Laser Roll Welding of Dissimilar Metal Joint of Zinc Coated Steel and Aluminum Alloy

Transcription

1 IIW Doc IV Laser Roll Welding of Dissimilar Metal Joint of Zinc Coated Steel and Aluminum Alloy Muneharu KUTSUNA Nagoya University Hitoshi OZAKI Nagoya University Shigeyuki NAKAGAWA Nissan Motor Co. Kenji MIYAMOTO Nissan Motor Co.

2 ~Laser Roll Welding facility Laser Roll Roll Welding facility Laser Roll Welding was developed for joining of dissimilar metals by M.Kutsuna and M.Rathod in A pressure roller was mounted on a 2.4 kw pulse CO 2 laser facility. It is desirable that thermal cycle for joining can be shortened by heating of laser. Therefore, the formation of the brittle intermetallic compound can be easily controlled. Furthermore, good contact of steel sheet and aluminum sheet and rapid heat transfer from a steel sheet to an aluminum alloy sheet are conducted by a pressure roller.

3 Introduction Low Low fuel fuel consumption by by lightening the the body Safety improvement There is is Multi material car body concept. An An example of of multi multi material car car body body Use of high strength steel Lightening with improving strength Use of aluminum alloy Lightening However It It is is difficult to to join steel to aluminum by fusion welding.

4 ~Fusion welding of steel and aluminum Problems in in fusion fusion welding of of steel steel and and aluminum In fusion welding, brittle intermetallic compound at weld interface Ductile IMC s Brittle IMC s M.Yasuyama, et al., 1996 Brittle intermetallic compound could be controlled by application of Laser Roll Welding

5 ~Characterization of Laser Roll Welding Classification Classification of of welding welding processes processes 1. Fusion welding 4. Laser Roll Welding Joint by melting and alloying a part of both base metal 2.Solid-state welding Joint by the use of interdiffusion and the plastic flow phenomenon in the interface By heating and pressurizing only metal A, heat is conducted to metal B, molten phase is formed at the faying surface and jointed 3. Brazing Joint that uses brazing filler metal of low melting point for faying surface It It is is not not pertinent to to any any among among left left three three welding processes. It It can can be be said said the the fourth fourth welding process.

6 ~Purpose of this study Laser Roll Welding of zinc coated steel and aluminum alloy was investigated, and the process parameters such as travel speed were studied. Cause effect diagram of of LRW The influences of the process parameters on the weldability, the formation of intermetallic compound layer and the mechanical properties of joints.

7 Experimental procedure ~Materials Zinc coated steel Dimension: mm Galvanized steel sheet (GI) Table 1 Chemical composition and coating weight of steel sheet Material Elements (mass%) C Mn P S Coating weight (g / m 2 ) GI <0.15 <0.60 <0.05 < Aluminum alloy Dimension: mm 6000 series aluminum alloy sheet (A6000) Table 2 Chemical composition of aluminum alloy sheet Material Si Fe Cu Elements (mass%) Mn Mg Cr Zn Ti Al A Bal.

8 ~Process parameters Table 3 Process parameters for Laser Roll Welding Laser type Laser peak power Pulse CO 2 Laser 2.0 kw Duty cycles 75% Frequency Travel speed Overlapped width Roll pressure Center shielding gas Side shielding gas 150 Hz 0.2~0.7 m / min 3 mm 150 MPa Ar : 25 l / min Ar : 25 l / min Surface pretreatment Upper surface of the steel: Coated with graphite Faying surface of the steel: Degreased with ethanol Faying surface of the aluminum alloy: Polished, degreased and coated with aluminum brazing flux

9 Experimental results and discussions ~Video of Laser Roll Welding Laser peak power = 2.0 kw Travel speed = 0.5 m / min Overlapped width = 3 mm Roll pressure = 150 MPa Side view Oblique view Zinc vapor Zinc vapor

10 ~Bead appearance and cross-section Laser peak power = 2.0 kw Travel speed = 0.5 m / min Overlapped width = 3 mm Roll pressure = 150 MPa GI GI Top Top bead bead Cross-section Bottom Bottom bead bead GI GI GI A6000 3mm A6000 2mm A6000 3mm A6000 View from the top 10mm A6000 View from the bottom 10mm

11 ~Effect of travel speed on intermetallic compound layer thickness Laser peak power = 2.0 kw Overlapped width = 3 mm Roll pressure = 150 MPa Interface layer thickness (μm) GI GI // A Travel speed (m/min) Observed Observed position position Intermetallic compound layer thickness decreases significantly from 22 to 5 μm when the travel speed increases from 0.3 to 0.6 m / min. 10μm

12 Effect of heat input on intermetallic compound layer thickness Laser peak power = 2.0 kw Overlapped width = 3 mm Roll pressure = 150 MPa Interface layer thickness (μm) GI GI // A Travel speed (m/min) Heat input (J/cm) Observed Observed position position Heat input and intermetallic compound layer thickness decrease significantly when the travel speed increase from 0.3 to 0.6 m / min. 10μm

13 Thermal cycle measurement result Laser peak power = 2.0 kw Travel speed = 0.5 m / min Overlapped width = 3 mm Roll pressure = 150 MPa Measured Measured position position Thermocouple (Pt-13%PtRh,φ=0.3mm) Measurement Measurement result result Stage Ⅰ Ⅱ Ⅲ Ⅳ 800 A Temperature ( ) B C Time (sec) StageⅠ:Rapid heating with laser StageⅡ:Evaporation of zinc / Heat conduction to Al StageⅢ:Heat conduction by roll pressure (Rapid cooling) StageⅣ:Natural cooling

