Laser Welding of Thin Sheet Steel with Surface Oxidation

Transcription

1 Laser Welding f Thin Sheet Steel with Surface Oxidatin Surface xidatin enhances the absrptin f laser energy significantly and has little influence n the mechanical prperties f laser welds BY J. XIE AND A. KAR ABSTRACT. The jining f thin sheet steel generally invlves cnductin mde welding, in which the reflectin f the laser beam by the sheet surface is high. The absrptin f laser energy by the wrkpiece increases significantly during keyhle laser welding, in which a vaprplasma-filled cavity is frmed. The reflectivity f cld-rlled thin sheet steel was fund t be in the range f 65-80% in CO 2 laser welding. The reflectivity decreased t abut 30% when the sheet surface was xidized befre laser welding. In the laser welds with surface xidatin, the xygen inclusins and prsity were nt fund, but the grain size was large. Hwever, the tensile strength f all welds -- with r withut surface xidatin -- was higher than the base metal. The tughness f the welds with surface xidatin degraded, beca use f the small amunt f xygen cntent, but it was still cmparable t the tughness f the welds withut surface xidatin. The xygen cntent in the welds with surface xidatin was fund t be slightly higher than in the welds withut surface xidatin. The mechanical prperties f the welds with surface xidatin were fund t imprve when steel pwders cnsisting f manganese and silicn were used during welding. Intrductin Thin steel sheets are extensively used in industries prducing prducts such as washer bdies, medical and electrnic cmpnents. Laser welding is cnsidered t be a suitable manufacturing prcess fr thin sheet steel structures J. XIE is with Edisn Welding Institute, Clumbus, Ohi. A. KAR is with Center fr Research and Educatin in Optics and Lasers (CREOL) and Department f Mechanical, Materials and Aerspace Engineering, University f Central Flrida, Orland, Fla. because f several advantages: lw heat input, little distrtin, small heat-affected zne (HAZ), gd mechanical prperties and excellent repeatability f welds. It has been reprted that laser welds decreased sheet frmability by 10-18%, while mash-seam welds decreased frmability by 29-35% (Ref. 1). Fatigue life perfrmance f laser welds was increased by % ver resistance welds (Ref. 2). Therefre, in the future, it is expected that laser welding will be used mre widely t jin a variety f materials in industries. Mst thin sheet steels are cld-rlled lw-carbn r mild steels. The weldability f such materials is usually excellent and lasers can be utilized t achieve gd quality welds. The cnductin welding mde (rather than the typical keyhle welding mde) is likely t be en~ cuntered in the laser welding f thin sheet steel (Ref. 3), when the laser beam spt size is generally cmparable t the thickness f the sheet r laser intensity is insufficient t bil the materials. Cnductin welding has been reprted t be well suited t weld materials under apprximately 0.5 mm (Ref. 3). Hwever, a few prblems in laser welding f thin sheet steel, such as the high reflectin f KEY WORDS Laser Welding Cnductin Mde Surface Oxidatin Sheet Steel Cld Rlled CO 2 Laser Reflectivity Pwder Mechanical Prperty Oxygen laser beams by the sheet surface and strict requirement fr wrkpiece fitup, limit the applicatins f laser welding in industries. The effect f multiple reflectins f the laser beam in the keyhle, which increases the absrptin f laser energy by mre than 90% in keyhle welding, is nt bserved in cnductin mde welding. A small prtin f the laser energy is absrbed by the surface and the rest is reflected in laser cnductin welding. This prcess is knwn as Fresnel absrptin (Ref. 4). A number f investigatins n the reflectin f laser beams by metal surfaces was carried ut, but mst wrk was fr plished metals (Refs. 5-8). Theretical values f reflectivity f plished metals were usually estimated based n Drude thery. Sme theretical and experimental data fr bth CO 2 and Nd:YAG lasers listed in Tables 1 and 2 shw that althugh the reflectivity was quite high fr plished metals, it decreased with an increase in