Thermal stress analysis for laser cutting corner with a fluctuant cutting speed in steel plate

Transcription

1 Journal of Physcs: Conference Seres PAPER OPEN ACCESS Thermal stress analyss for laser cuttng corner wth a fluctuant cuttng speed n steel plate To cte ths artcle: H B Xu et al 2015 J. Phys.: Conf. Ser Vew the artcle onlne for updates and enhancements. Ths content was downloaded from IP address on 21/07/2018 at 08:22

2 Thermal stress analyss for laser cuttng corner wth a fluctuant cuttng speed n steel plate H.B. Xu 1, Jun Hu 1, Hong Sheng 1, Z.C Du 1 1 School of Mechancal Engneerng, Shangha Jao Tong Unversty, Chna E-mal: xuhebng@sjtu.edu.cn Abstract. The temperature and thermal stress feld are mportant factors for controllng laser cuttng qualty. In the present study, thermal stress feld analyss for laser cuttng corner wth a fluctuant cuttng speed s carred out. Frstly, a three-dmensonal (3-D) fnte element model s establshed to smulate the temperature and thermal stress feld. Then the factors of nfluencng the cuttng speed fluctuaton such as corner angle and cuttng speed are studed n laser cuttng corner. It s found that the constraned cuttng speed at the corner ntroduces the phenomenon of corner burnng and uneven resdual stress dstrbuton. The predcted temperature, thermal stress results agree well wth the experment data through the laser cuttng corner experment of 3 mm steel plate. 1. Introducton Laser cuttng s a knd of non-contact thermal cuttng process and has many advantages such as good cuttng qualty, hgh cuttng speed, small heat-affected zone, lttle polluton and so on. And t has been wdely used n the ndustral producton. However, there s a certan decrease of cuttng speed n order to avodng cuttng machne shock at the cuttng corner [1, 2]. And ths results n bad cuttng qualty such as corner burnng, heat concentraton and uneven resdual stress dstrbuton. So t s n dre need of solvng ths problem to mprove laser cuttng effcency and extend applcaton. The heat transfer durng laser cuttng has been analysed by many scholars. Sheng and Josh [3] analysed the heat affected zone formaton for laser cuttng of stanless steel. The energy balance at the cuttng front was used to derve the steady-state kerf shape, and the extent of the heat affected zone (HAZ) can be determned from the area where propertes of the workpece materal were sgnfcantly affected by the resultng temperature feld. Prusa, Venktachalam [4] presented a mathematcal model to estmate the heat conducton losses durng laser cuttng. They predcted the thermal feld n the heat affected zone (HAZ). The nfluence of gas jet velocty n laser heatng wth a movng workpece was studed by Shuja and Ylbas [5]. They found that the effect of assstng gas jet velocty was more pronounced n the coolng cycle than n the heatng cycle of the laser heatng process. The temperature feld durng laser formng of plate metal was analysed by J and Wu [6]. An ncrease n the maxmum values of temperature was found wth an ncrease n laser output power, whle ncreasng the cuttng speed and workpece thckness has an nverse mpact on the temperature. Thermal stress of the workpece n laser cuttng s always the result of the thermal load whch s determned by the coupled temperature feld. Ylbas, Arf [7] establshed a three-dmensonal fnte model to predct thermal Content from ths work may be used under the terms of the Creatve Commons Attrbuton 3.0 lcence. Any further dstrbuton of ths work must mantan attrbuton to the author(s) and the ttle of the work, journal ctaton and DOI. Publshed under lcence by IOP Publshng Ltd 1

