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

Transcription

1 ALAW2007 Laser Roll Welding of Dissimilar Metal Joint of Zinc Coated Steel and Aluminum Alloy Hitoshi Ozaki and Muneharu Kutsuna Nagoya University, Japan Shigeyuki Nakagawa and Kenji Miyamoto Nissan Motor Co., Ltd., Japan

2 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

3 1.1 Background ~Issues in automotive industry~ Low Low fuel fuel consumption by by lightening Safety improvement Concept of hybrid body structure An An example of of hybrid body structure 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 to aluminum by fusion welding.

4 1.2 Fusion welding of steel to aluminum Problems in in fusion fusion welding of of steel steel to to aluminum Brittle intermetallic compounds (IMC's) are formed at weld interface. Ductile IMC s Brittle IMC s M.Yasuyama, et al., 1996 Brittle IMC's should be minimized by application of Laser Roll Welding.

5 1.3 Objectives of this study 1) Laser Roll Welding of zinc coated steel to aluminum alloy was investigated to make a reliable dissimilar joint. The following process parameters should be controlled. Parameter tree treeof of LRW 2) The effects of the process parameters on the joinability, the formation of IMC layer and the mechanical properties of joints should be made clear.

6 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

7 2.1 What is Laser Roll Welding? Laser Roll Roll Welding facility Laser Roll Welding was developed for joining of dissimilar metals by M.Kutsuna, M.Rathod and A.Tsuboi in A pressure roller was mounted on a 2.4 kw pulse CO 2 laser facility. The thermal cycle for joining should be shortened by laser heating with a low heat input. Because, the formation of brittle IMC s should be avoided. Furthermore, good contact of two metal sheets and rapid heat transfer from a upper sheet to a lower sheet are conducted in one time by this pressure roller.

8 2.2 Characterization of Laser Roll Welding Classification of of welding processes 4. Laser Roll Welding 1. Fusion 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. 3. Brazing Joint that uses brazing filler metal of low melting point for faying surface. By heating only metal A and pressurizing the both metals, the heat is conducted to metal B. Molten phase is formed at the faying surface and joining is finished by cooling. It It is is not not pertinent to to any any among among left left three three welding processes. It It should should be be classified as as the the fourth fourth welding category.

9 2.3 Joining mechanisms of Laser Roll Welding Type ⅠⅠ Combination of metals with large difference in their melting points. Type Ⅱ Combination of metals with eutectic reaction at lower temperature. ex) Fe Al ex) Ti Fe Only the metal of a high melting point is heated up to elevated temperature for avoid brittle IMC s. The interlayer is formed and cooled rapidly to minimize the thickness. The eutectic reaction occurs at a lower temperature than those of both metals. The interlayer of the eutectic phase is formed.

10 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

11 3.1.1 Materials Zinc coated steel Dimension : mm GI type of galvanized steel sheet (= GI) Table 1 Chemical compositions and coating weight of steel sheet Material C Elements (mass%) 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 compositions of aluminum alloy sheet Material Si Elements (mass%) Mg Cu Al A <0.1 Bal.

12 3.1.2 Process parameters Table Process parameters for Laser Roll Welding Laser type Laser peak power Duty cycles Frequency Beam spot size D LR Pulsed CO 2 Laser 2.0 kw 75 % 150 Hz Major axis : 3.5 mm Minor axis : 2.5 mm 25 mm Travel speed Overlapped width Roll pressure Shielding gas Flux 0.2 ~ 0.7 m/min 3 mm 100 ~ 175 MPa Ar : 25 l/min KAlF 4 :K 2 AlF 5 H 2 O D LR : Distance between Laser beam spot and Roller Surface preparation Top surface of steel sheet : Coated with graphite spray Faying surface of steel sheet : Degreased with ethanol alcohol Faying surface of aluminum sheet : Polished, degreased and coated by flux

13 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

14 Laser peak power = 2.0 kw Travel speed = 0.5 m/min Bead appearance and cross-section 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

15 3.2.2 Microstructures at interface Laser peak power = 2.0 kw Travel speed = 0.4 m/min Overlapped width = 3 mm Roll pressure = 150 MPa Low Low magnification Laser beam GI A mm High High magnification 10μm GI GI GI GI GI IMC layer thickness = A μm A μm A6000 A6000 A μm 9.8μm 4.9μm

