LASER WELDING OF THIN SHEET HEAT-TREATABLE STEEL

Slobodan Kralj, Branko Bauer, Zoran Kožuh

Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia

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ABSTRACT

A report on the welding of heat-treatable steel 25 CrMo 4 and 42 CrMo 4, 2mm thick sheet, with high power Nd:YAG laser is presented. Laser and process parameters were explored. Argon and Helium were used as shielding gases. Metalographic analysis was done to asses weld geometry on bead on plate welded samples. Optimum focus position and full penetration welding speed range was determined for each combination of material and shielding gas.

Keywords: Laser welding, steel, Nd:YAG laser, Parameters

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1. Introduction

Laser welding of thin sheet structures is a joining process of materials having ever growing application in electronics, aeronautics, automotive and military industries. Thin sheets made of steels for heat treatment are offering advantage of reduced weight of structure due to their increased strength. Despite the reduced wall thickness, stiffness of structure is improved. Prior to any laser welding procedure to be implemented in the production line, optimal position of focus related to the surface of workpice to be welded must be established. Also, welding speed producing full penetration, type and flow of shielding gas are determined through trials [1].

The intention of the research has been to establish the optimum focus position and feasible range of welding speeds providing full penetration welds on 2 mm thick sheets made of 25 CrMo 4 i 42 CrMo 4 steel grade. Additionally, research has been targeted to establish whether effects of material quality and shielding gases (argon or helium) upon the studied parameters can be noted.

2. Experimental Work

2.1. Equipment

Welding trials were performed using 2kW Nd:YAG laser of “ROFIN CW 020”. 600m optic cable is used for beam transfer, and focusing optics 120/120mm. The beam diameter in the focus is 0.6mm. Focusing optics is fixed to the robot arm “IGM limat RT 280”.

2.2. Experimental Procedure

When the maximum laser power is applied, maximum welding speed is attained, this being interest of industry. For this reason, laser power has been preset at 1800 W, i.e. at 90 % of the unit rated power. Welding has been performed on the samples of 100 x 25 mm size. Prior to welding, samples have been cleaned applying emery paper and ethanol. The samples have been fixed in the jigging tool. To determine welded joint geometry, macro-etches of joint cross sections have been prepared. Following features have been measured: penetration depth, weld width, width of HAZ, joint cross section area.

During the research, argon and helium, the inert shielding gases have been applied. The reactive gases such as nitrogen and carbon dioxide have been rejected since they have effects upon the metallurgical processes in the weld and produce increased hardness in the weld joint [2]. To supply the gas to the weld location, coaxial nozzle has been used.

2.3. Determination of Focus Position Relative to Workpiece Surface

For each tested material and shielding gas, series of bead-on-plate welds have been made, applying constant power rating of 1800 W and welding speed of 200 cm/min. Position of focus have been changed moving the lens of the focusing assembly in steps of 0,2 mm. Welding speed of 200 cm/min has been selected to avoid full penetration, since the penetration depth is the feature that is to be measured.

2.4. Determination of Welding SpeedRange Producing Full Penetration

The highest welding speed producing full penetration has been determined applying constant parameters, and only welding speed has been increased. Series of welds have been made applying different welding speeds, and root side of weld has been inspected to check the full penetration. For the maximum speed, uniform appearance of root face still must be visible. For the lowest welding speed, shape of the weld must be of the form characteristic for the laser welding. Welding speeds in range from 80 to 170 cm/min for each material and shielding gas have been tested, step being 10 cm/min. Welding has been conducted applying optimal position of focus determined in preliminary testing, and using parameters applicable for this condition.

3. Results and Discussion

3.1. Optimum Focus Position

Measuring penetration depth on the macro-etches applying image analyzer, an interval located from 0 mm up to 1,4 mm beneath the surface of the plate has been revealed, in which penetration is approximately equal, for both tested materials and shielding gases. Medium value of this interval is frequently used as the optimum focus position, securing that small vertical movements of focus will not be reflected in significant change of penetration depth [1].

Optimum focus position is in the middle of revealed interval and is located at 0,7 mm beneath plate surface, Fig.1.

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Fig.1.Optimum focus position: steel 25CrMo4,shielding gas He

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This result is in accordance with data in references, where it is suggested that maximum penetration is achieved if focus is located at depth of 1/3 of plate thickness beneath the surface [3].Results are indicating that effects of type of material and shielding gas upon the position of focus providing maximum penetration are very limited. When helium is used as the shielding gas, maximum penetration is achieved if focus is located slightly deeper in the material.

Moreover, there are examples where location of maximum penetration produces unsatisfactory outer appearance of the weld. In such case, criterion for selecting the optimal focus position is not penetration but weld surface appearance. In welding of certain aluminium alloys, optimal position of focus is located above the material surface [4].

3.2. Welding speed range

In the case of bead-on-plate welding on steel 25 CrMo 4, welding speeds are ranging from 110 to 160 cm/min for argon and helium gas shielding, Fig. 2. In subsequent welding of butt joint applying welding speed of 160 cm/min full penetration along the length of weld has not been attained. Welding speed range is reduced to values from 110 to 150 cm/min. This corresponds to heat input of 72 to 98 J/mm.