14 ~Thermal cycle measured Laser peak power = 2.0 kw Overlapped width = 3 mm Roll pressure = 150 MPa GI GI // A6000 Temperature ( ) m/min 0.5 m/min 0.7 m/min Time (sec) Thermocouple (Pt-13%PtRh,φ=0.3mm) Measured Measured position position The thermal cycle at the interface affects on the formation of the intermetallic compound layer. When the travel speed increases from 0.3 to 0.7 m / min, the peak temperature decreases from 850 to 680, and holding time more than 500 shortage at the weld interface.

15 Layer B : Variation in composition of the intermetallic compound layer is seen as stepped lines. From the position of stepped lines, it is suggested that most intermetallic compound layer is formed mainly by brittle FeAl. Electron-probe microanalysis (EPMA) Laser peak power = 2.0 kw Travel speed = 0.5 m / min Overlapped width = 3 mm Roll pressure = 150 MPa GI GI // A6000 Fe Al Layer A : Fe decreases rapidly, and Al rises. Signal intensity A B C O Layer C : Fe decreases further, and Al rises. Zn 10μm GI A6000

16 ~Electron-probe microanalysis (EPMA) Laser peak power = 2.0 kw Overlapped width = 3 mm Roll pressure = 150 MPa GI GI // A6000 Fe Al Fe Al Fe Al Signal intensity O Signal intensity O Signal intensity O Zn 10μm Zn 10μm Zn 10μm GI A6000 GI A6000 GI A6000 (a) Travel speed = 0.3 m/min (b) 0.5 m/min (c) 0.6 m/min it is suggested that most of intermetallic compound layer are brittle FeAl 3. When the travel speed is faster than 0.6m / min, zinc can be seen in aluminum alloy base metal.

17 Interdiffusion coefficient [ ] Interdiffusion Coefficient (m 2 /s) Temperature ( ) Al in Fe Zn in Fe Fe in Al Zn in Al

18 Laser peak power = 2.0 kw Travel speed = 0.5 m / min ~Ultrafine Vickers hardness measurement Overlapped width = 3 mm Roll pressure = 150 MPa GI GI GI // A Hv 940 Hv 94 Hv A6000 Vickers hardness FeAl Fe 2 Al FeAl 470 Fe 3 Al 330 It is thought that FeAl 3 and Fe 2 Al 5 are mainly formed at the interface. M.Yasuyama, et al., 1996

19 Method of tensile shear test Tensile Tensile shear shear test test specimen specimen Set-up Set-up of of tensile tensile shear shear test test specimen specimen

20 Laser peak power = 2.0 kw Overlapped width = 3 mm ~Effect of travel speed on tensile strength Roll pressure = 150 MPa GI GI // A6000 Failure in base metal of zinc coated steel Failure in base metal of zinc coated steel Failure in interface Failure in interface Tensile load (N) Failure in interface 10μm Interface layer thickness (μm) Travel speed (m/min) Failure in base metal When intermetallic compound layer was less than 10 μm, specimens were failure in base metal of zinc coated steel. 3mm

21 Method of Erichsen cupping test Set-up Set-up of of Erichsen Erichsencupping cupping test test specimen specimen Cupping height was evaluated as Erichsen value.

22 ~Erichsen cupping test Laser peak power = 2.0 kw Travel speed = 0.5 m/min Overlapped width = 3 mm Roll pressure = 150 MPa Base metal GI Laser Roll Welded joint GI Failure Erichsen value = 11.9 mm A6000 A6000 HAZ failure Erichsen value = 7.9 mm Failure Erichsen value = 8.6 mm 2 mm

23 Conclusions The present study is focused on joining a dissimilar metal combination of zinc coated steel and 6000 series aluminum alloy. (1) Increase in travel speed led to decrease the thickness of the intermetallic compound layer at the interface. (2) Increase in travel speed led to lowering of the peak temperature and shortening of the holding time more than 500 at the interface. (3) It is suggested that intermetallic compound layer that is formed mainly is brittle FeAl 3. As the travel speed is faster than 0.6m/min, zinc is confirmed in aluminum alloy. (4) When intermetallic compound layer was less than 10μm, failure of specimen occurred at base metal of zinc coated steel in tensile shear test. The thickness of intermetallic compound layer can be controlled by increasing the travel speed, and the tensile load of welded joints have increased.

24 Thank you very much for your kind attentions!

25 How to distinguish IMC Table Composition of compounds formed between iron and aluminum Compound at% Fe at% Al wt% Fe wt% Al Fe 3 Al FeAl FeAl Fe 2 Al FeAl

26 Fe-Zn binary equilibrium diagram

27 Comparison of physical properties Al Fe Zn Atomic number Atomic mass Crystal structure fcc bcc hcp Melting point Boiling point Density of solid Mg / m Specific heat J / kg K Thermal conductivity W / m K Coefficient of linear expansion / K 900~1076 (20~400 ) 238 (20~400 ) (20~400 ) 444~791 (20~800 ) 73.3~29.7 (20~800 ) (20~800 ) 389~444 (20~400 ) 113~96 (20~400 ) (20~300 )