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Bridging the joint gap with wire feed laser welding
Journal of Materials Processing Technology 87 (1999) 213 – 222
Bridging the joint gap with wire feed laser welding Z. Sun *, M. Kuo Gintic Institute of Manufacturing Technology, 71 Nanyang Dri6e, Singapore 638075, Singapore Received 3 September 1998
Abstract Laser welding of carbon and stainless steel using wire feed has been investigated using a 3.0 kW CO2 laser system. A straight thin tube nozzle, attached to the laser beam nozzle, was used to deliver the wire to the weld zone. This technique was found to be able to accommodate larger joint gaps than autogenous laser welding. A gap of up to 1.0 mm for 2.0 mm thick butt joint has been welded successfully. This is a significant improvement over autogenous laser welding. Mixing analysis revealed that the wire is uniformly distributed in the weld metal although a larger joint gap tends to accommodate more wire. The practical implication of these results is that tight fit-up requirements for laser welding can be relaxed somewhat for many demanding industrial applications. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Laser welding; Carbon steel; Stainless steel; Filler wire; Hardness; Microstructure; Tensile strength; Joint gap; Weld metal; Heat affected zone
1. Introduction Laser welding has recently received growing attention due to its special features and potential. It has been widely implemented in industrial applications, e.g. in the automotive industry [1 – 6]. However, many more applications could arise following further investigation. Laser welding is a high energy-density, low heat-input process with specific advantages over conventional fusion welding processes. These include high welding speed, narrow heat-affected zone (HAZ), low distortion, ease of automation, single-pass thick section capability and enhanced design flexibility. One of the many features of laser welding is the capability to weld without filler material (autogenous welding) for square butt joints. Although autogenous welding offers distinct advantages, filler addition, in many cases, can enhance its capabilities whilst retaining its many features. Firstly, filler addition can be used to tailor the weld properties by modifying the fusion zone composition. Secondly, the tight joint fit-up requirement for laser welding can be relaxed somewhat by using filler material. Thirdly, as thick section structures can be welded by a multi* Corresponding [email protected]
author.
Fax:
+ 65
7925362;
e-mail:
pass technique, the need for expensive very high-power lasers can be obviated. Several types of filler materials may be introduced, before or during welding, such as wire, profile and powder. Wire feed, which is considered to be the most versatile, has received much attention. A major concern with wire feed laser welding is the wire mixing behaviour in the weld metal which determines the microstructure and thus the uniformity of the weld properties. Although studies on wire feed laser welding technique have been extensively documented [7–25], information on the wire mixing behaviour is still limited. The objective of the present paper is to report findings on wire feed laser welding and the wire mixing behavior.
2. Experimental
2.1. Materials The base metals used in this investigation included 3.0 mm thick 0.2% carbon steel and 2.0 mm thick 304 stainless steel, laser cut to 150× 60 mm. The edges to be welded were ground and cleaned to ensure their cleanliness and edge dimension. Filler wires used included * 1.0 mm ER70S-6 copper coated manganese-
0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00346-X
Z. Sun, M. Kuo / Journal of Materials Processing Technology 87 (1999) 213–222
214
Table 1 Typical weld metal composition of ER70S-6 wire (wt. %) C
Si
Mn
Fe
0.10
0.8
1.2
Bal.
silicon bearing C–Mn steel wire and * 1.2 mm ER308 Si stainless steel wire. Their chemical compositions are given in Tables 1 and 2, respectively. Fig. 1. Schematic illustration showing the wire feed and laser beam arrangement.
2.2. Set-up for wire feed laser welding Compared with autogenous laser welding, wire feed laser welding requires an additional wire feed system. Fig. 1 shows schematically the laser beam and wire feed arrangement. The filler wire was fed parallel to the welding seam in the vertical plane containing the axis of the laser beam. The wire feed can be performed in such a way that the wire is directed to the interaction point between the laser beam and the parent material. The angle between the beam and the wire was kept at around 50°. The wire was fed by a Cyclomatic DWF-3 TIG wire feeder at a pre-set feed rate. The wire feed start and stop command was controlled manually. In order to keep the wire as straight as possible before entering the interaction point, a long copper nozzle (around 200 mm) was used to straighten the wire, as shown in Fig. 2. Straightness of wire is very important because it determines whether the wire will interact properly with the laser beam and hence the effectiveness of the wire feed process. A fixture was used to clamp butt joint samples (Fig. 2). Two steel plates to be welded can be placed in the fixture, which was equipped with two retention bars to prevent warpage of the welded samples. Various steel plate thickness can be accommodated and fixed by adjusting screws. Joint gaps were pre-set with feeler gauges. The alignment of the beam relative to the gap centre was checked by a He – Ne laser beam. When welding with wire feed, the mechanism of laser power absorption is different from that in autogenous welding because a part of the laser beam will be reflected by the wire. The interaction between the laser beam, the filler wire and the workpiece becomes a complicated phenomenon. The extent of interaction between the laser beam and filler wire depends on where the wire is directed. If the filler wire is fed to a
point some distance away from the laser beam axis, it can only be melted by the molten weld pool because it does not interact with the laser beam. Ideally, direct interaction between the laser beam and the wire should occur when the wire intersects the focused laser beam at a point along the beam axis. Therefore, the laser beam would melt both the wire and the base metal to form the weld.
