Ž .Applied Surface Science 140 1999 19–23

Microstructure and properties of laser-clad high-temperaturewear-resistant alloys 1

Yongqiang Yang )

South China UniÕersity of Technology, Department of Mechatronic Engineering, Guangzhou 510641, China

Received 17 December 1997; accepted 26 May 1998

Abstract

A 2-kW CO laser with a powder feeder was used to produce alloy coatings with high temperature-wear resistance on the2

surface of steel substrates. To analyze the microstructure and microchemical composition of the laser-clad layers, a scanningŽ .electron microscope SEM equipped with an energy dispersive X-ray microanalysis system was employed. X-ray diffraction

techniques were applied to characterize the phases formed during the cladding process. The results show that theŽ Ž . .microstructure of the cladding alloy consists mainly of many dispersed particles W C, W,Ti C , WC , a lamellar2 1yx

Ž .eutectic carbide M C, and an f.c.c matrix. Hardness tested at room and high temperature showed that the laser-clad zone12

has a moderate room temperature hardness and relatively higher elevated temperature hardness. The application of thelaser-clad layer to a hot tool was very successful, and its operational life span was prolonged 1 to 4 times. q 1999 Publishedby Elsevier Science B.V. All rights reserved.

Keywords: High temperature wear-resistant alloy; Laser cladding; Microstructure; Elevated temperature hardness; Hot tool

1. Introduction

Laser cladding is one of many laser surface modi-fication techniques. In the laser cladding procedure,not only the clad powder but also the substrate ismelted by the laser beam, so a new surface coatingwhose composition differs from that of either thepowder or the substrate is produced. Although there

w xhave been some reports 1–3 of applying lasercladding techniques to industrial components, theapplication of laser-clad high temperature wear-re-

) Tel.rFax: q86 20 87114484; E-mail: meyqyang@scut.edu.cn1 Sponsored by the National Science Foundation of Guandong

province, China.

sistant alloys to tools operated at elevated tempera-tures has not been common.

In this paper, a coating produced by laser claddingis used on the surface of hot tools. Our uniquetechnological feature is the use of a powder feedingmethod to achieve a coating of high temperaturealloys. The microstructure, metal phases and micro-composition have been investigated and the hardnessat room and elevated temperatures tested and ana-lyzed.

2. Experimental methods

The alloy powders used in laser cladding includea mixture of 70% of a Ni-based high temperature

0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00320-1

( )Y. YangrApplied Surface Science 140 1999 19–2320

Table 1Composition of the powders tested by EDAX

Materials Wt.%

W Ni Fe Co Cr Ti Mo Al

Ni-based alloy 16.42 48.14 4.54 8.51 5.09 2.68 4.81 9.81WC 100

Carbon cannot be tested by EDAX, the amount of C in the complete powder, by chemical analysis, is 1.62 wt.%.

alloy and 30% WC particles. The Ni-based alloyparticles are globe-shaped and the WC particles arelump-shaped. The two metal powders were mixed bymechanically. The particulate size is y200q300mesh. The composition is shown in Table 1. The

Ž .laser cladding device consists of two parts Fig. 1 .The first part is the laser optical system, whichincludes a 2 kW multimode CW CO laser, a GaAs2

lens of focal length 300 mm, and a nozzle. Themelting pool is located 50 mm from the focal pointand the laser beam diameter at the melting pool isabout 5 mm. The second part is powder feedingsystem, which includes a powder feeder and a pow-der-carried copper tube of inner diameter 3 mm. Thecarrying gas is Argon at a flow rate 0.2 M3rh, andboth parts employed Ar as the shielding gas.

The substrate materials were 4Cr MoV Si and 455 1

steel, in whose original microstructure the 4Cr -5

MoV Si material presents tempered martensite and1

45 steel pearlite q ferrite. The compositions ofsubstrate materials shown in Table 2. The specimen

Ž .size: 40=40=9 mm .

Fig. 1. Schematic diagram of laser cladding configuration.

The devices for investigation and analysis in-Ž .cluded SEM scanning electronic microscope ,

EDAX with an electron probe diameter of 1 mm andan X-ray diffraction analysis device with a Cu targetwith Es30 V, Is30 mA, and a scanning rate of

.48rmin . Hardness tests were carried out on a mi-crosclerometer and a vacuum high temperature Vick-ers sclerometer.

