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Effect of dual beam laser welding on microstructure–property relationships of hot-rolled complex phase steel sheets
archives of civil and mechanical engineering 17 (2017) 145–153
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/acme
Original Research Article
Effect of dual beam laser welding on microstructure– property relationships of hot-rolled complex phase steel sheets Mateusz Morawiec a, Maciej Różański b, Adam Grajcar a,*, Sebastian Stano b a
Silesian University of Technology, Institute of Engineering Materials and Biomaterials, 18a Konarskiego Street, 44-100 Gliwice, Poland b Institute of Welding, 16-18 Bl. Czesława Street, 44-100 Gliwice, Poland
article info
abstract
Article history:
The work addresses microstructure–property relationships of hot-rolled complex phase
Received 21 June 2016
steel sheets subjected to various conditions of dual beam laser welding. The article also
Accepted 26 September 2016
contains a comparison between single-spot and twin-spot laser welding. Test-related joints
Available online 19 October 2016
were made using a Yb:YAG disc laser having a maximum power of 12 kW and a welding head enabling the focusing of a laser beam on two spots. The tests involved investigating the
Keywords:
effect of power distribution on the macrostructure, microstructure and mechanical proper-
Hot-rolled steel sheet
ties of joints. Microscopic investigations revealed the stabilisation of some fraction of
Dual beam laser welding
retained austenite in the intercritical heat affected zone (ICHAZ) of joints. The application
Complex phase steel
of the second beam resulted in tempering-like effects in the martensite of the fusion zone
Weldability
and in the HAZ, favourably reducing the hardness of the joint. The use of bifocal welding
Hardness reduction
enabled the obtainment of a 10% hardness reduction in the fusion zone as against singlespot laser beam welding. # 2016 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
1.
Introduction
Advanced high strength steels (AHSS), combining high strength and plasticity, belong to structural materials enabling the reduction of car body weight [1–6]. Additional mass reduction requires using the advanced metal forming of sheets [7–9] and methods enabling the joining of individual auto-body units [10]. The structural whole of a car body including drawings and thin-walled sections of diversified
geometry enables the effective absorption of energy in the frontal or side zones of a car during impact collisions [11]. Until today, primarily because of its efficiency, spot resistance welding has dominated in the automotive industry [12]. The use of tailored blanks and tailored tubes technology enabled the effective reduction of car body weight, mainly due to the use of laser welding [13]. The above named technology is well developed as regards conventional mild steels and microalloyed steels [10,14]. However, the greatest reduction of car body weight accompanies the use of AHSS. Usually,
* Corresponding author. Fax: +48 32 2372281. E-mail address: [email protected] (A. Grajcar). http://dx.doi.org/10.1016/j.acme.2016.09.007 1644-9665/# 2016 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
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archives of civil and mechanical engineering 17 (2017) 145–153
research is concerned with dual phase (DP) steels. Within the strength range of up to approximately 600 MPa, the welding of these steels is free from any special problems [15]. In cases of DP steels characterised by greater strength, problems include the relatively high hardness of the fusion zone (FZ) and of the heat affected zone (HAZ) resulting from a diffusionless martensitic transformation [15–17]. Another problem is softening caused by martensite tempering in the zone between the HAZ and the base material [15,18]. Biro et al. [19] investigated the amount of heat input triggering the phenomenon of softening in dual phase steels. They reported that the heat input required for HAZ softening decreased along with an increase in a carbon content in martensite. The weldability of complex phase steels has not been the subject of extensive research until today. Recently, the weldability of TRIP steels has been investigated by Amirthalingam et al. [12] and Grajcar et al. [20,21]. Because of greater contents of carbon and alloying elements in these steels, problems related to the formation of joints characterised by high hardness are greater than in the case of DP steels. An additional issue, related to the significant amount of strong oxidisers (Al and Si), is the high susceptibility of TRIP steels to oxidation [22]. The least tested steels are complex phase (CP) steels. Park et al. [23] investigated the effect of boron contents on the microstructure, hardness and softening phenomenon of complex phase steels. They reported that the hardness of each zone showed a little change in relation to boron contents but the softening phenomenon occurred in the HAZ near the base material regardless of boron contents. Kong et al. [24] investigated the effect of boron contents on the tensile-share strength and on the type of fracture occurring in resistance spot-welded CP steels. They noticed that boron addition increased both the amount of martensite in the base material and the hardness of the softening zone after welding. One of relatively new functionalities offered by laser techniques is the possibility of splitting a laser beam [13,21]. Until today, this technique has not been used with complex phase steels. For this reason, this work is focused on the comparison of the microstructure and mechanical properties of joints made of CP steels, obtained using conventional laser welding and bifocal welding.
2.