2.3. Welding parameters Welding was conducted using a Trumpf 3000 W CO2 laser system, with a maximum continuous power output of 3.0 kW. The laser beam is delivered to the workpiece surface using water-cooled, coated copper mirrors. Focusing was performed using a lens with a 200 mm focal length. Argon shielding gas was provided coaxially with the laser beam at a flow rate of 20 l min − 1. The wire feed rate was determined according to the volume of the joint gap: the wider the joint gap, the greater the wire feed rate. In principle, 15% more wire is fed in consideration of the weld root and reinforcement. Tack welds were made at both ends of the butt joint in order to minimise distortion during welding. The welding parameters used to produce acceptable weld bead profiles are summarised in Table 3.
Table 2 Typical weld metal composition of ER308 SI stainless steel wire (wt. %) C
Si
Mn
Cr
Ni
Fe
0.06
0.8
1.7
20.0
10.5
Bal. Fig. 2. Laser beam and wire feed nozzle arrangement in this study.
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Table 3 Laser welding parameters used in the study Base metal
Joint gap (mm)
Power (kW)
Speed (mm min−1)
Wire feed rate (mm min−1)
Filler wire
Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Stainless steel Stainless steel Stainless steel Stainless steel Stainless steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel
0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0
3.0 2.7 2.85 3.0 3.0 2.4 2.4 2.4 2.4 2.55 3.0 2.7 2.85 3.0 3.0
1100 1250 1250 1150 1000 1250 1250 1250 1250 1250 1100 1250 1150 1000 900
635 1600 2400 2800 3050 500 1270 1780 1930 2360 900 1900 2800 3050 3430
ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER70-6 ER70-6 ER70-6 ER70-6 ER70-6
2.4. E6aluation procedures Cross-sections were prepared for optical and electron microscopy using standard procedures including grinding, polishing and etching. Hardness was measured on polished and etched metallographic samples using a Mitutoyo micro-hardness tester with a load of 0.3 kg. Compositional analysis across the weld metal was carried out with a Cambridge scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The analytical area for each point was about 2400 mm2. The elements analysed included Fe, Cr, Ni, Mo, Si, and Mn. The concentrations of these elements were normalised to 100 wt.%. The volume fraction of martensite or ferrite in the weld metal was measured with the aid of a permeability meter (Fisher Feritscope MP3C). Tensile testing was conducted using an Instron Tensile Tester (Model 4505) at room temperature with an strain rate of 2.5 cm min − 1.
Fig. 3. Cross section of a laser weld on 3 mm thick carbon steel with C–Mn steel wire (joint gap 0.6 mm).
The tensile test samples were machined from the joint and prepared according to the ASME standard [26].
3. Results and discussion
3.1. Weld bead Three types of joints were produced, i.e. carbon steel base metal with either stainless steel wire or C–Mn steel wire, and stainless steel base metal with stainless steel wire (Table 3). Visual examination of the weld beads indicated that all of the welds made with the various welding parameters (Table 3) showed complete joint penetration. The welds were uniform along the entire weld seam, which is indicative of precise control and good reproducibility. Fig. 3 shows the cross section of a typical weld. It is noted that the weld is wider than an autogenous weld due to the wide joint gap (0.6 mm)
Fig. 4. Cross section of a laser weld on 3 mm thick carbon steel with stainless steel wire (joint gap 1.0 mm).
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Fig. 5. Chemical analyses of wire feed laser welding of carbon steel with stainless steel wire (joint gap 0.6 mm) showing the vertical and transverse compositional distribution.
[27]. It also demonstrates the 100% filler wire usage efficiency, i.e. all the wire was melted and incorporated into the joint. This is different from powder feed laser welding in which a certain amount of powder is wasted without melting [28]. Fig. 4 shows the cross section of a carbon steel weld with stainless steel wire. The purpose of using stainless steel wire to weld carbon steel are twofold: firstly, to test the suitability of such a combination; and secondly, to provide an easy way to examine the wire mixing behaviour in the weld metal. The present results demonstrate that carbon steel can be welded with both C – Mn steel and stainless steel wires with satisfactory weld profile. Not surprisingly, stainless steel sheets can also be easily welded with stainless steel wire.