temperature. It was als fund that the reflectivity f AISI 4340 steels fr CO 2 lasers decreased frm 93.1 t 88.3% in the temperature range f C ( F) at an argn flw rate f 25 L/min (Ref. 9). Changes f reflectivity with CO 2 laser intensity fr 35NCD16 steels in slid, liquid, vapr and plasma states were studied (Ref. 10). The reflectivity f plished steels decreased cntinuusly frm 95% in slid state t 85% in liquid, 70% in vapr and 60% in plasma, at a laser intensity f 105 W/cm 2. When the laser intensity changed frm 105 W/cm 2 t 106 W/cm 2, the reflectivity f the plished steels declined frm 95 t 55% in slid state, frm 85 t 52% in liquid and frm 70 t 45% in vapr (Ref. 10). Incident angles f the laser beam have little influence n reflectivity if the angle is less than 20 deg (Ref. 4). Surface cnditins f metals influence the reflectin f laser beams signif- WELDING RESEARCH SUPPLEMENT I 343-s

2 Table 1 -- Experimental and Theretical Values f Reflectivity at 10.6 pm (Ref. 7) Metal Measured (%) Calculated (%) Fe (500 C) 94.7 (500 C) Ti Zr Cu AI 98.1 M Ta SS (0 C) (0 C) Ti-6AI-4V (400 C) 86.0 (400 C) Table 2 -- Experimental and Theretical Values f Reflectivity at 1.06 IJm Metal Measured (%) Calculated (%) (Ref. 7) (Ref. 5) Ti V Cr Mn Fe C Ni Cu (Liquid) AI (Liquid) Table 3 -- Chemical Cmpsitins f Filler Pwders (wt-%) (1400 C) (1400 C) (1400 C) (150 C) 1018 Steel Pure Irn Manganese Silicn Pwder Pwder icantly, and experiments indicated surface rughening and cating culd reduce the reflectin f laser beams (Refs ). The surfaces f the cldrlled sheets were smth and the reflectin f the laser beam during welding was very high, which resulted in the waste f laser energy, reductin f prductivity and safety prblems. It is imprtant t understand the reflectivity f cld-rlled steel and the influence f surface treatments n the reflectivity and laser welds; hwever, the reflectin by cld-rlled surfaces is still prly understd and the effect f surface xidatin n the reflectin f cld-rlled steel is seldm studied. The reflectin f a CO 2 laser beam by the cld-rlled sheet steel and the influence f surface xidatin n the reflectin were investigated in this study. As-received and xidized steel sheets were welded using a CO 2 laser beam and the mechanical prperties f the laser welds were analyzed. The effect f adding allyed steel pwders n the mechanical prperties f the laser welds was als studied. These prperties are reprted in this article. Experiments The reflectivity f cld-rlled sheet steel at the wavelength f CO 2 lasers (10.6 tjm) was measured by using an experimental setup as shwn in Fig. 1. The CO 2 laser beam irradiated n a wrkpiece at an incidence angle f 15 deg, and a pwer detectr was placed in the path f the reflected beam. The reflected laser pwer was measured using a pwer meter, and the reflectivity was determined by taking the rati f the reflected laser pwer t the incident laser pwer. A 400-W CO 2 laser was used fr measuring the reflectivity and fr welding lw-carbn sheet steel. The CO 2 laser beam was fcused with a ZnSe plancnvex lens with a fcal length f 3.5 in. (89 mm). Bth argn and helium were used as shielding gases, with a flw rate f 15 L/min. The laser pwer and travel speed in the measurement f reflectivity and welding were abut 360 W and 0.2 mm/s, respectively. When the filler pwder was used fr surface-xidized sheets, the air gap between tw sheets was abut 0.2 mm. The pwder was pre-placed in the gap between tw sheets. The cld-rlled sheets used in the experiment were AISI 1010 lw-carbn steel, with a thickness f 0.6 mm. The steel sheets were xidized by putting them int an air furnace at 0 C (1832 F) fr 20 and 40 s, respectively. Since xidized samples wuld have exhibited increased xygen cncentratins in the welds, filler pwders cnsisting f dexidizing elements such as manganese, silicn, titanium and zircnium were used fr laser welding f the xidized samples t imprve the mechanical prperties. In this experiment, manganese and silicn were added t the filler pwder because f the capability f these elements t reduce the xygen cntent in the weld metal. Tw types f pwder were