Experimental procedure
In order to compare the effect of single-spot and twin-spot laser beam welding on the properties and microstructure of joints made of steel CPW 800, it was necessary to use 2.5 mm sheets (150 mm 300 mm). Steel CP 800 is hot rolled steel subjected to controlled cooling, characterised by a limited carbon content (0.08%) and higher contents of Mn, Si and Cr, increasing the steel hardenability. Microadditions of Ti and Nb were added in order to additionally harden the material
through the grain refinement and precipitation strengthening [25]. The chemical composition of steel CP 800 is presented in Table 1. Before welding, an oxide layer was removed mechanically from the cut edges and the surfaces of the sheets were degreased with acetone over a width of 50 mm from the edge subjected to welding. The laser welding tests of the steel involved the use of keyhole welding and a solid state laser (Welding Institute in Gliwice). The primary equipment of the laboratory station included the following: Laser TruDisk 12002 – a Yb:YAG solid-state laser (Trumpf) having a maximum power of 12 kW and laser beam quality designated by the parameter of BPP ≤ 8 mm mrad. CFO head (Trumpf) used for single-spot laser welding. The head was connected to the laser source using an optical fibre having a diameter of 200 mm and a focusing lens having a focal length of fog = 300 mm. The diameter of the laser beam focus amounted to 300 mm. D70 head (Trumpf) provided with a system enabling the twin-spot focusing of a laser beam. The distribution of power density between two focuses was monitored using a UFF100 laser beam analyser (Prometec). The tests involved the selection of an optical fibre and a laser beam focusing lens enabling the obtainment of a laser beam focus having a diameter of 0.6 mm. The edges of the joint were subjected to square butt weld preparation and fixed without a spacing gap. Welding was performed in air atmosphere. In order to determine the effect of welding parameters using a single-spot and a twin-spot laser beam on the mechanical properties and the microstructure of welded joints, it was necessary to make butt joints. The adjustment of welding parameters was based on the results of initial tests involving the laser melting of 2.5 mm thick sheets. Table 2 presents the parameters for a single-spot and a twin-spot laser beam enabling the obtainment of proper quality joints, i.e. with the penetration of the entire base material thickness, without spatter, burn-throughs, undercuts, etc. The single-spot joints were formed under conditions of low- (1-LE) and high-energy (1-HE). As regards the twin-spot laser beam (2-HE, HE – high energy) welding was performed with the power distribution of 50:50, in the tandem system. The distance between the focuses amounted to 4 mm. The actual distribution of power density was monitored using a UFF100 laser beam analyser. Fig. 1 presents the results concerning measurements of the power distribution of the twin-spot beam. The preparation of specimens for macro- and microscopic metallographic tests as well as hardness measurements involved the cutting of the welded joints in the plane perpendicular to the weld axis, at the half of the fusion length. The specimens were included in epoxy resin, subjected to grinding by means of abrasive paper and next polished
Table 1 – Chemical composition of steel CPW 800, wt.%. C
Mn
Si
Cr
S
P
Al
Mo
N
Ti
Nb
0.08
1.72
0.56
0.34
0.003
0.010
0.29
0.016
0.002
0.125
0.005
archives of civil and mechanical engineering 17 (2017) 145–153
Table 2 – Welding process parameters for the single-spot and the twin-spot laser beam. Specimen designation Beam power, kW Welding rate, m/min Linear energy, kJ/mm Beam power distribution
1-HE 2 2 0.060 –
1-LE 5 7.8 0.037 –
2-HE 6 5.5 0.065 50:50
using diamond and next corundum slurry. The microstructure of the steel was revealed by etching in 3% Nital and in the aqueous solution of sodium pyrosulfate. The metallographic tests were performed using an MeF4 light microscope manufactured by Leica. Morphological details were revealed using a SUPRA 25 scanning electron microscope in the BSE observation mode. Cross-sectional hardness measurements (HV1) of the welded joints were performed using a KB50BVZ-FA testing machine manufactured by KB Prüftechnik.
3.
Results and discussion
3.1.
Macroscopic tests
147
Macroscopic metallographic tests were performed at 25 magnification. In each case, the width of the fusion zone (FZ) and that of the HAZ were measured. The widths of these areas were measured at the half of the sheet thickness. Fig. 2 presents the macrostructure of the joints welded using the single-spot beam having a linear energy of 0.037 kJ/mm and of 0.06 kJ/mm as well as the macrostructure of the joint welded using the twin-spot beam having a linear energy of 0.065 kJ/mm and the beam power distribution of 50:50. In the joints made using the single spot laser beam, the dependence of the width of the HAZ and that of the weld was linear. An increase in welding linear energy triggered an increase in the width of the weld and that of the HAZ. As
Fig. 1 – Graphic presentation of the power distribution of the twin-spot laser beam.
Fig. 2 – Macrostructures of the laser welded joints for (a) single-spot beam having a linear energy of 0.037 kJ/mm, (b) singlespot beam having a linear energy of 0.060 kJ/mm and (c) twin-spot beam having a linear energy of 0.065 kJ/mm.
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regards the steels subjected to analysis, the width of the weld and that of the HAZ made using a linear energy of 0.037 kJ/mm amounted to 0.6 mm and 0.15 mm respectively, whereas the width of the weld and that of the HAZ made using a linear energy of 0.06 kJ/mm amounted to 0.81 and 0.36 mm, respectively. The use of a twin-spot beam resulted in the increased width of the weld and of the HAZ, i.e. 0.9 mm and 0.47 mm respectively. As can be seen, an increase in linear energy led to an increase in the width of the weld and that of the HAZ (as a result of a greater heat input to the material). The obtained results were typical of laser welded joints [20,23]. It was observed that an increase in a welding rate, leading to a decrease in the heat input to the material, reduced the width of the welded joint. In addition, the macrostructures revealed that each specimen was characterised by the columnar orientation of crystals in the direction of the fastest heat discharge.