3.2. Weld composition and microstructure In order to gain a clear indication of the wire mixing behaviour in the weld, chemical analysis of the weld metal was conducted using the SEM/EDS for joints made of carbon steel and stainless steel wire. Since the compositional differences between the wire and base metal (carbon steel) are significant, it is easy to see if the mixing is homogenous or incomplete from the elemental spatial distribution. Cr and Ni contents were used to indicate the extent of mixing in the weld metal, because the wire contains 20% Cr and 10.5% Ni, whilst carbon steel has none of them. To provide an adequate picture of the distribution, both transverse and vertical analysis were performed across the weld metal. Figs. 5
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Fig. 6. Chemical analyses of wire feed laser welding of carbon steel with stainless steel wire (joint gap 1.0 mm) showing the vertical and transverse compositional distribution.
and 6 show the results for samples with a joint gap of 0.6 and 1.0 mm, respectively. With a joint gap of 0.6 mm (Fig. 5), the distribution of Cr and Ni is uniform which indicates that wire mixing is uniform from the top to the bottom of the weld. However, the Cr and Ni contents become lower when the measuring points are near to the fusion line of the weld in the transverse direction, particularly in the lower part of the weld. This is due to the expected higher dilution in the vicinity of the base metal. With a larger joint gap of 1.0 mm (Fig. 6), both vertical and transverse distribution of Cr and Ni are quite uniform which indicates an even more uniform wire mixing in the weld than that of the narrower gap joint. Hardness measurements in the weld metals also demonstrate the
uniform distribution along the centreline of both welds (Figs. 7 and 8). This is also an indication of uniform wire mixing in the welds. Comparison between Figs. 5 and 6 also shows that the Cr and Ni contents are greater in the weld with a larger joint gap using the higher wire feed rate. Furthermore, the Schaeffler Cr and Ni equivalents [29] for the two weld metals were calculated using the average contents of Cr, Ni, Si, Mn, and Mo from the EDS measurements (i.e. 21 measuring points in Figs. 5 and 6). The carbon content was determined from the average dilution calculated from the averaged Cr and Ni contents. Fig. 9 shows the positions of the welds, the base metal and the wire in the Schaeffler diagram. As discrepancies in the accuracy of the Schaeffler-diagram
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Fig. 7. Hardness profile of a carbon steel weld with stainless steel wire (joint gap 0.6 mm) showing a uniform hardness distribution along the weld centreline.
Fig. 8. Hardness profile of a carbon steel weld with stainless steel wire (joint gap 1.0 mm) showing a uniform hardness distribution along the weld centreline.
prediction for laser welds have been observed due to the rapid cooling rate of the process [30 – 33], the Schaeffler diagram will only be used qualitatively. It is noted that the welds are roughly in the middle of the connection line between the base metal and the wire. The weld with the 0.6 mm joint gap is located in the fully martensite zone, whilst the weld with 1.0 mm joint gap is in the austenite and martensite mixed zone. Although the former weld is located in the fully martensite zone, it may still possess a cer-
tain amount of austenite. In fact, feritscope measurements gave readings of 52 and 44% martensite for weld metals with 0.6 and 1.0 mm joint gap, respectively. The balance should be austenite. Hardness measurement further corroborated this finding. The average hardness was 453 Hv for the weld metal with a 0.6 mm gap and 406 Hv for the weld metal with a 1.0 mm gap. The reduced hardness observed in wider gap welds is attributed to the decreased martensite content.
Z. Sun, M. Kuo / Journal of Materials Processing Technology 87 (1999) 213–222
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Fig. 9. Positions of welds plotted on the Schaeffler diagram; , base metal; , wire; , weld with 0.6 mm joint gap; , weld with 1.0 mm joint gap.
Fig. 10. Hardness profile for a laser weld of carbon steel with stainless steel wire (joint gap 1.0 mm).
3.3. Gap tolerances in the butt joints As the main objective of this study is to demonstrate the joint gap bridging capability of the technique, the results obtained are very promising. The maximum joint gap accommodated is almost equal to the diameter of the wire. For example, when a * 1.2 stainless steel wire was used, a joint gap of at least 1.0 mm can be bridged satisfactorily (Fig. 4). It is also clearly seen that the gap can be accommodated easily even with an offset in the thickness direction which is very difficult for autogenous laser welding. This is particularly desirable for industrial applications where relative large
tolerance allowance is preferred. The same applies to the * 1.0 C–Mn steel wire which can also produce satisfactory welds with a 1.0 mm wide gap. With this gap tolerance, most gap related problems encountered in autogenous laser welding will be resolved. As a rule of thumb, the allowable joint gap tolerance is around 10% of the joint thickness. For example, 0.3 mm is the maximum allowable gap to produce a satisfactory joint for a 3 mm thick plate. Although machining of the joint edges can readily reduce joint gaps to an acceptable level, this will be very costly for mass production. In the present study, a 1.0 mm joint gap has been bridged with wire feed laser welding for 2.0 mm thick stainless steel
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Fig. 11. Hardness profile for a laser weld of carbon steel with C – Mn steel wire (joint gap 0.6 mm).
