Eigenschaften von Laser behandelten AISI-M2 Schneidwerkzeugen zum Schälen von Buchenholz

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  For many wood machining processes, the interest of tool steels remains very important because of their good tool edge accuracy and easy grinding. The main problem is their low resistance to wearing and corrosion. In order to increase their
  Improvement in wear characteristics of the AISI M2 by laser claddingand melting Wayan Darmawan a   Department of Forest Products, Faculty of Forestry, Bogor Agricultural University, Kampus IPB Darmaga, Bogor 16680, Indonesia Jean Quesada, Frédérique Rossi, and Rémy Marchal  LABOMAP-ENSAM, rue Porte de Paris, 71250 Cluny, France Frédérique Machi  IREPA LASER, Pôle API, 67400 Illkirch, France Hiroshi Usuki  Department of Natural Resources, Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan  Received 14 October 2008; accepted for publication 14 August 2009; published 5 March 2010  The interest of high speed steel   HSS   for wood cutting tools remains very important because of their good tool edge accuracy and easy grinding. The main problem is their low resistance to bothmechanical and chemical wearing. Resistance of HSS cutting tools to wearing is a primary concernin the applicability of the HSS cutting tools to a wood cutting operation. In order to increase theirperformance, a laser melting and cladding applied on the tool edges is presented in this paper. First,the annealed AISI M2 bar was melted, and the M2 powder was cladded onto the AISI L2 substrateby a laser beam. The microstructure and microhardness of the M2-clad and M2-melted werecharacterized. Second, their wear resistance was tested for cutting wood. The experimental resultsshowed that the microstructures on the clad zone   CZ   of M2-clad and melted zone   MZ   of theM2-melted reveal fine and homogeneous iron dendritic structure, in which whole primary carbideswere completely dissolved during laser cladding and melting. The energy dispersive spectroscopy  EDS   analysis indicated that the M2-clad reveals CZ microstructure with more uniform distributionof fine carbides compared to MZ microstructure. The M2-clad and M2-melted, which present almostthe same microhardness, have larger microhardness compared to the M2-conventional. The resultsof wearing tests showed that the M2-clad and M2-melted cutting tools are better in wear resistance,edge roughness, and suffer less edge fractures than the M2-conventional peeling tool. Laser meltingand cladding are considered to be valuable techniques to improve the performance of the M2 highspeed steel cutting tools for wood machining application. ©  2010 Laser Institute of America. Key words: M2 high speed tool steel, laser melting, laser cladding, microhardness, microstructure,wear resistance, edge roughness I. INTRODUCTION Actually high speed steel   HSS   is still manufactured forcutting tools in the woodworking industry. HSS is a commoncutting tool material for planer blades, molder and shaperknives, router bits, and veneer peeling knife. The interest of HSS for wood cutting tools remains very important becauseof their good tool edge accuracy and easy grinding. The mainproblem is their low resistance to both mechanical andchemical wearing. Resistance of HSS cutting tools to wear-ing is a primary concern in the applicability of the HSScutting tools to a wood cutting operation. Therefore, thechemical stability of the HSS cutting tools, as well as itshardness and toughness, must be considered. There are manydifferent grades of the HSS cutting tool. However, a researchresult revealed that AISI-M2 HSS cutting tool provide betterperformance than the other grades for machining woods dueto its good toughness and wear resistance. 1 In many instances, the production of high-quality HSScutting tools begins with the use of powder metals. Cur-rently, powder metal processing requires the powder to beformed by gas atomization. This process allows the HSSpowder particle to cool rapidly, forming a fine grain size anda higher carbide formation, which contribute to the improvedhardness and wear characteristics of powder metal highspeed steel. However, the additional processing required toform powder metal into bar stock for standard HSS cuttingtools production generates significant costs, which necessi-tates a selling price of $20 to $30 per pound of bar stock.This cost factor has severely limited its acceptance for stan-dard cutting tools production despite its significant attributes.In addition to this fact, improvement in wear characteristicsof the HSS cutting tools by surface coating of hard materials a  Tel.:  62-251-8621285; electronic mail: wayandar@indo.net.idJOURNAL OF LASER APPLICATIONS VOLUME 21, NUMBER 4 NOVEMBER 2009 1042-346X/2009/21  4   /176/7/$25.00 © 2010 Laser Institute of America176  for wood machining application was experimented. 