial loading in long-life fatigue. Fatigue Fract Eng Mater Struct 2002;25(8/9):765–73, http://dx.doi.org/10.1046/j.1460-2695.2002.00574.x. [4] Bachmann F. Industrial application of high power diode lasers materials processing. Appl Surf Sci 2003;208–209:p125, in http://dx.doi.org/10.1016/S0169-4332(02)01349-1. [5] Li L. Advances and characteristics of high-power diode laser materials processing. Opt Lasers Eng 2000;34:p231, http://dx.doi.org/10.1016/S0143-8166(00)00066-X. [6] Pantsar H, Kujanpää V. Diode laser beam absorption in laser transformation hardening of low alloy steel. J Laser Appl 2004;16(3):p147, http://dx.doi.org/10.2351/1.1710879. [7] Ganesh P, Sundar R, Kumar H, Kaul R, Ranganathan K, Hedaoo P, et al. Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening. Mater Design 2014;54:734–41, http://dx.doi.org/10.1016/j.matdes.2013.08.104. [8] Haag M, Rudlaff T. Assesment of different HPDL for materials processing. Laser Mater Proces 1997;3097:p717, http://dx.doi.org/10.1117/12.281120. [9] Klocke F, Demmer A, Zaboklicki A. Investigation into the use of high power diode lasers for hardening and thermal conduction welding of metals. Proc SPIE 1997;3097:592. [10] Ehlers B, Herfurth HJ, Heinmann S. Surface hardening with high power diode laser, proceedings of ICALEO’98 85G. Laser Inst Am 1998:p75.
271
[11] Ki Hyungson, So Sangwoo, Kim Sanseo. Laser transformation hardening of carbon steel sheets using a heat sink. J Mater Process Technol 2014;214(November (11)):2693–705, http://dx.doi.org/10.1016/j.jmatprotec.2014.06.013. [12] Gupta Nupur, Nataraj Neela. A dual weighted residual method for an optimal control problem of laser surface hardening of steel. Math Comput Simul 2014;103:12–32, http://dx.doi.org/10.1016/j.matcom.2013.12.007. ˇ [13] Cern y´ Ivo, Sís Jiˇrí. Fatigue strength of laser hardened 42CrMo4 steel considering effects of compressive residual stresses on short crack growth. Proc Eng 2014;74:417–20, http://dx.doi.org/10.1016/j.proeng.2014.06.292. [14] Wang Zhou, Jiang Chuanhai, Gan Xiaoyan, Chen Yanhua, Ji Vincent. Influence of shot peening on the fatigue life of laser hardened 17-4PH steel. Int J Fatigue 2011;33(4):549–56, http://dx.doi.org/10.1016/j.ijfatigue.2010.10.010. [15] Ren NF, Yang HM, Yuan SQ, Wang Y, Tang SX, Zheng LM, et al. High temperature mechanical properties and surface fatigue behavior improving of steel alloy via laser shock peening. Mater Design 2014;53(January):452–6, http://dx.doi.org/10.1016/j.matdes.2013.07.009. [16] Mahmoudi B, Torkamany MJ, Sabour Rouh Aghdam AR, Sabbaghzade J. Laser surface hardening of AISI 420 stainless steel treated by pulsed Nd:YAG laser. Mater Design 2010;31(May (5)):2553–60, http://dx.doi.org/10.1016/j.matdes.2009.11.034. [17] Gisario A, Barletta M, Boschetto A. Characterization of laser treated steels using instrumented indentation by cylindrical flat punch. Surf Coat Technol 2008;202(12):2557–69, http://dx.doi.org/10.1016/j.surfcoat.2007.09.024. ˜ ˜ [18] Lusquinosa F, Condea JC, Bonssb S, Riveiroa A, Quinteroa F, Comesanaa R, et al. Theoretical and experimental analysis of high power diode laser (HPDL) hardening of AISI 1045 steel. Appl Surf Sci 2007;254(4):948–54, http://dx.doi.org/10.1016/j.apsusc.2007.07.200. [19] Guarino S, Barletta M, Afilal A. Fatigue life improvement of steel components by surface diode laser treatments. In: International Conference on Innovative Technologies IN-TECH. 2011. [20] Incropera FP, Dewitt DP, Bergman TL, Lavine AS. Fundamentals of Heat and Mass Transfer. sixth ed. New York: John Wiley & Sons; 2007. [21] Gisario A, Barletta M, Conti C, Guarino S. Springback control in sheet metal bending by laser-assisted bending: experimental analysis, empirical and neural network modelling. Opt Laser Eng 2011;49(12):1372–83, http://dx.doi.org/10.1016/j.optlaseng.2011.07.010.