In the case of bead-on-plate welding on steel 42 CrMo 4, welding speeds are ranging from 90 to 150 cm/min, for argon and helium gas shielding Fig. 3. In subsequent welding of butt joint applying welding speeds of 140 and 150 cm/min full penetration along the length of weld has not been attained. Welding speed range is reduced to values from 90 to 130 cm/min.

This corresponds to heat input of 83 to 120 J/mm.

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a) Argon / b) Helium

Fig. 2. Cross-sections, material 25 CrMo 4, maximum speed 160 cm/min, focus position z=-0,7 mm

a) Argon / b) Helium

Fig. 3. Cross-sections, material 42 CrMo 4, maximum speed 150 cm/min, focus position z=-0,7 mm

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The obtained results indicate existence of the relationship between type of material and welding speed, this being in conformance with other investigations [5]. The reason for this phenomenon may be attributed to size and distribution of globular cementite within ferrite, phases making the bulk of microstructure of base material. Influence of shielding gas upon the welding speed is not registered.

In the industrial production, target is to apply the maximum speed, though such approach may have certatin drawbacks, since the more precise joint fitup and laser beam manipulation is required. Maximum welding speed gives minimal heat input, having favourable effects upon distortions and negative effects considering occurrence of increased hardness. Therefore, welding speed range ensuring full penetration has been investigated. In such case, welding speed that is associated with most suitable heat input for given application can be selected.

Geometric characteristics of welded joint, joint cross-section area, weld width, root width and HAZ width, measured for same welding speeds for both material grades are slightly greater when helium shielding is applied.

The obtained results indicate that approximately same amount of heat input is required to attain full penetration when applying both shielding gases. This is a contrast to gas shielded arc welding processes (MIG/MAG).

Effect of shielding gas upon the weld pattern is significant in the case of lower power densities (2 to 5 x105 W/cm2), while in the case of higher power densities (2 x106 W/cm2) it is less marked. In this trial welding, power density was at 6,4 x105 W/cm2 level and this value is ranked into the high density field.

Effect of shielding gas upon the joint geometry (weld width and penetration) is less marked [6]. According the [6] sources, it is generally accepted that both argon and helium shielding are providing approximately equal penetration depth and weld width in the case of using Nd:YAG laser.

Higher ionization energy required for helium is a higher resistance to plasma generation. As there is no generation of plasma during Nd:YAG laser welding [7], there is no significant difference in effects of argon and helium upon the penetration depth.

4.Conclusion

From the investigation of parameters for laser welding of 2 mm thick heat-treatable steel 25 CrMo 4 and 42 CrMo 4, applying argon and helium shielding atmosphere following conclusions can be made:

-interval of focus position in which maximum penetration can be achieved is within 0 to 1,4 mm below the plate surface, the optimum distance of focus being 0,7 mm beneath the plate surface. Effects of type of base material and shielding gas upon the optimal focus position have not been detected.

-the optimum range of welding speed for steel grade 25 CrMo 4 welded applying argon and helium shield is between 110 and 150 cm/min, wile for steel grade 42 CrMo 4 this range is between 90 and 130 cm/min. These results suggest that there is influence of type of material, but there are no effects of type of shielding gas upon the welding speed.

This investigation has been targeted to determine focus position interval and speed range producing full penetration, but heat input was different. However, exactly heat input is the major factor affecting the hardness rise in the welded joint. Determination of the effects of heat input upon the hardness changes in the welded joint may be topic of the further research, so providing a recommendation for the most suitable combination of focus position and welding speed taking into consideration the maximum hardness in the welded joint.

References

[1] …, ANSI/AWS C7.2, Recommended Practices for Laser Beam Welding, Cutting and Drilling, AWS, ISBN 0-87171-562-7, Miami, 1998.

[2]M. Faerber, Process Gases for Laser Welding, Proc. of 6e CISFFEL, pp. 837-841, Toulon, Jun,1998

[3]P. Limley, Parameter beim Laserstrahlschweißen, In: Laserstrahlschweißen und Prüfen und Bewerten von Laserstrahlschweißungen, pp 1-16, SLV München, München, May, 2000.

[4] S. Kralj, Z. Kožuh, B. Bauer, WeldingofAluminiumAlloyswithaHighPowerNd:YAGLaser, Proc. of 11thInternationalDAAAMSymposium, pp. 243-244, ISBN 3-901509-13-5, Opatija, October, 2000.

[5] A.W.E. Nentwig, UntersuchungenzurSchweißeignungvonStählenzumLaserstrahlschweißenmitCO2Hochleistungslasern, AiF-Bericht Nr. 9803, SLV München, 1997.

[6] ..., VDI-Technologiezentrum Physikalische Technologien, Schweissen mit Festkoerperlasern, Handbuchreihe: Laser in der Materialbearbeitung, Band 2, VDI Verlag, ISBN 3-18-401407-X, Düsseldorf, 1995.

[7] L. Dorn, H. Grutzeck, S. Jafari, Schweißen und Löten mit Festkörperlasern, Springer-Verlag, ISBN 3-540-55543-9, Berlin, 1992.

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