3.2.
Hardness measurements
The identification of the microstructure of the steel in individual zones of joints was preceded by measurements of hardness and mechanical properties. The measurements were performed from the weld axis in both directions, with distances between measurement points amounting to 0.2 mm each. The measurement line was located at the half of the depth of a weld. The cross-sectional hardness measurement results related to welded joints made using the single-spot and the twin-spot laser beam are presented in Fig. 3. The cross-sectional hardness measurements of the joints made using a single-spot laser beam and various process parameters revealed that each joint underwent phase transformations leading to a significant increase in hardness of the weld material and that of the HAZ. The increase in hardness was confirmed by the results of microscopic metallographic
Fig. 3 – Curves presenting hardness changes in the crosssection of the joint made using single-spot and twin-spot laser beam welding.
tests revealing the presence of martensite in almost the entire volume of the weld metal. The greatest hardness (393 HV1) was measured in the weld made using the single spot laser beam having a linear energy of 0.037 kJ/mm (specimen no. 1-LE). The specific nature of laser welding characterised by the significant density of laser beam power and a high welding rate (very high cooling rate) was responsible for a martensitic transformation in the entire volume of the material. The joints made using a higher linear energy of 0.06 kJ/mm were characterised by the similar hardness of the weld metal. The reason for the relatively low hardness of martensite in steel CPW 800 was its relatively low carbon content of 0.08% and, consequently, the low supersaturation of the solid solution with carbon.
Table 3 – Results of tensile tests involving the joints made of steel CPW 800, welded using the single-spot and the twin-spot laser beam. Specimen
1-LE 1-HE 2-HE
Tensile strength, UTS, MPa 1
2
3
4
5
861 856 858
826 860 855
872 863 858
830 856 858
844 847 857
Average UTS, MPa
Standard deviation
847 857 857
19.8 6.2 1.3
Fig. 4 – Photographs of exemplary laser welded specimens.
archives of civil and mechanical engineering 17 (2017) 145–153
149
Fig. 5 – Microstructure of the base material of steel CPW 800.
It is also necessary to pay attention to the clearly visible effect of the width of the weld and that of the HAZ on the distribution of hardness in the cross-section of a joint. In the weld made with the greatest linear energy (0.06 kJ/mm) (specimen no. 1-HE), the width of the area characterised by higher hardness in relation to the base material amounted to
1.1 mm from the weld axis. The width of the area characterised by higher hardness in the joint made with a linear energy of 0.037 kJ/mm amounted to 0.7 mm from the weld axis. As regards, twin-spot laser welding, the maximum hardness obtained for a linear energy of 0.065 kJ/mm amounted to 365 HV1, approximately 30 HV1 less than that obtained when
Fig. 6 – Images obtained using light and scanning electron microscopes and presenting the microstructure of the fusion zone for (a, b) single-spot beam having a linear energy of 0.037 kJ/mm, (c, d) single-spot beam having a linear energy of 0.060 kJ/ mm and (e, f) twin-spot beam having a linear energy of 0.065 kJ/mm.
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single-spot laser welding was used. Twin-spot laser welding was performed using the power distribution of 50:50. Therefore, it is possible to state that the weld was melted two times or that the second temperature cycle caused the heating of martensite, leading to the initiation of tempering processes reducing the hardness of this phase. Similar results were obtained by Cretteur et al. [26] in steel TRIP subjected to an additional heat impulse directly following resistance welding. It should also be noted that, as regards the joints welded using the twin-spot laser beam, the zone characterised by greater hardness in relation to the base material was much larger than that of the joints welded using the single-spot beam; the width of the above named zone, regardless of welding linear energy, amounted to approximately 1.6 mm from the weld axis. The increased width of the area characterised by greater hardness was undoubtedly caused by the bifocal effect of the twin-spot laser beam and by a significant increase in the temperature of the base material over a greater width from the molten edges. As can be seen, the joints subjected to analysis did not contain a ‘‘soft zone’’ in the
Fig. 7 – Morphology of martensite laths in the fusion zone (twin-spot beam).
Fig. 8 – Images obtained using light and scanning electron microscopes and presenting the HAZ microstructure for (a, b) single-spot beam having a linear energy of 0.037 kJ/mm, (c, d) single-spot beam having a linear energy of 0.060 kJ/mm, (e, f) twin-spot beam having a linear energy of 0.065 kJ/mm.
archives of civil and mechanical engineering 17 (2017) 145–153
151
heat affected zone. Kim et al. [18] reported that the ‘‘soft zone’’ in the HAZ was usually present in steels characterised by tensile strength exceeding 1100 MPa, which was attributed to tempered martensite. Similar results were obtained by Kong et al. [24] during the resistance spot welding of steel CP. In that case, the ‘‘soft zone’’ was located on the boundary between the base material and HAZ. Taking into consideration the fact that the steel subjected to analysis was characterised by a strength of approximately 800 MPa, the condition of UTS ≥1100 MPa was not present (the lack of the ‘‘soft zone’’).
specimens subjected to tensile tests ruptured in the base material, at a significant distance from the weld (on average 20–30 mm) – Fig. 4. In each case, the strength of the welded joint was greater than that of the base material. The tensile strength values of the specimens amounted to 847–857 MPa as regards the joints welded using the single-spot laser beam and approximately 857 MPa in the case of the joint welded using the twin-spot laser beam.