Fig. 12. Hardness profile of a carbon steel weld with C–Mn steel wire (joint gap 0.6 mm) showing a relative uniform hardness distribution along the weld centreline.
sheet which is impossible for autogenous laser welding. This improvement can significantly reduce the tight joint gap fit-up requirement.
3.4. Weld properties Fig. 10 shows the hardness profile of carbon steel welded with stainless steel wire. It can be seen that the average hardness in the weld metal is much higher than that in the base metal. Since weld metal is composed of
base metal and filler wire, it has the microstructure of martensite or a mixture of austenite and martensite according to the Schaeffler diagram [29]. Therefore, high hardness (around HV400) can be expected. The high hardness value in the HAZ was related to the phase transformations, since the high cooling rate associated with laser welding may result in the formation of hard phases such as martensite. Because the hardness is proportional to the strength of a material, the hardness of the weld metal indicates that it possesses sufficient
Z. Sun, M. Kuo / Journal of Materials Processing Technology 87 (1999) 213–222
strength. Therefore, there will be no strength problem in the joints. However, their toughness may be adversely affected by the hard weld metal. Fig. 11 shows the hardness profile of a carbon steel weld produced with C – Mn steel wire. Although there is not much difference in chemical composition between the carbon steel and the C – Mn steel wire, the high cooling rate associated with laser welding still produces much harder phases in the weld metal. Further measurement gave a relative uniform vertical hardness distribution, which suggests a uniform wire distribution from the top to the bottom of the carbon steel weld (Fig. 12). Tensile testing was conducted for the wire feed welds. Since a weld produced with stainless steel wire will be stronger than the base metal, there is no need to test such joints. Therefore, only welds produced with C– Mn steel wire were tensile tested because they have similar strengths. All of the joints tested fractured at the base metal, which indicated sufficient strength of the joints. In fact, the strength obtained was representative of the base metal itself. Although laser welding with wire feed widens the application ranges, it is more complex as there are more variables to control. The accuracy of set-up, particularly the relative positioning of the wire and the beam incident location, is very critical. If these variables are not properly controlled, the results might be even worse than without wire feed. For the present tests, manual control of the wire feed worked well, but for production it may encounter some difficulties. Therefore, integration of the laser beam and the wire feed control is an important issue for practical industrial applications.
4. Conclusions Laser welding with wire feed has been successfully conducted using a 3 kW CO2 laser system. The present investigation has clearly demonstrated the improvement in reducing joint gap tolerance requirement by wire feed welding. For a 2.0 mm thick butt joint, 1.0 mm gap can be accommodated to produce a satisfactory weld. This can enlarge the range of applications for laser welding in many cases. The present study shows that the distribution of wire is generally uniform from the top to the bottom of the weld. Therefore, uniform weld properties can be expected. As for the quality of joints, the welds made by wire feed laser welding possess sufficient strength. The key factor in the technique is to precisely control the relative positions of the laser beam and the wire feed direction in order to obtain sufficient interaction. To optimise the process, further development in integrating laser beam and wire filler control is required.
221
Acknowledgements The authors would like to thank KM Teh, J Sun and FL Ng for their help in the experiments and Dr SK Chang for his comments.
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[17] S.E. Nielsen, L.E. Hansen, J.K Kristensen, High power laser welding of C/Mn steel—The effects of using different filler materials and plasma control gases, in: Proceedings of the Fifth International Conference on Welding and Melting by Electron and Laser Beams, La Baule, France, vol. 1, 1993, 203–210. [18] I.A Jones, Laser welding of structural steel with cold and hot wire feed techniques, Proceedings of the Fifth International Conference on Welding and Melting by Electron and Laser Beams, La Baule, France, vol. 1, 1993, pp. 195–201. [19] U. Dilthey, J. Schneegans, Studies into laser beam welding with filler wire addition of unalloyed and low-alloyed steels, Weld. Cut. 46 (3) (1994) E40–E42. [20] A.S. Salminen, V.P. Kujanpa¨a¨, T.J.I. Moisio, Effect of use of filler wire on requirements of laser welded butt joint, Proceedings of the International Conference ICALEO’94, Laser Institute of America, Orlando, FL, 1994. [21] J.M. Roman, D. Kechemair, J.P. Ricaud, CO2 laser welding of very large thickness materials with wire filler, Weld. Int. 8 (5) (1994) 376 – 379. [22] E.A. Metzbower, H.K.D.H. Bhadeshia, R.H. Phillips, Microstructures in hot wire laser beam welding of HY 80 steel, Mater. Sci. Technol. 10 (1) (1994) 56–59. [23] I. Jones, Lasers make advances in welding of automotive aluminium alloys, TWI Bulletin 2, TWI, Cambridge, UK, 1995. [24] A.S. Salminen, V.P. Kujanpa¨a¨, T. Moisio, Beam-wire interactions during laser welding with filler wire, Weld. J. 75 (1) (1996) 9– 13.