designed fr laser welding f xidized steel samples, as shwn in Table 3. The mesh size f the steel pwder was in the range f 80-. The strength and ductility f the laser welds were evaluated by cnducting tensile tests. T btain the mechanical prperties f such small laser welds, tw ntches were created at bth thin sides f the welded samples (Fig. 2) t ensure the ccurrence f rupture in the weld zne. Since the ntched samples were nt f standard size fr tensile testing, the test results f the ntched samples were nt cmpared with thse f standard cupns. The tughness f the laser welds was estimated by calculating the area under the stress-strain curves btained in tensile test. The xygen cncentratins in the weld and base metals were measured by using the Auger Electrn Spectrscpic (AES) technique, fr which the plished surfaces f laser welds were sputtered fr 10 min t remve the surface cntaminants befre the measurement. Results and Discussin Reflectivity Reflectivity f plished metals is an ptical prperty f the metals and it is quite high fr mst metals (Ref. 4). Reflectivity decreases when the tempera- 344-S I OCTOBER 1999

3 ture f the wrkpiece is increased and the reflectivity at melting temperature is lwer than at rm temperature because f the higher resistivity at the melting temperature. Reflectivity generally decreases as resistivity increases (Refs. 4, 11, 14). The temperature-dependent reflectivity fr plished metals has been theretically btained in an earlier wrk (Ref. 14), as shwn in Table 4. It has been fund that the absrptivity f Nd:YAG lasers is abut 3.16 times higher than that f CO 2 lasers and the reflectivity f plished metals fr bth CO 2 and Nd:YAG lasers is quite high -- even at the melting temperatures. Therefre, mst f the laser energy is reflected away during laser cnductin welding. The surface cnditins f cld-rlled sheet metals are different frm plished metals. The cld-rlled surface f metals is nt as smth as the plished surface and the presence f il r water film n the surface can enhance the absrptin f laser energy. The reflectivity f cldrlled sheet metals is expected t be lwer than that f plished metals. The reflectivity f cld-rlled sheet metals was measured fr CO 2 lasers and the results are listed in Table 5. Since the reflectivity in slid and melting states is different, surface melting f varius metals under the CO 2 laser irradiatin f 360 W is shwn in Table 5. The surface melting is usually achieved by a fcused laser beam fr many metals, but it is als pssible fr the surface t be melted by an unfcused laser beam (raw beam) fr stainless steels and Incnel allys because f their lw thermal cnductivity. There was a small plasma abve the wrkpieces when surface melting ccurred under the irradiatin f the fcused CO 2 laser beam. Table 5 shws aluminum and cpper sheets have the highest reflectivities and the values fr mild steels are als quite high. A lt f laser energy is lst during laser cnductin welding f the cld-rlled metal sheets. The change in reflectivity with rughness fr cpper and aluminum is shwn in Fig. 3, while the metal surface was rughened by sandpapers with varius grits. Since the reflectivity is reduced by the irregular reflectins f laser beams n the rugh surface, the reflectivity decreases with increasing rughness. Experimental results fr the reflectivity f cld-rlled sheet steel with different surface treatments are shwn in Fig. 4. The as-received sheet steel has the highest reflectivity in the range f 65-80%. Fr the surface-xidized samples, the reflectivity decreases significantly -- as lw as 35% depending n the surface xidatin time -- because f the high electrical DC (direct current) re- sistivity f the surface xide films. By placing steel pwders n the substrate surface, the reflectivity is fund t be in the range f 20-35%, because f the irregular reflectins f the laser beam by the steel pwders. In the laser cnductin welding f thin sheet steel, strng reflectin f the CO 2 laser beam results in the waste f laser energy and safety prblems. The xidized samples reduce the reflectin f the laser energy; hwever, it is necessary t understand the influence f the surface xides n the mechanical prperties f the laser welds. As-received, surface-xidized and pwder preplaced samples were welded by using CO 2 lasers. The mechanical prperties f these welds are cmpared in the fllwing sectin. Laser Weld Laser pwer meter Fig An experimental setup fr measuring reflectivity. 