3.3.
The microscopic metallographic tests involved the base material and three areas of welded joints, i.e. the fusion zone, the heat affected zone and the area located on the side of the base material. Fig. 5 presents the microstructure of the base material after subjecting the specimen to a thermo-mechanical processing. It can be noticed that the microstructure of steel CPW 800 is characterised by the significant dispersion of individual phases. The ferritic–bainitic (F + B) matrix contains martensitic–austenitic islands (MA) of various sizes. In addition, it is
Mechanical properties of the welded joints
Tensile tests were performed at room temperature. Specimens used in the tensile tests were prepared in accordance with the requirements of PN-EN ISO 4163:2013. Each joint was subjected to five tensile tests. The results of the tensile tests involving the joints made using single and twin-spot welding are presented in Table 3. The tensile test results revealed that in each case, i.e. when welding using the single-spot or the twin-spot laser beam, the
3.4.
Microstructure
Fig. 9 – Images obtained using light and scanning electron microscopes and presenting the microstructure of the transition zone between the HAZ and the BM for (a, b) single-spot beam having a linear energy of 0.037 kJ/mm, (c, d) single-spot beam having a linear energy of 0.060 kJ/mm, (e, f) twin-spot beam having a linear energy of 0.065 kJ/mm.
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possible to identify a slight content of retained austenite (RA) having a bright shade. Fig. 6 presents the microstructures of the joint made using single-spot and twin-spot laser beam welding. In each case, it can be noticed that the fusion zone structure contains lath martensite of significant lath thicknesses. However, the use of the twin-spot beam (Fig. 6e and f) resulted in the more diversified orientation of laths packets. The morphology of martensite laths indicates the initiation of tempering processes. Biro et al. [19] reported that the decomposition of martensite did not immediately follow the obtainment of the temperature of Ac1 but proceeded along with an increasing hold time. This, in turn, explains the presence of tempered martensite in the fusion zone of the specimen affected by the twin-spot laser beam. The second beam provided an additional heat input, extending the hold time of martensite and initiating processes typical of tempering. Morphological details of defragmented martensite laths observed in the specimen subjected to twin-spot laser beam welding are presented in Fig. 7. It can be seen that martensite underwent defragmentation (caused by the action of the second beam) and that precipitation processes were initiated. These phenomena are visible particularly well at the top right-hand corner of the image. This area contains many point-like morphological elements, possibly precipitates formed as a result of martensite tempering affected by the second beam. The microstructure of the heat affected zone is presented in Fig. 8. As can be seen, the single-spot beam-related HAZ is composed of fine lath martensite. The use of the twin-spot beam resulted in the greater dispersion of martensite laths, typical of tempered structures. The single-spot beam-related HAZ microstructure is composed of relatively homogenous martensite. As regards the use of a twin-spot beam, it is possible to notice a fine-grained martensitic-bainitic structure without clearly visible laths, typical of martensitic structure (Fig. 8f). The microstructure of the transition zone between the HAZ and BM is presented in Fig. 9. The transition zone between the HAZ and the base material is characterised by a mixture of fine-grained ferrite, bainite, martensite and retained austenite (RA). In the significant part of this zone, the temperature of the welded material is restricted within an intercritical range. The content of ferrite grows along with a decreasing distance from the base metal (Fig. 9a, c and e). The presence of retained austenite in the form of characteristic polygonal grains [27] is greater than that observed in the base material, which can be attributed to enrichment with carbon in the range of intercritical temperature (Fig. 9b, d and f). The particularly high content of fine grains can be related to dual beam welding (Fig. 9f).
4.
Conclusions
The results of the tests involving joints welded using the single-spot or the twin-spot laser beam justified the formulation of the following conclusions:
The use of the twin-spot beam favourably reduced the hardness of welded joints, thus increasing their ductility. The hardness obtained when using the twin-spot laser beam was by approximately 10% lower than that obtained using the single-spot beam. The microstructure of the fusion zone contained lath martensite of relatively thick laths. Subjected to the twinspot laser beam, martensite underwent partial tempering, which led to a decrease in hardness in this zone. The heat affected zone was characterised by fine-grained martensite, where the greater degree of dispersion of individual lath packets followed the use of bifocal welding. In the transition zone, it was possible to stabilise a significant amount of retained austenite, which can improve the ductility of the joint. An increase in linear energy, and as a result, an increase in heat input to the material, increased the width of the welded joint. In the test, for a linear energy of 0.037 kJ/mm, the width amounted to 0.7 mm, whereas for a linear energy of 0.060 kJ/mm, the width amounted to 1.1 mm. For the dual beam having a linear energy of 0.065 kJ/mm, the width of the welded joint amounted to 1.6 mm. The obtained welded joints were characterised by tensile strength equal to or greater than that of the base material, which was reflected in the static tensile tests. In each specimen, the rupture took place in the base material.