3 stress feld for dfferent materals. The thermal stress formaton durng cuttng n dfferent shapes such as crcle, sharp edges has been studed [8-11]. In ther studes, a constant temperature heat source at the meltng temperature of materal for the laser beam was ntroduced n the smulaton. Comparatvely, Nyon, Nyeoh [12] used the same method to smulate kerf wdth formaton and thermal stress durng the laser cuttng process. And a Guassan-dstrbuted laser heat source was adopted to model the heat nput durng the laser rradaton. However, n above studes few research studes have been conducted on the factor of cuttng speed fluctuaton at corner. So n ths paper, the nfluence of cuttng speed fluctuaton for thermal stress feld s carefully analysed, and a Guassan-dstrbuted heat source s also adopted as laser heat source n the 3-D fnte smulaton model. Meanwhle, the process of materal meltng s smulated by the method of element death and brth n ANSYS that the temperature of fnte element over the melt temperature wll be deactvated. A deactvated element remans n the model but contrbutes a near-zero conductvty value to the overall matrx. 2. Model of cuttng speed fluctuaton at corner Accordng to reference [13], the corner sze and maxmum speed for the machne tool and the dstance of acceleraton and deceleraton must be consdered to calculate the cuttng speed at corner. The workpece sze and B, C, D locatons are shown n Fg.1. Fg.1 Workpece sze and B, C, D locatons (unt: mm) Consderng the corner sze, then v!!!!"#!!"#!! where T s nterpolaton perod, s maxmum acceleraton, s corner sze. (7) Consderng the maxmum speed v of the cuttng machne, then max v! v!"# (8) Consderng the dstance of acceleraton and deceleraton, then v!!! v!!! + a! L! (9) v!!! v!!! + 2a! L!!! (10) where v s the ntal velocty at acceleraton dstance L to the corner, s the end velocty at deceleraton dstance L away from the corner. m v + 1 a s the maxmum acceleraton of cuttng machne. Combnng Eqs.(7)-(10), the cuttng speed at corner can be calculated as v!!!!!"!!"#!! (11) 2

4 In ths study, specfed cuttng speedν 0 s 50mm/s, and maxmum acceleraton s 8000mm/s2, and max the nterpolaton perod s 5ms. The cuttng speed at locaton B, C, D can be calculated by speed fluctuaton model as: ν B = 36.8 mm/s, ν C =21.92mm/s, ν D =36.8mm/s. It s obvous that the speed at locaton C decreases most. The cuttng speed varaton along the laser scannng path for locaton B, C, D s shown n Fg Cuttng speed varaton a V(mm /s) t(s) Fg.2 The curve of cuttng speed varaton 3. Fnte element smulaton The cuttng parameters used n the smulaton are lsted n Table1 whch are the same wth that n the experment for comparson. The process of laser beam movng along the corner s realzed by program as shown n Fg.3, and only half part of the cuttng kerf s modeled due to ts symmetry. The temperature dstrbuton and the resulted thermal stress n the workpece are performed by the FEM software ANSYS usng ANSYS parametrc Language (APDL). The mesh n fnte element analyss s shown as Fg.3. The mesh s more ntensve near the cuttng edge. The mesh comprses about elements. Fg.3 Model and meshng The movng dstance of laser source n each tme step s the length of an element. The cuttng parameters and materal mechancal propertes n smulaton model are shown n Table 1. Temperature dependent thermal propertes of plate are shown n Table 2. Cuttng Speed (mm/s) Table 1. Cuttng parameters and materal propertes used n the smulaton Laser Plate elastcty thermal expanson Posson power thckness modulus coeffcent α (1/K) rato ν (W) (mm) E(GPa) ρ (kg/m 3 ) x