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[25] Z. Sun, A.S. Salminen, Current status of laser welding with wire feed, Mater. Manuf. Process. 12 (5) (1997) 759 – 777. [26] ASME Boiler and Pressure Vessel Code Section IX, The American Society of Mechanical Engineers (ASME), New York, 1986. [27] M. Kuo, Z. Sun, D. Pan, Laser welding of thin sheet materials. Report C96-P-034A, Gintic Institute of Manufacturing Technology, Singapore, 1997. [28] Z. Sun, M. Kuo, Laser welding with powder feed, Sci. Technol. Welding Joining, in press. [29] A.L. Schaeffler, Constitutional diagram for stainless steel weld metal, Metal Progr. 56 (5) (1949) 680 – 680. [30] S. Katayama, A. Matsunawa, Solidification microstructure of laser welded stainless steels, Proceedings of ICALEO’84, Laser Institute of America, Boston, MA, 1984, pp. 60 – 67. [31] S.A. David, J.M. Vitek, T.L. Hebble, Effect of rapid solidification on stainless steel weld metal microstructures and its implications on the Schaeffler diagram, Weld. J. 66 (10) (1987) 289s – 300. [32] Y. Nakao, K. Nishimoto, W.P. Zhang, Effect of rapid solidification by laser surface melting on solidification modes and microstructure of stainless steels, Trans. JWS 19 (2) (1988) 20–26. [33] J.W. Elmer, T.W. Eagar, S.M. Allen, Single-phase solidification during rapid-resolidification of stainless steel alloys, in: R.A. Patterson, K.W. Mahin (Eds.), Proceedings of the Materials Weldability Symposium, October 1990, Detroit, MI, ASM International, Materials Park, Ohio, 1990, pp. 143 – 150.
Bridging the joint gap with wire feed laser welding Z. Sun *, M. Kuo Gintic Institute of Manufacturing Technology, 71 Nanyang Dri6e, Singapore 638075, Singapore Received 3 September 1998
Abstract Laser welding of carbon and stainless steel using wire feed has been investigated using a 3.0 kW CO2 laser system. A straight thin tube nozzle, attached to the laser beam nozzle, was used to deliver the wire to the weld zone. This technique was found to be able to accommodate larger joint gaps than autogenous laser welding. A gap of up to 1.0 mm for 2.0 mm thick butt joint has been welded successfully. This is a significant improvement over autogenous laser welding. Mixing analysis revealed that the wire is uniformly distributed in the weld metal although a larger joint gap tends to accommodate more wire. The practical implication of these results is that tight fit-up requirements for laser welding can be relaxed somewhat for many demanding industrial applications. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Laser welding; Carbon steel; Stainless steel; Filler wire; Hardness; Microstructure; Tensile strength; Joint gap; Weld metal; Heat affected zone
1. Introduction Laser welding has recently received growing attention due to its special features and potential. It has been widely implemented in industrial applications, e.g. in the automotive industry [1 – 6]. However, many more applications could arise following further investigation. Laser welding is a high energy-density, low heat-input process with specific advantages over conventional fusion welding processes. These include high welding speed, narrow heat-affected zone (HAZ), low distortion, ease of automation, single-pass thick section capability and enhanced design flexibility. One of the many features of laser welding is the capability to weld without filler material (autogenous welding) for square butt joints. Although autogenous welding offers distinct advantages, filler addition, in many cases, can enhance its capabilities whilst retaining its many features. Firstly, filler addition can be used to tailor the weld properties by modifying the fusion zone composition. Secondly, the tight joint fit-up requirement for laser welding can be relaxed somewhat by using filler material. Thirdly, as thick section structures can be welded by a multi* Corresponding [email protected]
author.
Fax:
+ 65
7925362;
e-mail:
pass technique, the need for expensive very high-power lasers can be obviated. Several types of filler materials may be introduced, before or during welding, such as wire, profile and powder. Wire feed, which is considered to be the most versatile, has received much attention. A major concern with wire feed laser welding is the wire mixing behaviour in the weld metal which determines the microstructure and thus the uniformity of the weld properties. Although studies on wire feed laser welding technique have been extensively documented [7–25], information on the wire mixing behaviour is still limited. The objective of the present paper is to report findings on wire feed laser welding and the wire mixing behavior.