3. Results and analysis

The multi-pass overlapping method was used toobtain a wide zone of laser-clad material. Itsschematic is shown in Fig. 2. Under the conditionsof laser power, that is, 1500 W, a powder feedingrate of 10 grmin, a laser beam scanning rate of 3mmrs, and an overlap of 50%, a surface coating 0.9mm thick and 40=40 mm2 area is produced.

3.1. Microstructure of the laser clad layer

Metallographic investigation shows that thelaser-clad layer does not have a uniform microstruc-ture. The whole layer consists of two zones, an upperzone and a lower zone. The thickness of the upperzone is about 0.6 mm, and that of the lower zone is0.3 mm. In the upper zone, see Fig. 3a, X-ray andEDAX analysis identifies the metal phases to include

Ž . Ž .the g f.c.c phase, WC, W C and W,Ti C parti-2 1yxŽ .cles and lamellar eutectic carbide M C. W, Ti12

C is type of MC carbide, and its atom ratio of C1yxŽ .to W, Ti is less than 1. The carbide M C has a12

˚lattice constant a s10.80 A and its molecular for-0Ž . Ž .mula is Fe,Cr,Ni,Co W,Mo,Ti C.6 6

( )Y. YangrApplied Surface Science 140 1999 19–23 21

Table 2Compositions of 4Cr MoV Si and 45 steel5 1

Steel Chemical composition

C Si Mn P S Cr Mo V

4Cr MoV Si 0.38 1.10 0.33 0.020 0.015 5.15 1.43 0.555 1

45a steel 0.45 0.21 0.63 0.021 0.018 0.15 – –

In the lower zone, the main part of the microstruc-ture is similar to that of the upper zone except for alack of WC, but also the distribution of each phase is

Ždifferent, see Fig. 3b. The amount of W C and W,2.Ti C is less than that in the upper zone, but the1yx

amount of M C is greater. Another difference be-12

tween the upper and lower zones is the amount ofFe, see Table 3. There is more Fe in the lower zone.The reason for the difference is the dilution effect ofthe substrate.

3.2. Hardness of the laser-clad layers

We tested both room temperature and elevatedtemperature hardness. The results are shown in Figs.4 and 5. Fig. 4 shows the hardness of the laser-cladspecimen from surface to substrate at room tempera-

Žture. In the laser clad zone distance 0 to 1 mm from.the surface the average hardness is about 520 Hv0.2.

In the heat-affected zone, it is different between twosubstrates.

The hardness of the heat-affected zone in the4Cr MoV Si substrate is even higher than in the5 1

Ž .laser-clad zone curve 1 . Curve 2 corresponds to the45 steel substrate.

We tested the elevated temperature hardness 600Ž .to 10508C see Fig. 5 . The hardness at 6008C is the

Fig. 2. Schematic diagram of multi-pass laser cladding.

highest, even higher than at room temperature, up to550–580 Hv0.2. At 8008C, the hardness is 380–550Hv0.2; at 9508C, 100–200 Hv0.2; and at 10508C,50–90 Hv0.2. These results demonstrate that the

Ž .Fig. 3. SEM photographs of the laser cladding layer, a upperŽ .zone, b lower zone.

( )Y. YangrApplied Surface Science 140 1999 19–2322

Table 3Composition of the laser clad layer as determined by EDAX

Wt.%

W Ni Fe Co Cr Ti Mo Al

Upper zone, Fig. 3 33.35 26.14 23.55 5.82 4.57 0.98 2.66 2.93Lower zone, Fig. 4 29.75 21.79 33.07 3.70 4.69 0.93 3.54 2.53

coating has the potential to function at high tempera-tures.

4. Discussion

4.1. The formation of upper zone and lower zone

Two zones accurately describe the laser-clad layer.w xA heat and mass transfer model 4 elucidates the

reason why two zones are formed. Suppose the entireenergy of the laser beam melts the powder andsubstrate materials. While the laser beam interactswith the powder, the melting pool is a uniform

w xsolution 5 . As the cooling rate is very high duringlaser cladding, the melted zone solidifies rapidly andreleases crystalline latent heat, which will melt thelower substrate material. Then there is no time touniformly mix this layer with the upper solution. Asa result, the upper zone has the dominant composi-tion of the laser cladding and the lower zone that ofthe substrate material. In this experiment, the thick-ness of upper zone is larger than that of the lower

Fig. 4. Hardness of the claddings at room temperature.

zone, 0.6 mm and 0.3 mm, respectively, which meansthat laser beam melts the powders dominantly andthe clad ingredient composes the main part of thelaser clad layer.