2,3 Though the surface coating provides benefits for the HSS insome wood machining applications, however the surfacecoating will generate high cost for large dimension  2000 mm  300 mm  16 mm   of wood veneer peelingknife.Developments made in laser technology over the past 20years have enabled laser processing an established activity inindustry. In the manufacturing sector, laser processes aregaining widespread acceptance for repair, refurbishment, andrapid prototyping. 4 Mainly the laser applications directed toimprove the surface properties of materials involve surfacealloying and melting. 5 Laser surface melting is a very prom-ising technique to improve the hardness and microstructureof tool steels with retaining the good toughness of the tool-bulk. It was reported that a fine microstructure is producedafter a sinteredAISI M42 was melted using a laser beam, andits microhardness increases up to 1100HV compared to itsconventional microhardness of 720HV. 6,7 The increase in thewear resistance of a melted AISI M2 for cutting a steel ma-terial was reported due to improvement in hardness, in cor-rosive, erosive and fatigue resistance, and in coefficient of friction. 8 The life of laser-melted AISI M35 bit for cutting asteel material was to be 20%–125% higher than if the bit wasconventionally hardened. 9 The wear resistance and wear pat-tern of the T1-melted cutting tool in peeling Beech was bet-ter than those of the T1-conventional peeling tool. 10 It couldbe summarized that the changes in microstructure and theincreases in hardness due to laser melted as described above,which lead to the increase in the wear and corrosion resis-tances of the laser-melted steels, were attributed to the for-mation of a very fine microstructure, small proportion of retained austenite, and a higher precipitation of fine carbideswithin martensite.Moreover, surface cladding is now being a viable alter-native to improve the quality of the surface properties. Thiscladding process typically involves the use of powder that isefficiently melted by a laser beam and precisely depositedonto the substrate material to form a well-bound clad layer.The microstructure of the clad depends on the alloy that ischosen to form it. Clad layer samples produced using a diodelaser have an entirely fine dendritic microstructure. 11 In an-other report microstructure of AISI M2-clad is formed by acellular-dendritic zone composed of fine dendrites trans-formed to martensite and carbide, and AISI 431-clad pre-sented a microstructure formed mainly by martensite withretained austenite. 12 The laser AISI M2-clad showed greaterwear resistance than the AISI 431-clad due to its dense net-work of carbides, which effectively support the load appliedduring the test. In the AISI M2-clad, plastic deformation wasthe dominant wear mechanism, and oxidation the secondarymechanism. Otherwise the mechanism for the samples clad-ded with AISI 431 was predominantly plastic deformationwith a high contribution of abrasion. The friction coefficientof both layers remained approximately constant during thetest, not experiencing significant changes with the load. Itwas also detailed that characteristics of laser cladding pro-cesses include low thermal distortion, minimal metallurgicaldegradation to the substrate, relatively high deposition rates,and rapid solidification rates associated with the deposit. 13 These combinations of attributes make the laser claddingprocess an ideal candidate for a wide variety of applications.However, the ability to obtain the demanding properties re-quired for wood cutting tool purposes has provided a formi-dable challenge.Realizing the potential of laser processing technology, avaluable research was embarked to identify possible applica-tions, evaluate, and characterize laser melting and claddingfor an existing wood cutting tool material. Initial researchwas conducted in conjunction with the IREPA Laser, Stras-bourg, France. AISI M2, which is widely used for woodmachining purposes in form of drills, cutters, and end mills,was a tool material