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Available online at www.sciencedirect.com
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Original Research Article
Effect of dual beam laser welding on microstructure– property relationships of hot-rolled complex phase steel sheets Mateusz Morawiec a, Maciej Różański b, Adam Grajcar a,*, Sebastian Stano b a
Silesian University of Technology, Institute of Engineering Materials and Biomaterials, 18a Konarskiego Street, 44-100 Gliwice, Poland b Institute of Welding, 16-18 Bl. Czesława Street, 44-100 Gliwice, Poland
article info
abstract
Article history:
The work addresses microstructure–property relationships of hot-rolled complex phase
Received 21 June 2016
steel sheets subjected to various conditions of dual beam laser welding. The article also
Accepted 26 September 2016
contains a comparison between single-spot and twin-spot laser welding. Test-related joints
Available online 19 October 2016
were made using a Yb:YAG disc laser having a maximum power of 12 kW and a welding head enabling the focusing of a laser beam on two spots. The tests involved investigating the
Keywords:
effect of power distribution on the macrostructure, microstructure and mechanical proper-
Hot-rolled steel sheet
ties of joints. Microscopic investigations revealed the stabilisation of some fraction of
Dual beam laser welding
retained austenite in the intercritical heat affected zone (ICHAZ) of joints. The application
Complex phase steel
of the second beam resulted in tempering-like effects in the martensite of the fusion zone
Weldability
and in the HAZ, favourably reducing the hardness of the joint. The use of bifocal welding
Hardness reduction
enabled the obtainment of a 10% hardness reduction in the fusion zone as against singlespot laser beam welding. # 2016 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
1.
Introduction
Advanced high strength steels (AHSS), combining high strength and plasticity, belong to structural materials enabling the reduction of car body weight [1–6]. Additional mass reduction requires using the advanced metal forming of sheets [7–9] and methods enabling the joining of individual auto-body units [10]. The structural whole of a car body including drawings and thin-walled sections of diversified
geometry enables the effective absorption of energy in the frontal or side zones of a car during impact collisions [11]. Until today, primarily because of its efficiency, spot resistance welding has dominated in the automotive industry [12]. The use of tailored blanks and tailored tubes technology enabled the effective reduction of car body weight, mainly due to the use of laser welding [13]. The above named technology is well developed as regards conventional mild steels and microalloyed steels [10,14]. However, the greatest reduction of car body weight accompanies the use of AHSS. Usually,
* Corresponding author. Fax: +48 32 2372281. E-mail address: [email protected] (A. Grajcar). http://dx.doi.org/10.1016/j.acme.2016.09.007 1644-9665/# 2016 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
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research is concerned with dual phase (DP) steels. Within the strength range of up to approximately 600 MPa, the welding of these steels is free from any special problems [15]. In cases of DP steels characterised by greater strength, problems include the relatively high hardness of the fusion zone (FZ) and of the heat affected zone (HAZ) resulting from a diffusionless martensitic transformation [15–17]. Another problem is softening caused by martensite tempering in the zone between the HAZ and the base material [15,18]. Biro et al. [19] investigated the amount of heat input triggering the phenomenon of softening in dual phase steels. They reported that the heat input required for HAZ softening decreased along with an increase in a carbon content in martensite. The weldability of complex phase steels has not been the subject of extensive research until today. Recently, the weldability of TRIP steels has been investigated by Amirthalingam et al. [12] and Grajcar et al. [20,21]. Because of greater contents of carbon and alloying elements in these steels, problems related to the formation of joints characterised by high hardness are greater than in the case of DP steels. An additional issue, related to the significant amount of strong oxidisers (Al and Si), is the high susceptibility of TRIP steels to oxidation [22]. The least tested steels are complex phase (CP) steels. Park et al. [23] investigated the effect of boron contents on the microstructure, hardness and softening phenomenon of complex phase steels. They reported that the hardness of each zone showed a little change in relation to boron contents but the softening phenomenon occurred in the HAZ near the base material regardless of boron contents. Kong et al. [24] investigated the effect of boron contents on the tensile-share strength and on the type of fracture occurring in resistance spot-welded CP steels. They noticed that boron addition increased both the amount of martensite in the base material and the hardness of the softening zone after welding. One of relatively new functionalities offered by laser techniques is the possibility of splitting a laser beam [13,21]. Until today, this technique has not been used with complex phase steels. For this reason, this work is focused on the comparison of the microstructure and mechanical properties of joints made of CP steels, obtained using conventional laser welding and bifocal welding.
2.