16 3.2.3 Results of Electron Probe Micro-Analysis (EPMA) and Vickers hardness Laser peak power = 2.0 kw Travel speed = 0.5 m/min Overlapped width = 3 mm Roll pressure = 150 MPa EPMA Vickers hardness Fe Al GI 137 Hv Signal intensity O 940 Hv Zn 10μm 94 Hv GI A6000 A6000 Vickers hardness FeAl Fe 2 Al FeAl 3 and Fe 2 Al 5 are mainly formed at the interface. FeAl Fe 3 Al M.Yasuyama, et al., 1996

17 3.2.4 EPMA mapping results Laser peak power = 2.0 kw Travel speed = 0.5 m/min Overlapped width = 3 mm Roll pressure = 150 MPa COMP image GI A6000 IMC layer GI A6000 IMC layer Diffused Fe Fe Fe GI GI IMC layer IMC layer Al Al Zn Zn A6000 A6000 Diffused Zn

18 3.2.5 Summary 1) Partial melting and spreading of molten metal on the steel sheet occurs in the aluminum sheet in the observation of the joint crosssection. 2) Existence of IMC layer was confirmed at weld interface. The IMC layer thickness right under the center of the beam spot was the thickest. 3) It is suggested that most of the IMC's are brittle FeAl 3 and Fe 2 Al 5 from the EPMA results and the Vickers hardness measurement. 4) Diffused Fe atoms and Zn atoms were confirmed in the fusion zone of A6000 series aluminum alloy from the EPMA mapping results.

19 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

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

21 3.3.2 Measurement of weld thermal cycle Set-up of of thermal cycle measurement A cone-shaped hole of 2 mm in diameter was drilled at the center of the overlapped width in the aluminum sheet.

22 3.3.3 Thermal cycle during Laser Roll Welding Laser peak power = 2.0 kw Travel speed = 0.5 m/min Overlapped width = 3 mm Roll pressure = 150 MPa Measurement result Schematic drawing Stage Ⅰ Ⅱ Ⅲ Ⅳ 800 A Temperature ( ) B C 0 Stage Stage Ⅰ: Ⅰ: Rapid Rapid heating heating of of steel steel surface surface Time (sec) Stage Stage Ⅱ: Ⅱ: Stage StageⅢ: Ⅲ: Evaporation Evaporation of of Heat Heat conduction conduction zinc zinc // Heat Heat by by roll roll pressure pressure conduction conduction to to Al Al (Rapid (Rapid cooling) cooling) Stage StageⅣ: Ⅳ: Natural Natural cooling cooling

23 3.3.4 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) When the travel speed increases from 0.3 to 0.7 m/min, three different thermal cycles were obtained. Thermocouple (Pt-13%PtRh,φ=0.3mm) Measured Measured position position Point aimed at: 1. Peak temperature 2. Holding time more than 500

24 3.3.5 Change of peak temperature and holding time more than 500 Laser peak power = 2.0 kw Overlapped width = 3 mm Roll pressure = 150 MPa GI GI // A6000 Peak temperature ( ) Peak temperature Holding time Travel speed (m/min) Holding time more than 500 (sec) Thermocouple (Pt-13%PtRh,φ=0.3mm) Measured Measured position position The thermal cycle at the interface affects on the formation of the IMC 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 is shortened at the weld interface.

25 3.3.6 Results of 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 IMC layer thickness decreases when the travel speed increases from 0.3 to 0.6 m/min. When the travel speed is faster than 0.6m/min, zinc can be seen into aluminum alloy base metal.

26 3.3.7 Summary 1) Increase in the travel speed led to decrease of the thickness of the IMC layer at the interface. 2) Thermal cycles of Laser Roll Welding has three inflection points. 3) When the travel speed increases from 0.3 to 0.7 m/min, the peak temperature decreases, and holding time more than 500 is shortened at the weld interface. 4) When the travel speed is faster than 0.6 m/min, zinc is diffused confirmed into aluminum alloy.

27 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

28 Laser peak power = 2.0 kw Travel speed = 0.5 m/min Effect of roll pressure on IMC layer thicknessoverlapped width = 3 mm IMC layer thickness (μm) GI GI // A Roll pressure (MPa) IMC layer thickness decreases from 10 to 4 μm when the roll pressure decreases from 150 to 100 MPa. Observed Observed position position 10μm

29 3.4.2 Rolling reduction measured Laser peak power = 2.0 kw Travel speed = 0.5 m/min Overlapped width = 3 mm Definition of of rolling reduction GI GI // A6000 R = t o t R : Rolling reduction t o : Original thickness t f : Final thickness t o f Rolling reduction (%) GI sheet A6000 sheet Roll pressure (MPa) When the roll pressure increases from 100 to 175 MPa, the rolling reduction of both GI sheet and A6000 sheet increase.