5 Table 2. Thermal propertes parameters of plate Temp(K) K(W/mg K) Cp(J/kgK) Results and dscusson 4.1. Effects of corner sze o o Frstly, the corner sze declnes from 48.3 to The acceleraton s 5000mm/s 2, and the cuttng speeds for locaton B, C, D are calculated as:b=34.45mm/s, C=21.16mm/s, D=34.45mm/s. It can be found that the cuttng speed for locaton C declnes because of the smaller corner sze. Temperature dstrbuton contrast along Y-axs at locaton C for two knds of corner sze s separately shown n Fg.4 (a) and (b).the temperature dstrbuton along Y-axs for two knds of corner sze s almost the same when the laser beam s at locaton C n Fg.4 (a). However, the temperature for smaller corner sze at cut edge s hgher when laser cuttng fnshes, as shown n Fg.4 (b). The reason for ths s that smaller corner sze leads to smaller heat dffuson area. Meanwhle, smaller corner sze results n bgger declne of cuttng speed and the corner attans more energy. Consequently, the corner burnng s more serous. (a) (b) o Fg.4 Temperature dstrbuton contrast along Y-axs at locaton C α = 48.3 (C0), o α = 38.5 (C1) (a) Laser beam at locaton C (b) after cuttng Von Mses stress dstrbuon along Y-axs contrast wth dfferent corner sze for locaton C s shown n Fg.5. Tme T2 s when the laser beam s located at pont C, and tme T4 s when the workpece s completely cool. The Von Mses stress has always rose from dstance 0 to 0.056m at tme T2 for dfferent corner sze. But the von mses stress has declned from dstance 0.056m to cuttng edge because of modulus of elastcty declnng obvously near the meltng temperature. It also should be noted that the Von Mses stress s smaller for smaller corner sze because of more serous heat concentretonand and smaller temperature gradent. The resdual stress has the same varaton trend as Von Mses stress at tme T4. The reason for ths s that resdual stress results from the Von Mses stress. Meanwhle, the corner wth smaller sze also has smaller resdual stress. The maxmum resdual stress of dfferent corner sze at dfferent locactons s shown n Table 3. It can be found that the resdual stress reach the maxmum at locaton B, D. Meanwhle, resdual stress has ncreased when the corner sze declnes. Table 3 resdual stress at locatons B, C, D B C D C=48.3rad 275MPa 130MPa 270MPa C=38.05rad 290MPa 120MPa 280MPa 4

6 Fg.5 Von Mses stress dstrbuton contrast along Y-axs for locaton C 4.2. Effects of cuttng speed The cuttng speed declnes from 50mm/s to 30mm/s. The acceleraton s 5000mm/s 2. Then the cuttng speed for locaton B, C, D s changed to 30mm/s, 21.92mm/s, 30mm/s. Temperature dstrbuton contrast along Y-axs at locaton C for two knds of cuttng speeds s shown n Fg.6. The temperature dstrbuton along Y-axs for two cuttng speeds near cuttng edge s almost the same when the laser beam s at locaton C n Fg.6 (a). It should be noted that the temperature s hgher for smaller cuttng speed at dstance from 0.036mm to 0.055mm. The reason for ths s that smaller cuttng speed leads to the materal attanng more energy. The temperature for two knds of cuttng speed at cut edge s the same when laser cuttng fnshed n Fg.6 (b). The reason for ths s that the cuttng speed at corner declnes almost to the same value. And ths corner burnt phenomenon s the same for two cuttng speeds. However, the temperature s hgher for smaller cuttng speed at dstance 0.02mm to 0.051mm for the same reason that smaller cuttng speed leads to more absorbed energy. (a) (b) Fg.6 Temperature dstrbuton along Y-axs contrast at locaton C, 50mm/s(C0), (a) Laser beam at locaton C (b) after cuttng The Von Mses stress dstrbuon along Y-axs contrast wth dfferent cuttng speed for locaton C s shown n Fg.7. Tme T2 s when the laser beam s at locaton C, and tme T4 s when the workpece s completely cool. It can be found that the Von Mses stress s smaller for smaller cuttng speed because of more serous heat concentraton. Meanwhle, the corner wth smaller cuttng speed also has a smaller resdual stress.the maxmum resdual stress of dfferent cuttng speed at dfferent locactons s shown n Table 4. Resdual stress has declned when the cuttng speed declned. Table 4 resdual stress at locatons B, C, D B C D V=50mm/s 275MPa 130MPa 270MPa V=30mm/s 265MPa 120MPa 265MPa 5