Experimental procedure
In order to compare the effect of single-spot and twin-spot laser beam welding on the properties and microstructure of joints made of steel CPW 800, it was necessary to use 2.5 mm sheets (150 mm 300 mm). Steel CP 800 is hot rolled steel subjected to controlled cooling, characterised by a limited carbon content (0.08%) and higher contents of Mn, Si and Cr, increasing the steel hardenability. Microadditions of Ti and Nb were added in order to additionally harden the material
through the grain refinement and precipitation strengthening [25]. The chemical composition of steel CP 800 is presented in Table 1. Before welding, an oxide layer was removed mechanically from the cut edges and the surfaces of the sheets were degreased with acetone over a width of 50 mm from the edge subjected to welding. The laser welding tests of the steel involved the use of keyhole welding and a solid state laser (Welding Institute in Gliwice). The primary equipment of the laboratory station included the following: Laser TruDisk 12002 – a Yb:YAG solid-state laser (Trumpf) having a maximum power of 12 kW and laser beam quality designated by the parameter of BPP ≤ 8 mm mrad. CFO head (Trumpf) used for single-spot laser welding. The head was connected to the laser source using an optical fibre having a diameter of 200 mm and a focusing lens having a focal length of fog = 300 mm. The diameter of the laser beam focus amounted to 300 mm. D70 head (Trumpf) provided with a system enabling the twin-spot focusing of a laser beam. The distribution of power density between two focuses was monitored using a UFF100 laser beam analyser (Prometec). The tests involved the selection of an optical fibre and a laser beam focusing lens enabling the obtainment of a laser beam focus having a diameter of 0.6 mm. The edges of the joint were subjected to square butt weld preparation and fixed without a spacing gap. Welding was performed in air atmosphere. In order to determine the effect of welding parameters using a single-spot and a twin-spot laser beam on the mechanical properties and the microstructure of welded joints, it was necessary to make butt joints. The adjustment of welding parameters was based on the results of initial tests involving the laser melting of 2.5 mm thick sheets. Table 2 presents the parameters for a single-spot and a twin-spot laser beam enabling the obtainment of proper quality joints, i.e. with the penetration of the entire base material thickness, without spatter, burn-throughs, undercuts, etc. The single-spot joints were formed under conditions of low- (1-LE) and high-energy (1-HE). As regards the twin-spot laser beam (2-HE, HE – high energy) welding was performed with the power distribution of 50:50, in the tandem system. The distance between the focuses amounted to 4 mm. The actual distribution of power density was monitored using a UFF100 laser beam analyser. Fig. 1 presents the results concerning measurements of the power distribution of the twin-spot beam. The preparation of specimens for macro- and microscopic metallographic tests as well as hardness measurements involved the cutting of the welded joints in the plane perpendicular to the weld axis, at the half of the fusion length. The specimens were included in epoxy resin, subjected to grinding by means of abrasive paper and next polished
Table 1 – Chemical composition of steel CPW 800, wt.%. C
Mn
Si
Cr
S
P
Al
Mo
N
Ti
Nb
0.08
1.72
0.56
0.34
0.003
0.010
0.29
0.016
0.002
0.125
0.005
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Table 2 – Welding process parameters for the single-spot and the twin-spot laser beam. Specimen designation Beam power, kW Welding rate, m/min Linear energy, kJ/mm Beam power distribution
1-HE 2 2 0.060 –
1-LE 5 7.8 0.037 –
2-HE 6 5.5 0.065 50:50
using diamond and next corundum slurry. The microstructure of the steel was revealed by etching in 3% Nital and in the aqueous solution of sodium pyrosulfate. The metallographic tests were performed using an MeF4 light microscope manufactured by Leica. Morphological details were revealed using a SUPRA 25 scanning electron microscope in the BSE observation mode. Cross-sectional hardness measurements (HV1) of the welded joints were performed using a KB50BVZ-FA testing machine manufactured by KB Prüftechnik.
3.
Results and discussion
3.1.
Macroscopic tests
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Macroscopic metallographic tests were performed at 25 magnification. In each case, the width of the fusion zone (FZ) and that of the HAZ were measured. The widths of these areas were measured at the half of the sheet thickness. Fig. 2 presents the macrostructure of the joints welded using the single-spot beam having a linear energy of 0.037 kJ/mm and of 0.06 kJ/mm as well as the macrostructure of the joint welded using the twin-spot beam having a linear energy of 0.065 kJ/mm and the beam power distribution of 50:50. In the joints made using the single spot laser beam, the dependence of the width of the HAZ and that of the weld was linear. An increase in welding linear energy triggered an increase in the width of the weld and that of the HAZ. As
Fig. 1 – Graphic presentation of the power distribution of the twin-spot laser beam.
Fig. 2 – Macrostructures of the laser welded joints for (a) single-spot beam having a linear energy of 0.037 kJ/mm, (b) singlespot beam having a linear energy of 0.060 kJ/mm and (c) twin-spot beam having a linear energy of 0.065 kJ/mm.
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regards the steels subjected to analysis, the width of the weld and that of the HAZ made using a linear energy of 0.037 kJ/mm amounted to 0.6 mm and 0.15 mm respectively, whereas the width of the weld and that of the HAZ made using a linear energy of 0.06 kJ/mm amounted to 0.81 and 0.36 mm, respectively. The use of a twin-spot beam resulted in the increased width of the weld and of the HAZ, i.e. 0.9 mm and 0.47 mm respectively. As can be seen, an increase in linear energy led to an increase in the width of the weld and that of the HAZ (as a result of a greater heat input to the material). The obtained results were typical of laser welded joints [20,23]. It was observed that an increase in a welding rate, leading to a decrease in the heat input to the material, reduced the width of the welded joint. In addition, the macrostructures revealed that each specimen was characterised by the columnar orientation of crystals in the direction of the fastest heat discharge.