12 2mm 90deg 120 mm Fig A ntched specimen fr tensile tests. Laser welds with surface xi- 30 datin were crss sectined t analyze the defects in the welds. N xide inclusins and prsity were fund in the weld metal, but the grain size was quite large, as shwn in Fig. 5. It seems that the xide film decmpses during laser irradiatin and a small amunt f xygen is disslved in the weld metal. The irn xide decmpses int irn and xygen, with mst f the xygen being carried away by the shielding gas. Only a small amunt f the xygen is retained in the weld pl because f the lw slubility f xygen in liquid irn (Ref. 15). The small amunt f xygen presented in the weld metal is prbably due t the lw 9O O I O Cu, statinary Cu, mving A AI, statinary AI, mving " g Cld-rlled sheets Ar shielding gas CO 2 laser pwer: 360 W ij As-received Rughness, Grit! r velcity, v Fig Effect f surface rughness n reflectivity fr cpper and aluminum. welding speed used in the welding experiments f this study. The lw welding speed prvides sufficient time fr the xide film t decmpse cmpletely, which may be the reasn fr the absence f xide inclusins and prsity in the welds. The relative xygen atmic cncentratins in the base metal and welds that were determined by Auger Electrn Spectrscpy (AES) are shwn in Fig. 6. There is little xygen cntent in the base metal and the value f the xygen cn- WELDING RESEARCH SUPPLEMENT I 345-s

4 - Reflectivity - Oxygen - A 80 v=40 ram/rain., Argn v=150 mm/min., Argn O v=40 ram/rain., Helium O v=150 mm/min., Helium Pwder, Argn secj0 C Surface xidatin, Cld-rlled steel: sec./l~ C ~ Sheet thickness: 0.6 mm Surface xidatin CO 2 laser pwer: 360 W Pwder i - i.... i.... i.... i... Types f Samples Fig. 5 - laser weld made with surface xidatin. Fig. 4 - f cld-rlled sheet steel with varius surface cnditins i.... i.... i.... i.... I 84 Weld with surface xidatin O.dk-'" Weld with Oxidatin /t, and Filler Pwder :~ Weld withut Oxidatin,,'i I : / Weld with Oxidatin 6 54 :t 4 f~ 400 //? y N.q Base Metal 0 Types f Samples Strain Fig. 6 - cntent in laser welds. Fig Typical stress-strain curves btained in tensile tests i.... i.... i.... i.... l I 7OO Base ~ I, 5OO 200 i Base Metal [ Welds withut surface xidatin I Welds with surface xidatin [ Welds with xidatin and pwder 1 Welds with xidatin and pwder 2 i 4OO 3OO 2OO i - i i i i i.... i.... i.... Types f Samples 0 Types f Samples Fig Ultimate tensile strength f laser welds and base metal. Fig Tughness f laser welds and base metal. 346-S I OCTOBER 1999

5 Table 4 -- Reflectivity f Plished Metals at the Rm and Melting Temperatures (%) (Ref. 14) Rm Temperature CO 2 COIL (a) Nd:YAG Wavelength (IJm) Aluminum Cpper Irn Nickel Titanium Carbn Steel Stainless Steel (a)chemical Oxygen-Idine Laser. (b)values at the temperature just belw melting pint. Melting Temperature (Liquid Phase) CO 2 COIL (a) Nd:YAG (b) 61.1(b) 56.7 (b) (b) 60.3(b) 55.8(b) centratin in Fig. 6 is backgrund nise frm the AES instrument. The xygen cncentratin is slightly higher in the weld with surface xidatin than withut xidatin. The additin f steel pwder t the weld zne during welding decreases the xygen cncentratin in the weld because f the presence f dexidizing elements Mn and Si in the steel pwder. Typical stress-strain curves btained in tensile tests are shwn in Fig. 7. The base metal has the highest elngatin f all the welds (Fig. 7), but its ultimate strength is lwer than all the welds-- Fig. 8. The tensile strength f the welds with surface xidatin is slightly higher than thse withut xidatin, pssibly because the disslved xygen atms in the welds act as slutes in the metal matrix, defrming the metal lattices, and the defrmed lattices enhance the strength f the weld metals. This hardening mechanism is similar t the slid-slutin hardening. When