selected for this experiment. An annealedM2 bar was melted and M2 powders were cladded onto thesurface of low carbon steel substrate   AISI L2   by using aDiode Laser Beam. A combination of parameters   laserpower, scanning speed, diameter of laser spot, and powderfeed rate  , which is considered to be important in affectingthe performance of the laser treatment, was fixed after somepreliminary tests. The purpose of this work was to character-ize the microstructure and microhardness of the M2-meltedand M2-clad in comparison with the M2-conventional, andto determine their wear resistance. II. MATERIALS AND METHODS AISI M2 high speed tool steel for the investigation,which was produced by hot isostatic pressing, was in an-nealed condition in form of bar   Table I  . M2 samples forlaser melting were prepared in the size of 80 mm by 40 mmby 10 mm from the annealed bar. Prior to laser melting, allM2 samples were cleaned. The laser melting was carried outat LABOMAP-ENSAM Cluny using a continuous wave of CO 2  laser with generated beam power up to 4000 W. Thesurfaces of the M2 samples were scanned by the laser beamat laser power of 2800 W, at scanning speed of 600 mm/min TABLE I. Specifications of AISI M2 high speed steel for investigation.Steel materials for investigationChemical Composition  wt %  Dimension or sizeC Si Mn P Cr W Mo V FeAnnealed M2 bar 0.88 0.25 0.30 0.02 4.04 6.13 4.92 1.85 Bal 500  40  10 mmM2 powder 0.85 0.20 0.30 0.02 4.35 6.30 5.00 1.90 Bal 53–150    mL2 steel bar 0.45 0.35 0.15 0.80 0.20 0.15 Bal 500  40  10 mm 177J. Laser Appl., Vol. 21, No. 4, November 2009 Darmawan  et al.   Table II  . The laser beam, which produces the energy den-sity redistribution, was positioned above the surface of theM2 samples and created a laser spot of 4 mm in diameterover the surface   Fig. 1  a  . Laser scanning on the surface at4 mm from an edge of the M2 samples was done in paralleldirection to the width of the sample, under helium atmo-sphere, in order to protect against oxidation.End groove in the depth of 2 mm and width of 20 mmwas prepared in the L2 substrates. M2 powders, whose com-position is indicated in Table I, were used to build multiple clad layers on the groove of the substrate. Numerical con-trolled coaxial laser cladding system was applied in this ex-periment   Fig. 2  a  . The laser used in this study was a con-tinuous wave diode type with an output power up to 3 kW.Its configuration consisted of two diode stacks with wave-lengths of 808 and 940 nm. The laser beam was focused 15mm above the substrate resulting in a laser spot diameter of 3.2 mm. The powder was injected into the nozzle by meansof an argon gas stream, which transported it from a feedingunit. An additional stream of the same gas was flowingthrough the nozzle in order to avoid surface oxidation. Inorder to create a clad layer of 2 mm in height, the powderfeed flow was adjusted to be 23 g/min, and the laser beamwas set up at speed of 600 mm/min and power of 2800 W.Overlapped tracks were made by successive scans until thegroove covered by the M2-clad layer.An M2-clad and M2-melted samples were sectionedtransverse to the laser scanning direction for microstructureand microhardness investigation. The sections of both M2-clad and M2-melted were triple tempered one hour each at560 °C in a furnace. The tempered sections were examinedunder optical video microscope for investigation of the pos-sible defects   crack, pores  , and were analyzed under VickersHardness Tester for determination of their microhardness.Then scanning electron microscopy   SEM  —energy disper-sive spectroscopy   EDS   were made to characterize their mi-crostructures.M2 cutting tools with 20° sharpness angle and 0° clear-ance angle were prepared from a M2-melted sample, M2-clad sample, and M2-conventionally hardened bar   annealedat 900 °C, austenitized at 1220 °C, triple tempered 1 h eachat 560 °C   by cutting and grinding technique. Cutting edgeof the M2-melted and M2-clad cutting tools was made in themelted and the clad surfaces respectively as shown in Figs.1  b   and 2  b  . Both the M2-melted and M2-clad cutting toolswere triple tempered one hour each at 560 °C. The wearresistance tests for the M2 cutting tools were performed in apeeling microlathe. At specified cutting lengths of 1 and 2km, the average amount of edge recession of the tested toolswas measured under an optical video microscope   Fig. 3  . Atcutting lengths of 0 km   before cutting test  , 1 km, and 