3.2.
Hardness measurements
The identification of the microstructure of the steel in individual zones of joints was preceded by measurements of hardness and mechanical properties. The measurements were performed from the weld axis in both directions, with distances between measurement points amounting to 0.2 mm each. The measurement line was located at the half of the depth of a weld. The cross-sectional hardness measurement results related to welded joints made using the single-spot and the twin-spot laser beam are presented in Fig. 3. The cross-sectional hardness measurements of the joints made using a single-spot laser beam and various process parameters revealed that each joint underwent phase transformations leading to a significant increase in hardness of the weld material and that of the HAZ. The increase in hardness was confirmed by the results of microscopic metallographic
Fig. 3 – Curves presenting hardness changes in the crosssection of the joint made using single-spot and twin-spot laser beam welding.
tests revealing the presence of martensite in almost the entire volume of the weld metal. The greatest hardness (393 HV1) was measured in the weld made using the single spot laser beam having a linear energy of 0.037 kJ/mm (specimen no. 1-LE). The specific nature of laser welding characterised by the significant density of laser beam power and a high welding rate (very high cooling rate) was responsible for a martensitic transformation in the entire volume of the material. The joints made using a higher linear energy of 0.06 kJ/mm were characterised by the similar hardness of the weld metal. The reason for the relatively low hardness of martensite in steel CPW 800 was its relatively low carbon content of 0.08% and, consequently, the low supersaturation of the solid solution with carbon.
Table 3 – Results of tensile tests involving the joints made of steel CPW 800, welded using the single-spot and the twin-spot laser beam. Specimen
1-LE 1-HE 2-HE
Tensile strength, UTS, MPa 1
2
3
4
5
861 856 858
826 860 855
872 863 858
830 856 858
844 847 857
Average UTS, MPa
Standard deviation
847 857 857
19.8 6.2 1.3
Fig. 4 – Photographs of exemplary laser welded specimens.
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Fig. 5 – Microstructure of the base material of steel CPW 800.
It is also necessary to pay attention to the clearly visible effect of the width of the weld and that of the HAZ on the distribution of hardness in the cross-section of a joint. In the weld made with the greatest linear energy (0.06 kJ/mm) (specimen no. 1-HE), the width of the area characterised by higher hardness in relation to the base material amounted to
1.1 mm from the weld axis. The width of the area characterised by higher hardness in the joint made with a linear energy of 0.037 kJ/mm amounted to 0.7 mm from the weld axis. As regards, twin-spot laser welding, the maximum hardness obtained for a linear energy of 0.065 kJ/mm amounted to 365 HV1, approximately 30 HV1 less than that obtained when
Fig. 6 – Images obtained using light and scanning electron microscopes and presenting the microstructure of the fusion zone for (a, b) single-spot beam having a linear energy of 0.037 kJ/mm, (c, d) single-spot beam having a linear energy of 0.060 kJ/ mm and (e, f) twin-spot beam having a linear energy of 0.065 kJ/mm.
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single-spot laser welding was used. Twin-spot laser welding was performed using the power distribution of 50:50. Therefore, it is possible to state that the weld was melted two times or that the second temperature cycle caused the heating of martensite, leading to the initiation of tempering processes reducing the hardness of this phase. Similar results were obtained by Cretteur et al. [26] in steel TRIP subjected to an additional heat impulse directly following resistance welding. It should also be noted that, as regards the joints welded using the twin-spot laser beam, the zone characterised by greater hardness in relation to the base material was much larger than that of the joints welded using the single-spot beam; the width of the above named zone, regardless of welding linear energy, amounted to approximately 1.6 mm from the weld axis. The increased width of the area characterised by greater hardness was undoubtedly caused by the bifocal effect of the twin-spot laser beam and by a significant increase in the temperature of the base material over a greater width from the molten edges. As can be seen, the joints subjected to analysis did not contain a ‘‘soft zone’’ in the
Fig. 7 – Morphology of martensite laths in the fusion zone (twin-spot beam).
Fig. 8 – Images obtained using light and scanning electron microscopes and presenting the HAZ microstructure for (a, b) single-spot beam having a linear energy of 0.037 kJ/mm, (c, d) single-spot beam having a linear energy of 0.060 kJ/mm, (e, f) twin-spot beam having a linear energy of 0.065 kJ/mm.
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heat affected zone. Kim et al. [18] reported that the ‘‘soft zone’’ in the HAZ was usually present in steels characterised by tensile strength exceeding 1100 MPa, which was attributed to tempered martensite. Similar results were obtained by Kong et al. [24] during the resistance spot welding of steel CP. In that case, the ‘‘soft zone’’ was located on the boundary between the base material and HAZ. Taking into consideration the fact that the steel subjected to analysis was characterised by a strength of approximately 800 MPa, the condition of UTS ≥1100 MPa was not present (the lack of the ‘‘soft zone’’).
specimens subjected to tensile tests ruptured in the base material, at a significant distance from the weld (on average 20–30 mm) – Fig. 4. In each case, the strength of the welded joint was greater than that of the base material. The tensile strength values of the specimens amounted to 847–857 MPa as regards the joints welded using the single-spot laser beam and approximately 857 MPa in the case of the joint welded using the twin-spot laser beam.