steel pwders are added during welding, the xygen cntent in the welds decreases because f chemical reactins between the dexidizing elements and irn xide; therefre, the hardening effect f the xygen atms becmes insignificant. As a result, the strength f the samples welded by pwder 1 (with less Mn and Si than pwder 2) is clse t the strength f the samples withut surface xidatin and is slightly lwer than the samples with surface xidatin. When pwder 2 (with mre dexidizing elements than pwder 1) is used, the strength f the weld is enhanced by the allying elements, Mn and Si, instead f the xygen atms. The tughness f the welds is apprximated by calculating the area under the stress-strain curves btained in tensile tests and is presented in Fig. 9, which shws that the base metal has the highest tughness. It is ntable that the tughness f the weld with surface xidatin is nly slightly lwer than the weld withut xidatin. Table 5 -- Reflectivity f As-Received Cld-Rlled Sheet Metals fr CO 2 Lasers Materials Reflectivity (%) Surface Melting Fcused Beam Aluminum Yes Yes Cpper N Yes Mild Steel Yes Yes Stainless Steel (304) Titanium Yes Yes N Yes Incnel N N I ncnel Yes Yes The tughness f the welds imprves with the additin f allying pwder. In general, surface xidatin decreases the reflectin f laser beams and intrduces a small amunt f xygen atms int the welds. The tensile strength f the weld with surface xidatin is gd and the tughness is acceptable. These results indicate that surface xidatin is a pssible means f reducing the reflectin f laser beams withut sacrificing the mechanical prperties f the welds. Hwever, xidizing the surface f the wrkpiece befre welding is difficult fr certain industrial applicatins. Surface xidatin culd be achieved by adding reactive gases in the shielding gas. It was reprted that mixed shielding gas, with an added 10% xygen in argn, culd increase the weld depth significantly (Ref. 16), and the CO 2 gas culd be used as a shielding gas (Ref. 17). In additin, the results btained frm the study imply that little r n shielding gases may be used fr welding lw-carbn steel because acceptable laser welds can be prduced in an active shielding envirnment. If these results are verified by further studies, substantial manufacturing csts can be saved in prductin. Cnclusins 1) The reflectivity f as-received cld-rlled sheet steel is in the range f 65-80% in CO 2 laser cnductin welding. Surface xidatin culd reduce the reflectivity t apprximately 30%. 2) In laser welds with surface xidatin, xygen inclusins and prsity are nt present, but the grain size is large. The xygen cncentratin in the welds with surface xidatin is slightly higher than in the welds withut surface xidatin. 3) The tensile strength f the welds is higher than that f the base metal. The tughness f the welds with surface xidatin is slightly degraded, because f the slight presence f xygen in the weld; hwever, it is still cmparable t the tughness f welds withut surface xidatin. 4) The additin f allyed steel pwders cntaining silicn and manganese can imprve the mechanical prperties and reduce the xygen cntent in the welds. Acknwledgment The authrs gratefully acknwledge the financial supprt prvided by the Center f Research and Educatin in Optics and Lasers (CREOL) and the Department f Mechanical, Materials and Aerspace Engineering at the University f Central Flrida. References 1. Hallum, D What's new in welding f sheet-metal assemblies? Welding Design & Fabricatin 66(4): WELDING RESEARCH SUPPLEMENT[ 347-S