2. Experimental
2.1. Materials The base metals used in this investigation included 3.0 mm thick 0.2% carbon steel and 2.0 mm thick 304 stainless steel, laser cut to 150× 60 mm. The edges to be welded were ground and cleaned to ensure their cleanliness and edge dimension. Filler wires used included * 1.0 mm ER70S-6 copper coated manganese-
0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00346-X
Z. Sun, M. Kuo / Journal of Materials Processing Technology 87 (1999) 213–222
214
Table 1 Typical weld metal composition of ER70S-6 wire (wt. %) C
Si
Mn
Fe
0.10
0.8
1.2
Bal.
silicon bearing C–Mn steel wire and * 1.2 mm ER308 Si stainless steel wire. Their chemical compositions are given in Tables 1 and 2, respectively. Fig. 1. Schematic illustration showing the wire feed and laser beam arrangement.
2.2. Set-up for wire feed laser welding Compared with autogenous laser welding, wire feed laser welding requires an additional wire feed system. Fig. 1 shows schematically the laser beam and wire feed arrangement. The filler wire was fed parallel to the welding seam in the vertical plane containing the axis of the laser beam. The wire feed can be performed in such a way that the wire is directed to the interaction point between the laser beam and the parent material. The angle between the beam and the wire was kept at around 50°. The wire was fed by a Cyclomatic DWF-3 TIG wire feeder at a pre-set feed rate. The wire feed start and stop command was controlled manually. In order to keep the wire as straight as possible before entering the interaction point, a long copper nozzle (around 200 mm) was used to straighten the wire, as shown in Fig. 2. Straightness of wire is very important because it determines whether the wire will interact properly with the laser beam and hence the effectiveness of the wire feed process. A fixture was used to clamp butt joint samples (Fig. 2). Two steel plates to be welded can be placed in the fixture, which was equipped with two retention bars to prevent warpage of the welded samples. Various steel plate thickness can be accommodated and fixed by adjusting screws. Joint gaps were pre-set with feeler gauges. The alignment of the beam relative to the gap centre was checked by a He – Ne laser beam. When welding with wire feed, the mechanism of laser power absorption is different from that in autogenous welding because a part of the laser beam will be reflected by the wire. The interaction between the laser beam, the filler wire and the workpiece becomes a complicated phenomenon. The extent of interaction between the laser beam and filler wire depends on where the wire is directed. If the filler wire is fed to a
point some distance away from the laser beam axis, it can only be melted by the molten weld pool because it does not interact with the laser beam. Ideally, direct interaction between the laser beam and the wire should occur when the wire intersects the focused laser beam at a point along the beam axis. Therefore, the laser beam would melt both the wire and the base metal to form the weld.
2.3. Welding parameters Welding was conducted using a Trumpf 3000 W CO2 laser system, with a maximum continuous power output of 3.0 kW. The laser beam is delivered to the workpiece surface using water-cooled, coated copper mirrors. Focusing was performed using a lens with a 200 mm focal length. Argon shielding gas was provided coaxially with the laser beam at a flow rate of 20 l min − 1. The wire feed rate was determined according to the volume of the joint gap: the wider the joint gap, the greater the wire feed rate. In principle, 15% more wire is fed in consideration of the weld root and reinforcement. Tack welds were made at both ends of the butt joint in order to minimise distortion during welding. The welding parameters used to produce acceptable weld bead profiles are summarised in Table 3.
Table 2 Typical weld metal composition of ER308 SI stainless steel wire (wt. %) C
Si
Mn
Cr
Ni
Fe
0.06
0.8
1.7
20.0
10.5
Bal. Fig. 2. Laser beam and wire feed nozzle arrangement in this study.
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Table 3 Laser welding parameters used in the study Base metal
Joint gap (mm)
Power (kW)
Speed (mm min−1)
Wire feed rate (mm min−1)
Filler wire
Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Stainless steel Stainless steel Stainless steel Stainless steel Stainless steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel
0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0
3.0 2.7 2.85 3.0 3.0 2.4 2.4 2.4 2.4 2.55 3.0 2.7 2.85 3.0 3.0
1100 1250 1250 1150 1000 1250 1250 1250 1250 1250 1100 1250 1150 1000 900
635 1600 2400 2800 3050 500 1270 1780 1930 2360 900 1900 2800 3050 3430
ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER308 Si ER70-6 ER70-6 ER70-6 ER70-6 ER70-6
2.4. E6aluation procedures Cross-sections were prepared for optical and electron microscopy using standard procedures including grinding, polishing and etching. Hardness was measured on polished and etched metallographic samples using a Mitutoyo micro-hardness tester with a load of 0.3 kg. Compositional analysis across the weld metal was carried out with a Cambridge scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The analytical area for each point was about 2400 mm2. The elements analysed included Fe, Cr, Ni, Mo, Si, and Mn. The concentrations of these elements were normalised to 100 wt.%. The volume fraction of martensite or ferrite in the weld metal was measured with the aid of a permeability meter (Fisher Feritscope MP3C). Tensile testing was conducted using an Instron Tensile Tester (Model 4505) at room temperature with an strain rate of 2.5 cm min − 1.
Fig. 3. Cross section of a laser weld on 3 mm thick carbon steel with C–Mn steel wire (joint gap 0.6 mm).