4.2. Metallurgical analysis of laser clad layer

The evident features of laser cladding are rapidmelting and rapid solidifying. When powders and thesubstrate surface are melted by the laser beam andbecome liquid, some unmelted WC particles remainthat will form crystalline nuclei during subsequentsolidification. However, because the nucleation and

Ždevelopment of WC needs more C and W C and W,2.Ti C is easier to form than WC, in the liquid it1yx

will be difficult to form new WC in subsequentsolidification. Also because Ti forms carbides more

w xreadily than W 6 , almost all Ti exists as carbide.Other elements, Ni, Co and Cr are in the g phase aselements substituting for Fe, and are found less incarbides. The alloying element Mo functions like Wand exists as the carbide M C.12

Carbides absorb most of the W and Ti, whichcauses the concentration of W and Ti to decrease.Thus the remnant liquid solution undergoes a eutec-

Fig. 5. Hardness of the cladding at elevating temperature.

( )Y. YangrApplied Surface Science 140 1999 19–23 23

Fig. 6. Schematic diagram of hot tool used in rolling mill.

tic transformation in subsequent cooling. Accordingto the analysis above, the cooling transformationstake place in turn:

L™W Cq W,Ti C 1Ž . Ž .2 1yx

L™gqM C 2Ž .12

5. The application of high temperature alloy coat-ing to the hot tool of a rolling mill

Fig. 6 is a schematic diagram of a hot tool. Itsworking surface is on the middle of the tool, a widthof 20 mm. After the laser cladding procedure, acoating width of 30 mm and thickness of 2 mm, isobtained. The substrate material is 4Cr MoV Si steel.5 1

A 2008C and 2-h tempering treatment was carriedout after laser cladding, which relieves stress result-ing from the laser cladding procedure and avoidsbrittle fracture of the laser-clad tool at room tempera-ture.

The result in real working surroundings at 10008Cshows the capability of the laser cladding tool to be arolling steel quantity of 1000–1100 tons per pair.This is twice as good as bulk 4Cr MoV Si tools, and5 1

5 times better than plain-carbon 45 steel tools. Byusing the technique of laser cladding, the failed toolcan be re-used after another laser coating is applied.

Thus not only can the operational life span of thelaser clad hot tool be prolonged but also tool mate-rial can be saved.

6. Conclusions

Ž .1 Under the condition of laser power of 1500W, powder-feeding rate of 10 grmin, scanning rateof 3 mmrs, a wide laser cladding layer has beenproduced by multi-pass overlapping method.

Ž .2 The metal phases of laser cladding layer’sŽ .microstructure mainly consist of g-phase matrix ,

Ž .dispersed WC, W C, W, Ti C particles and eu-2 1yx

tectic M C carbides.12Ž .3 The coating produced by laser cladding has

moderate room temperature hardness and a relativelyhigher high temperature hardness.

Ž .4 Great success was achieved in the applicationof laser cladding layer of high temperature alloy to ahot tool. The operational life span was significantlyprolonged relative to those of bulk 4Cr MoV Si and5 1

45 steel, respectively.

References

w x1 O. Moriaki et al., Development of Laser Cladding Process.Proceeding of LAMP ’87, Osaka, 5, 1987, pp. 156–160.

w x2 T. Takeda, W.M. Steen, D.R.F. West, Instill Clad AlloyFormation by Laser Cladding, Proc. LIM2, Burmingham, UK,March, 1985, Publ. by IFS Publications, Bedford UK.

w x3 M. MacIntyre, Laser Hardfacing of RB211 Turbine BladeShroud Interlocks, Proc. 2nd Int. Conf. On Applications ofLasers in Material Processing, Jan. 1983 Los Angeles, USA.

w x Ž .4 L.J. Li, J. Mazumder, in: K. Mukherji, J. Mazumder Eds. ,Laser Processing of Materials, Published by TMS-AIME,1985, pp. 35–50.

w x Ž .5 T. Chande, J. Mazumder, Appl. Phys. Lett. 57 1982 2226.w x Ž .6 Y.Q. Li, J.Y. Liu Eds. , Space Phase of Grain Boundary of

Superalloy, Metallurgical Industry Press, China, 1990.