2 km,average edge roughness along the worn cutting edges  Fig. 3   were also investigated under an optical laser profilo-meter. III. RESULTS AND DISCUSSIONA. M2-clad and M2-melted structure The structure of the M2-clad and M2-melted on thecross sections is presented in Fig. 4. It appears from the TABLE II. Laser melting and cladding conditions.TreatmentCondition forMelting CladdingLaser power   watt   2800 2800Scanning speed   mm/min   200 600Laser spot diameter   mm   4 3.2Powder feeding rate   g/min   23Width of clad overlap   mm   2.2Tempering after treatment   °C  Triple temperedat 560Triple temperedat 560FIG. 1. Schematic diagram of the laser melting on the surfaces of the AISIM2.FIG. 2. Schematic diagram of laser cladding system.FIG. 3. Schematic diagram for the edge recession and edge roughness mea-surement. 178 J. Laser Appl., Vol. 21, No. 4, November 2009 Darmawan  et al.  results in Fig. 4 that different zones were observed both inthe cross section of the M2-clad and M2-melted. The zonesare cladded zone   CZ  , melted zone   MZ  , transition zone  TZ  , heat affected zone   HAZ  , and substrate   sub  . Theoptical microscopic analysis shows that L2 substratesuffered less dilution, as indicated by flat surface of the L2substrate under the M2-clad layer   Fig. 4  a  . It was alsofound that the transition zone in the M2-clad   TZc   exhibitsgood metallurgical bonding between the L2 substrate and theM2 coating. However, pores were revealed in the CZ. Thissuggests that the cladding conditions should be improved.Fortunately, fine structure without any defects is revealed inthe MZ. The transition zone in the M2-melted   TZm   was apartially melted zone and is considered to correspond withliquidus and solidus interval of the solidificationtemperature. B. Microstructures of the laser treated M2 SEM microstructures of the M2-clad and M2-melted atdifferent zones are shown in Fig. 5. The microstructure on the CZ of the M2-clad, in which powder metals werecompletely melted to form a clad layer on the surface of theL2 substrate, reveals fine iron dendritic structure   Fig. 5a1  .The same fine iron dendritic structure was also observed onthe MZ of the M2-melted, in which whole primary carbideswere completely dissolved during laser melting   Fig. 5b1  .Fine carbide networks   interdendritic zone   formed due todissolution of the primary carbide grains during lasercladding and melting were also observed in the CZ and MZmicrostructures. Investigation of the CZ and MZmicrostructures under SEM-EDS revealed that the carbidenetworks present lamellar eutectic structure composed of Fe 3 W 3 C carbides. Though the SEM results of the CZshowed no significant difference in microstructureappearance compared to the MZ; however, the EDS analysisin Fig. 6 shows that the profile of elemental distribution of the carbides for the CZ was different from that of the MZ.More uniform peak levels of carbides profile investigated inthe tempered CZ   Fig. 6  a   than that in the MZ   Fig. 6  b  give an indication that the carbides   W, Mo, V, and Cr  distribute more evenly in the structure of M2-clad. Thiscould result from even mixture of elementals powder easilyprepared before injection. This even mixture would causethe carbides to be available to diffuse into any iron lattice inthe dendrites.The result in Fig. 5a2 reveals the same microstructure asCZ in upper part of the TZc and same microstructure as L2substrate in under part of the TZc. The narrow zone of themiddle part of the TZc indicated that the injected powderswere completely melted and properly deposited onto thesurfaces of the L2 substrate. Different from the TZc, irondendritic crystals, which are surrounded by carbides, wereobserved in the TZm   Fig. 5b2  . In this transition zone,partially melted primary carbides were visible. They arevisible as bright precipitates of typical fish bone morphologyand their sizes were observed around 3–5    m. Under theSEM-EDS analysis the type of carbide in Fig. 5b2 wasidentified as M 6 C. The M 6 C carbide is predominantly reachin tungsten   W  , molybdenum   Mo  , vanadium   V  , andchromium   Cr  . Remarkable differences in microstructurewere observed between the MZ and the M2 substrate.Metallurgical structure obtained by conventional heattreatment in the M2 substrate is a ferritic polycrystallinewith coarse primary carbides   Fig. 5b3  . These carbidegrains are varied in size and shape, and are uneven indistribution. C. Microhardness of the