6 2. Irving, B Autmtive engineers plunge int tmrrw's jining prblems. Welding Jurna / 73 ( 12 ): Naker, P. M Lasers penetrate fabricating. Manufacturing Engineering (10): Steen, W. M Laser Materials Prcessing. New Yrk N.Y., Springer-Verlag, pp Jhnsn, P. B., and Christy, R. W Optical cnstant f cpper and nickel as a functin f temperature. Physical Review B: Slid State (11 ): Waiters, C. T., Tucker T. R., Ream, S. L., Clauer, A. H., and Gallant, D. J Thermal cupling f CO 2 laser radiatin t metals. Ed. by K. Mukherjee and J. Mazumder in Laser in Metallurgy, Presented at Prceedings f the 110th AIME Annual Meeting, Chicag, Ill. 7. Duley, W. W Laser materials interactins f relevance t metal surface treatment, edited by C. W. Draper and P. Mazzldi, Presented at Prceedings f NATO Advanced Study Institute n Laser Surface Treatment f Metals, Italy. 8. Stern, G Absrptivity f CW CO 2, CO and YAG-laser beams by different metallic allys, Presented at Pr- ceedings f the 3rd Eurpean Cnference n Laser Treatment f Materials (ECLAT '90), Germany. 9. Fung, C., Peng, K., and Dng J Study f surface temperature n laser cutting and welding pwer absrptin. Internatinal Cmmunicatin in Heat and Mass Transfer 17: Fabbr, R., Bermej, D., Orza, J., Sabatier, L., Leprince, L., and Granier, V CO 2 Laser and Applicatin II, Prc. ECO3, SPIE Vl. 1276, Ed. by H. Opwer. The Internatinal Sciety fr Optical Engineering, the Netherlands. 11. Arata, Y., and Miyamat, I Sme fundamental prperties f high pwer laser beam as a heat surce (reprt 2). Transactin f Japan Welding Sciety 3: Hpkins, J. A., Semak, V. V., and McCay, M. H Calrimetric measurements f the influence f surface preparatin n the absrptin f 10.6 IJm radiatin. Ed. by T. O. McCay, A. Matsunawa and H. Hugel, Presented at Prceedings f the Internatinal Cnference n Lasers and Electr- Optics (ICALEO '94), Laser Institute f America, Orland, Fla. 13. Patel, R. S., and Brewster, M. Q Effect f xidatin and plum frmatin n lw pwer Nd:YAG laser metal interactin. Jurnal f Heat Transfer, Transactin f ASME 112(1 ): Xie, J., and Kar, A Temperaturedependent absrptivity and cutting capability f CO2, Nd:YAG and chemical xygen-idine lasers. Jurnal f Laser Applicatins 9: Ku, S Welding Metallurgy, p. 66, Jhn Wiley & Sns, New Yrk, N.Y. 16. Jrgensen, M Increasing energy absrptin in laser welding. Metal Cnstructin 12(2): Abbtt, D. H., and AIbright C. E CO 2 shielding gas effects in laser welding mild steel. Jurnal f Laser Applicatins 6: Preparatin f Manuscripts fr Submissin t the Welding Jurnal Research Supplement All authrs shuld address themselves t the fllwing questins when writing papers fr submissin t the Welding Research Supplement: Why was the wrk dne? What was dne? What was fund? What is the significance f yur results? What are yur mst imprtant cnclusins? With thse questins in mind, mst authrs can lgically rganize their material alng the fllwing lines, using suitable headings and subheadings t divide the paper. 1) Abstract. A cncise summary f the majr elements f the presentatin, nt exceeding 200 wrds, t help the reader decide if the infrmatin is fr him r her. 2) Intrductin. A shrt statement giving relevant backgrund, purpse and scpe t help rient the reader. D nt duplicate the abstract. 3) Experimental Prcedure, Materials, Equipment. 4) Results, Discussin. The facts r data btained and their evaluatin. 5) Cnclusin. An evaluatin and interpretatin f yur results. Mst ften, this is what the readers remember. 6) Acknwledgment, References and Appendix. Keep in mind that prper use f terms, abbreviatins and symbls are imprtant cnsideratins in prcessing a manuscript fr publicatin. Fr welding terminlgy, the Welding Jurnal adheres t ANSI/AWS A3.0-94, Standard Welding Terms and Definitins. Papers submitted fr cnsideratin in the Welding Research Supplement are required t underg Peer Review befre acceptance fr publicatin. Submit an riginal and ne cpy (duble-spaced, with 1-in. margins n 8 ~ x 11 -in. r A4 paper) f the manuscript. Submit the abstract nly n a cmputer disk. The preferred frmat is frm any Macintsh wrd prcessr n a 3.5-in. duble- r high-density disk. Other acceptable frmats include ASCII text, Windws TM r DOS. A manuscript submissin frm shuld accmpany the manuscript. Tables and figures shuld be separate frm the manuscript cpy, and nly high-quality figures will be published. Figures shuld be riginal line art r glssy phts. Special instructins are required if figures are submitted by electrnic means. T receive cmplete instructins and the manuscript submissin frm, please cntact the Peer Review Crdinatr, Dreen Kubish, at (305) ext. 275; FAX ; r write t the American Welding Sciety, 550 NW LeJeune Rd., Miami, FL s [ OCTOBER 1999