The tensile test samples were machined from the joint and prepared according to the ASME standard [26].
3. Results and discussion
3.1. Weld bead Three types of joints were produced, i.e. carbon steel base metal with either stainless steel wire or C–Mn steel wire, and stainless steel base metal with stainless steel wire (Table 3). Visual examination of the weld beads indicated that all of the welds made with the various welding parameters (Table 3) showed complete joint penetration. The welds were uniform along the entire weld seam, which is indicative of precise control and good reproducibility. Fig. 3 shows the cross section of a typical weld. It is noted that the weld is wider than an autogenous weld due to the wide joint gap (0.6 mm)
Fig. 4. Cross section of a laser weld on 3 mm thick carbon steel with stainless steel wire (joint gap 1.0 mm).
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Fig. 5. Chemical analyses of wire feed laser welding of carbon steel with stainless steel wire (joint gap 0.6 mm) showing the vertical and transverse compositional distribution.
[27]. It also demonstrates the 100% filler wire usage efficiency, i.e. all the wire was melted and incorporated into the joint. This is different from powder feed laser welding in which a certain amount of powder is wasted without melting [28]. Fig. 4 shows the cross section of a carbon steel weld with stainless steel wire. The purpose of using stainless steel wire to weld carbon steel are twofold: firstly, to test the suitability of such a combination; and secondly, to provide an easy way to examine the wire mixing behaviour in the weld metal. The present results demonstrate that carbon steel can be welded with both C – Mn steel and stainless steel wires with satisfactory weld profile. Not surprisingly, stainless steel sheets can also be easily welded with stainless steel wire.
3.2. Weld composition and microstructure In order to gain a clear indication of the wire mixing behaviour in the weld, chemical analysis of the weld metal was conducted using the SEM/EDS for joints made of carbon steel and stainless steel wire. Since the compositional differences between the wire and base metal (carbon steel) are significant, it is easy to see if the mixing is homogenous or incomplete from the elemental spatial distribution. Cr and Ni contents were used to indicate the extent of mixing in the weld metal, because the wire contains 20% Cr and 10.5% Ni, whilst carbon steel has none of them. To provide an adequate picture of the distribution, both transverse and vertical analysis were performed across the weld metal. Figs. 5
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Fig. 6. Chemical analyses of wire feed laser welding of carbon steel with stainless steel wire (joint gap 1.0 mm) showing the vertical and transverse compositional distribution.
and 6 show the results for samples with a joint gap of 0.6 and 1.0 mm, respectively. With a joint gap of 0.6 mm (Fig. 5), the distribution of Cr and Ni is uniform which indicates that wire mixing is uniform from the top to the bottom of the weld. However, the Cr and Ni contents become lower when the measuring points are near to the fusion line of the weld in the transverse direction, particularly in the lower part of the weld. This is due to the expected higher dilution in the vicinity of the base metal. With a larger joint gap of 1.0 mm (Fig. 6), both vertical and transverse distribution of Cr and Ni are quite uniform which indicates an even more uniform wire mixing in the weld than that of the narrower gap joint. Hardness measurements in the weld metals also demonstrate the
uniform distribution along the centreline of both welds (Figs. 7 and 8). This is also an indication of uniform wire mixing in the welds. Comparison between Figs. 5 and 6 also shows that the Cr and Ni contents are greater in the weld with a larger joint gap using the higher wire feed rate. Furthermore, the Schaeffler Cr and Ni equivalents [29] for the two weld metals were calculated using the average contents of Cr, Ni, Si, Mn, and Mo from the EDS measurements (i.e. 21 measuring points in Figs. 5 and 6). The carbon content was determined from the average dilution calculated from the averaged Cr and Ni contents. Fig. 9 shows the positions of the welds, the base metal and the wire in the Schaeffler diagram. As discrepancies in the accuracy of the Schaeffler-diagram
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Fig. 7. Hardness profile of a carbon steel weld with stainless steel wire (joint gap 0.6 mm) showing a uniform hardness distribution along the weld centreline.
Fig. 8. Hardness profile of a carbon steel weld with stainless steel wire (joint gap 1.0 mm) showing a uniform hardness distribution along the weld centreline.
prediction for laser welds have been observed due to the rapid cooling rate of the process [30 – 33], the Schaeffler diagram will only be used qualitatively. It is noted that the welds are roughly in the middle of the connection line between the base metal and the wire. The weld with the 0.6 mm joint gap is located in the fully martensite zone, whilst the weld with 1.0 mm joint gap is in the austenite and martensite mixed zone. Although the former weld is located in the fully martensite zone, it may still possess a cer-
tain amount of austenite. In fact, feritscope measurements gave readings of 52 and 44% martensite for weld metals with 0.6 and 1.0 mm joint gap, respectively. The balance should be austenite. Hardness measurement further corroborated this finding. The average hardness was 453 Hv for the weld metal with a 0.6 mm gap and 406 Hv for the weld metal with a 1.0 mm gap. The reduced hardness observed in wider gap welds is attributed to the decreased martensite content.