laser treated M2 M2-clad and M2-melted sections were triple temperedin order to obtain higher hardness by secondaryprecipitation. Measurement of the microhardness was madein the cross sections starting from the surface of the CZ orMZ going down to the substrate, and the results arepresented in Fig. 7. The distribution of microhardness for untreated M2   conventional hardened M2  , which was madealong its thickness, is also put in the figure for comparison.The M2-conventional shows uniform microhardness alongits thickness and its average microhardness was measured tobe 815 HV 0.5 . The average microhardness of the M2-clad inthe CZ and M2-melted in the MZ was found to be the same FIG. 4. Structure of the M2-clad layer   a   and M2-melted surface   b  . Note:clad zone   CZ  , melted zone   MZ  , transition zone on M2-clad   TZc  , tran-sition zone on M2-melted   TZm  , heat affected zone   HAZ  , substrate   sub  . a1a2b3b1 DendriteInterdendritic CZMZ   X3,000 5 μ m b2 Partially melted carbide TZmTZc X3,000 5 μ m a3L2 Substrate M2 Substrate Primary carbide EDS captureEDS capture DendriteInterdendritic FIG. 5. SEM microstructures of the M2-clad   a   and M2-melted   b   for theMZ   a1,b1  , TZ   a2,b2  , and substrate   a3,b3  . 179J. Laser Appl., Vol. 21, No. 4, November 2009 Darmawan  et al.  of about 840 HV 0.5 . The increase of microhardness for theM2-clad and M2-melted can be explained as a result of theformation of fine resolidified microstructure with densedistribution of very fine interdendritic carbides, andprecipitation of thin carbides into the iron lattice. It alsoappears from the results in Fig. 7 that the microhardness of the M2-clad and M2-melted is decreased starting from thetransition zone toward the substrate. The same phenomenonwas also found during melting the AISI T1 high speedsteel. 10 D. Wear resistance of the laser treated M2 cuttingtools The results of wear tests for the cutting tools tested inpeeling the Beech wood are presented in Fig. 8. The resultsin Fig. 8 indicate that the amount of cutting edge recessionincreases with increasing in cutting length. The M2-clad andM2-melted retain smaller amount of cutting edge recessionthan the M2-conventional in peeling the Beach wood. TheM2-conventional cutting tool suffers edge recession of about62    m at the 2 km cutting length, otherwise, the edgerecession are about 50 and 48    m at the 2 km cutting lengthfor the M2-melted and M2-clad, respectively. The higher inhardness and better in microstructures of the M2-clad andM2-melted compared to the M2-conventional would be thereason for this phenomenon. It also appears from the resultsin Fig. 8 that the roughness of cutting edge   Ra   of the toolsincreases with increasing in the cutting length. Beforecutting   0 km cutting length  , M2-conventional presentsslightly rougher cutting edge then the M2-clad and M2-melted. This could be caused by the presence of carbidegrains in the edge of M2-conventional, which were notcompletely cut by the grinding machine. The difference inedge roughness between the M2-conventional and the M2laser treated   M2-clad and M2-melted   became larger as thecutting length was increased. At the 2 km cutting length, theedge roughness of the M2-conventional is twice larger thanthat of the M2 laser treated. Meanwhile, M2-clad andM2-melted retain almost the same edge roughness until the 2km cutting length.The results of wear patters in Fig. 9 shows that theM2-clad and M2-melted cutting tool suffer less fracture of cutting edges compared to the M2-conventional cutting tool.The worn edges of the M2-clad and M2-melted revealsmooth surfaces, indicating the action of abrasion withoutany fracture failures   Figs. 9  b   and 9  c  . Different wearmechanism was observed in the M2-conventional peeling FIG. 6. Profiles of the elemental distribution on the CZ of the M2-clad   a   and the MZ of the M2-melted.FIG. 7. Microhardness characteristics of the M2-clad and M2-melted atdifferent zones 01020304050607080 Cuttingtool       E      d     g     e     r     e     c     e     s     s      i     o     n      /     r     o     u     g      h     n     e     s     s      (            m      )  Averageedgerecession(µm) Averageedgeroughness(µm) M2-conv M2-melted M2-clad 0 1 2 0 1 2 0 1 2 km FIG. 8. Wear and roughness   Ra   of the cutting edge of the tools in Beechwood peeling. 180 J. Laser Appl., Vol. 21, No. 4, November 2009 Darmawan  et al.
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