30 3.4.3 Discussion about roll pressure process Low Low roll roll pressure High High roll roll pressure Heat conduction to aluminum alloy starts to occur at the point A. Heat conduction to aluminum alloy doesn t start to occur at the point A, but at the point A.

31 3.4.4 Summary 1) Decrease in the roll pressure from 150 to 100 MPa led to decrease the thickness of the IMC layer at the interface. 2) Increase in the roll pressure from 150 to 175 MPa led to decrease the thickness of the IMC layer at the interface. 3) When the roll pressure increases from 100 to 175 MPa, the rolling reduction of both GI sheet and A6000 sheet increase.

32 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

33 3.5.1 Method of Erichsen cupping test Set-up of of Erichsen cupping test test specimen Cupping height was evaluated as Erichsen value.

34 3.5.2 Results of 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 Roll Welded joint joint Failure GI Erichsen value = 11.9 mm A6000 A6000 HAZ failure Erichsen value = 7.9 mm Failure Erichsen value = 8.6 mm 2mm

35 3.5.3 Method of tensile shear test Tensile shear test test specimens Set-up of of tensile shear test test

36 Laser peak power = 2.0 kw Overlapped width = 3 mm Effect of travel speed on tensile load Roll pressure = 150 MPa GI GI // A6000 Failure Failure in in base base metal metal of of zinc zinc coated coated steel steel Failure Failure in in interface interface Tensile load (N) Travel speed (m/min) IMC layer thickness (μm) 10mm When the travel speed increase from 0.3 to 0.6 m/min, the IMC layer thickness decreases, and the tensile load increases.. 3mm

37 Laser peak power = 2.0 kw Travel speed = 0.5 m/min Effect of roll pressure on tensile load Overlapped width = 3 mm GI GI // A6000 Failure Failure in in base base metal metal of of zinc zinc coated coated steel steel Failure Failure in in interface interface Tensile load (N) Roll pressure (MPa) IMC layer thickness (μm) 10mm All specimens were failure in base metal of zinc coated steel except for the roll pressure 100 MPa. 3mm

38 Laser peak power = 2.0 kw Overlapped width = 3 mm Effect of IMC layer thickness on tensile load GI GI // A6000 Failure Failure in in base base metal metal of of zinc zinc coated coated steel steel Failure Failure in in interface interface 4000 Tensile load (N) μm IMC layer thickness (μm) Failure in base metal Failure in interface 10mm When IMC layer thickness was less than 10 μm, specimens were failure in base metal of zinc coated steel. 3mm

39 3.5.7 Summary 1) Specimens failed in the aluminum HAZ was obtained in the Erichsen cupping test. 2) Increase in the travel speed led to decrease of the thickness of the IMC layer at the interface, and the tensile load of welded joints increased. 3) All specimens were failure in base metal of zinc coated steel except for the roll pressure 100 MPa. 4) When the IMC layer thickness was less than 10 μm, failure of specimen occurred at base metal of zinc coated steel in tensile shear test.

40 Contents Introduction What is is Laser Roll Welding? Laser Roll Welding of of GI GI steel to to A6000 Al Al alloy Experimental procedure Bead appearance and and cross-section of of LRW welds Effect of of travel speed on on interlayer of of joint joint Effect of of roll roll pressure on on interlayer of of joint joint Mechanical properties of of Welded joint joint Conclusions

41 4. Conclusions The present study is focused on joining a dissimilar metal joint of zinc coated steel and 6000 series aluminum alloy. 1) Increase in travel speed led to decrease of the thickness of the IMC layer at the interface. 2) It is suggested that most of the IMC's are brittle FeAl 3 and Fe 2 Al 5. 3) Decrease in the roll pressure from 150 to 100 MPa led to decrease of the thickness of the IMC layer at the interface. 4) When IMC layer was less than 10μm, failure of specimen occurred at base metal of zinc coated steel in tensile shear test. The thickness of IMC layer can be controlled by the travel speed and the roll pressure, and the tensile load of welded joints can be increased.

42 Thank you very much for your kind attentions!