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Fig. 9. Positions of welds plotted on the Schaeffler diagram; , base metal; , wire; , weld with 0.6 mm joint gap; , weld with 1.0 mm joint gap.
Fig. 10. Hardness profile for a laser weld of carbon steel with stainless steel wire (joint gap 1.0 mm).
3.3. Gap tolerances in the butt joints As the main objective of this study is to demonstrate the joint gap bridging capability of the technique, the results obtained are very promising. The maximum joint gap accommodated is almost equal to the diameter of the wire. For example, when a * 1.2 stainless steel wire was used, a joint gap of at least 1.0 mm can be bridged satisfactorily (Fig. 4). It is also clearly seen that the gap can be accommodated easily even with an offset in the thickness direction which is very difficult for autogenous laser welding. This is particularly desirable for industrial applications where relative large
tolerance allowance is preferred. The same applies to the * 1.0 C–Mn steel wire which can also produce satisfactory welds with a 1.0 mm wide gap. With this gap tolerance, most gap related problems encountered in autogenous laser welding will be resolved. As a rule of thumb, the allowable joint gap tolerance is around 10% of the joint thickness. For example, 0.3 mm is the maximum allowable gap to produce a satisfactory joint for a 3 mm thick plate. Although machining of the joint edges can readily reduce joint gaps to an acceptable level, this will be very costly for mass production. In the present study, a 1.0 mm joint gap has been bridged with wire feed laser welding for 2.0 mm thick stainless steel
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Fig. 11. Hardness profile for a laser weld of carbon steel with C – Mn steel wire (joint gap 0.6 mm).
Fig. 12. Hardness profile of a carbon steel weld with C–Mn steel wire (joint gap 0.6 mm) showing a relative uniform hardness distribution along the weld centreline.
sheet which is impossible for autogenous laser welding. This improvement can significantly reduce the tight joint gap fit-up requirement.
3.4. Weld properties Fig. 10 shows the hardness profile of carbon steel welded with stainless steel wire. It can be seen that the average hardness in the weld metal is much higher than that in the base metal. Since weld metal is composed of
base metal and filler wire, it has the microstructure of martensite or a mixture of austenite and martensite according to the Schaeffler diagram [29]. Therefore, high hardness (around HV400) can be expected. The high hardness value in the HAZ was related to the phase transformations, since the high cooling rate associated with laser welding may result in the formation of hard phases such as martensite. Because the hardness is proportional to the strength of a material, the hardness of the weld metal indicates that it possesses sufficient
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strength. Therefore, there will be no strength problem in the joints. However, their toughness may be adversely affected by the hard weld metal. Fig. 11 shows the hardness profile of a carbon steel weld produced with C – Mn steel wire. Although there is not much difference in chemical composition between the carbon steel and the C – Mn steel wire, the high cooling rate associated with laser welding still produces much harder phases in the weld metal. Further measurement gave a relative uniform vertical hardness distribution, which suggests a uniform wire distribution from the top to the bottom of the carbon steel weld (Fig. 12). Tensile testing was conducted for the wire feed welds. Since a weld produced with stainless steel wire will be stronger than the base metal, there is no need to test such joints. Therefore, only welds produced with C– Mn steel wire were tensile tested because they have similar strengths. All of the joints tested fractured at the base metal, which indicated sufficient strength of the joints. In fact, the strength obtained was representative of the base metal itself. Although laser welding with wire feed widens the application ranges, it is more complex as there are more variables to control. The accuracy of set-up, particularly the relative positioning of the wire and the beam incident location, is very critical. If these variables are not properly controlled, the results might be even worse than without wire feed. For the present tests, manual control of the wire feed worked well, but for production it may encounter some difficulties. Therefore, integration of the laser beam and the wire feed control is an important issue for practical industrial applications.
4. Conclusions Laser welding with wire feed has been successfully conducted using a 3 kW CO2 laser system. The present investigation has clearly demonstrated the improvement in reducing joint gap tolerance requirement by wire feed welding. For a 2.0 mm thick butt joint, 1.0 mm gap can be accommodated to produce a satisfactory weld. This can enlarge the range of applications for laser welding in many cases. The present study shows that the distribution of wire is generally uniform from the top to the bottom of the weld. Therefore, uniform weld properties can be expected. As for the quality of joints, the welds made by wire feed laser welding possess sufficient strength. The key factor in the technique is to precisely control the relative positions of the laser beam and the wire feed direction in order to obtain sufficient interaction. To optimise the process, further development in integrating laser beam and wire filler control is required.
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Acknowledgements The authors would like to thank KM Teh, J Sun and FL Ng for their help in the experiments and Dr SK Chang for his comments.
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