3.3.
The microscopic metallographic tests involved the base material and three areas of welded joints, i.e. the fusion zone, the heat affected zone and the area located on the side of the base material. Fig. 5 presents the microstructure of the base material after subjecting the specimen to a thermo-mechanical processing. It can be noticed that the microstructure of steel CPW 800 is characterised by the significant dispersion of individual phases. The ferritic–bainitic (F + B) matrix contains martensitic–austenitic islands (MA) of various sizes. In addition, it is
Mechanical properties of the welded joints
Tensile tests were performed at room temperature. Specimens used in the tensile tests were prepared in accordance with the requirements of PN-EN ISO 4163:2013. Each joint was subjected to five tensile tests. The results of the tensile tests involving the joints made using single and twin-spot welding are presented in Table 3. The tensile test results revealed that in each case, i.e. when welding using the single-spot or the twin-spot laser beam, the
3.4.
Microstructure
Fig. 9 – Images obtained using light and scanning electron microscopes and presenting the microstructure of the transition zone between the HAZ and the BM for (a, b) single-spot beam having a linear energy of 0.037 kJ/mm, (c, d) single-spot beam having a linear energy of 0.060 kJ/mm, (e, f) twin-spot beam having a linear energy of 0.065 kJ/mm.
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possible to identify a slight content of retained austenite (RA) having a bright shade. Fig. 6 presents the microstructures of the joint made using single-spot and twin-spot laser beam welding. In each case, it can be noticed that the fusion zone structure contains lath martensite of significant lath thicknesses. However, the use of the twin-spot beam (Fig. 6e and f) resulted in the more diversified orientation of laths packets. The morphology of martensite laths indicates the initiation of tempering processes. Biro et al. [19] reported that the decomposition of martensite did not immediately follow the obtainment of the temperature of Ac1 but proceeded along with an increasing hold time. This, in turn, explains the presence of tempered martensite in the fusion zone of the specimen affected by the twin-spot laser beam. The second beam provided an additional heat input, extending the hold time of martensite and initiating processes typical of tempering. Morphological details of defragmented martensite laths observed in the specimen subjected to twin-spot laser beam welding are presented in Fig. 7. It can be seen that martensite underwent defragmentation (caused by the action of the second beam) and that precipitation processes were initiated. These phenomena are visible particularly well at the top right-hand corner of the image. This area contains many point-like morphological elements, possibly precipitates formed as a result of martensite tempering affected by the second beam. The microstructure of the heat affected zone is presented in Fig. 8. As can be seen, the single-spot beam-related HAZ is composed of fine lath martensite. The use of the twin-spot beam resulted in the greater dispersion of martensite laths, typical of tempered structures. The single-spot beam-related HAZ microstructure is composed of relatively homogenous martensite. As regards the use of a twin-spot beam, it is possible to notice a fine-grained martensitic-bainitic structure without clearly visible laths, typical of martensitic structure (Fig. 8f). The microstructure of the transition zone between the HAZ and BM is presented in Fig. 9. The transition zone between the HAZ and the base material is characterised by a mixture of fine-grained ferrite, bainite, martensite and retained austenite (RA). In the significant part of this zone, the temperature of the welded material is restricted within an intercritical range. The content of ferrite grows along with a decreasing distance from the base metal (Fig. 9a, c and e). The presence of retained austenite in the form of characteristic polygonal grains [27] is greater than that observed in the base material, which can be attributed to enrichment with carbon in the range of intercritical temperature (Fig. 9b, d and f). The particularly high content of fine grains can be related to dual beam welding (Fig. 9f).
4.
Conclusions
The results of the tests involving joints welded using the single-spot or the twin-spot laser beam justified the formulation of the following conclusions:
The use of the twin-spot beam favourably reduced the hardness of welded joints, thus increasing their ductility. The hardness obtained when using the twin-spot laser beam was by approximately 10% lower than that obtained using the single-spot beam. The microstructure of the fusion zone contained lath martensite of relatively thick laths. Subjected to the twinspot laser beam, martensite underwent partial tempering, which led to a decrease in hardness in this zone. The heat affected zone was characterised by fine-grained martensite, where the greater degree of dispersion of individual lath packets followed the use of bifocal welding. In the transition zone, it was possible to stabilise a significant amount of retained austenite, which can improve the ductility of the joint. An increase in linear energy, and as a result, an increase in heat input to the material, increased the width of the welded joint. In the test, for a linear energy of 0.037 kJ/mm, the width amounted to 0.7 mm, whereas for a linear energy of 0.060 kJ/mm, the width amounted to 1.1 mm. For the dual beam having a linear energy of 0.065 kJ/mm, the width of the welded joint amounted to 1.6 mm. The obtained welded joints were characterised by tensile strength equal to or greater than that of the base material, which was reflected in the static tensile tests. In each specimen, the rupture took place in the base material.
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