7 Fg.7 Von Mses stress dstrbuton contrast along the y-axs at locaton C 5. Experment The laser used n the experment s contnuous CO2 laser and cuttng parameters are the same wth that used n the smulaton. The temperature data s measured by thermocouple. And the resdual stress s obtaned by XRD. Then the surface qualty of corner can be ganed by optcal mcroscope. Fg.8 shows the temperature varaton curve of locaton A for smulaton and experment. It can be found that the smulaton has good agreement wth the experment. The resdual stress at dfferent locatons for smulaton and experment s shown n Table 5. It also can be found that the smulaton has good agreement wth the experment. Fg.9 shows the surface qualty of corner. The surface has a poor qualty n Fg.9. At the same tme, t can be found that there exst serous corner burnng and large adherng slag at bottom. Fg.8 Temperature varaton curve of locaton A, C Table 5 resdual stress (Mpa) at locatons B, C, D B C D Experment(X-axs) ± ± ± 8.19 Model predct(x-axs) Experment(Y-axs) ± ± ± Model predct(y-axs)

8 4th Internatonal Conference on Mathematcal Modelng n Physcal Scences (IC-MSquare2015) IOP Publshng Fg.9 Surface qualty at the corner 6. Concluson A 3D model of laser cuttng corner wth a fluctuant cuttng speed s establshed, and the cuttng process of laser heat source movng along the corner trajectory s smulated. Then the temperature and thermal stress feld are analysed, and also the effects of changng cuttng parameters such as corner sze and cuttng speed are studed. 1) 2) Based on heat transfer, thermodynamcs theory and speed fluctuaton model, the modelng approach for laser cuttng corner wth a fluctuant speed s analyzed. The phenomenon of slower heat dffuson at the corner and corner burnt are been found n the 3D smulaton model. At the same tme, the nfluence of fluctuant speed on laser cuttng qualty s confrmed. The nfluence of laser cuttng parameters n the 3D smulaton model s carred out. It s found that the temperature changes more slowly at the corner and the phenomenon of corner burnng happens. Smaller thermal stress s obtaned when the corner sze decreases. And f the cuttng speed decreases, the same phenomenon can also been found. 7. Acknowledgements Ths work was supported by Research Project of State Key Laboratory of Mechancal System and Vbraton (MSV201204). 8. References [1] Farn, G.E., J. Hoschek, and M.-S. Km 2002 Handbook of computer aded geometrc desgn (North Holland) [2] Tsa, M.-S., H.-W. Nen, and H.-T. Yau 2008 Comput.-Aded Des [3] Sheng, P.S. and V.S. Josh 1995 Journal of materals processng technology [4] Prusa, J.M., G. Venktachalam, and P.A. Molan 1999 Internatonal Journal of Machne Tools and Manufacture [5] Shuja, S. and B. Ylbas 2000 Proceedngs of the Insttuton of Mechancal Engneers, Part C: Journal of Mechancal Engneerng Scence [6] J, Z. and S. Wu 1998 Journal of Materals Processng Technology [7] Ylbas, B., A. Arf, and B. Aleem 2009 Optcs and Lasers n Engneerng [8] Ylbas, B. and S. Akhtar 2012 Optcs and Lasers n Engneerng [9] Ylbas, B., S. Akhtar, and C. Karatas 2011 Journal of Materals Processng Technology [10] Ylbas, B., A. Arf, and B. Abdul Aleem 2010 Optcs and Lasers n Engneerng [11] Ylbas, B., A. Arf, and B.A. Aleem 2010 Journal of materals engneerng and performance [12] Nyon, K., et al The Internatonal Journal of Advanced Manufacturng Technology [13] Hu, J., et al The Internatonal Journal of Advanced Manufacturng Technology