Welding Solutions for Advanced High-Strength Steels Menachem Kimchi 1 A. Peer 1, Y. Lu 1, W. Zhang 1, C. Ji 2, Y. Park 2, T. Abke 3, S. Malcom 3 1 The Ohio State University, 2 Dong-Eui University, 3 Honda R&D Americas, Inc. Sponsored Work by: Honda R&D Americas, Inc. & WorldAutoSteel GDIS2017
Global Formability Diagram for Today s AHSS Grades 3
4 Issues in RSW of AHSS Hot Stamped Boron Steel Case Study 1: Coating Effects Parameter Optimization Technique Case Study 2: Failure Behavior Outline Initial Development of Criterion for Crash Simulation
Failure Modes on Resistance Spot Welds in AHSS Destructive inspection Chisel test Peel test Modes of failure Implied cooling rates (1000s of o C/s) Martensite formation FB PIF FIF 5
In AHSS applications, the acceptable strength can be achieved with PP or IF failure Button pulled without evidence of interfacial fracture Interfacial fracture with button pull Fracture Behaviors of RSW Partial thickness fracture with button pull Interfacial fracture with partial thickness fracture Partial thickness fracture with no button pull Full interfacial Fracture Interfacial fracture with button pull and partial thickness fracture No fusion 6
Presence of tensile stress Liquid Metal Embrittlement (LME) in AHSS Presence of liquid metals(low melting alloy from coating) Susceptible Microstructure(Austenite in TRIP,TWIP Steels) 7
Possible Solutions to Improve Failure Mode in AHSS Long weld time Pre/post pulsing Down sloping Short hold time Weld and temper Pulsation (0.87 mm, 980 MPa) 8 Dilution (AHSS to Mild Steel, HSLA) Increased minimum weld size
Case Study 1 Hot Stamped Boron Steel Coating Effects Parameter Optimization Technique C. Ji, M. Kimchi, Y. Kim and Y. Park, "The application of pulsed current in resistance spot welding of zn-coated hot-stamped boron steels," in Advances in Resistance Welding, Miami, FL, 2016. 9
Previous Research of Hot-Stamped Boron Steel Al-Si coated Hot stamped boron steel High speed Camera 10
Analysis of Hot-Stamped Boron Steel Coating Layer (Zn) 11
Pulsed Current Approach Based on Coating layer behavior & Contact area during Initial welding time Heat Generation Pattern & Nugget formation Nugget growth and Expulsion behavior 12
Welding Current (ka) Welding Current (ka) Heat generation pattern of 1 st Pulse 1 st Pulse Welding Time(Cy) Welding Time(Cy) 13
1 st Pulse Coating melting behavior & nugget formation of 1 st pulse 14
4 ka, 3 cycle 1 st Pulse - Results 8 ka, 1 cycle Molten Coating layer Molten B.M 15
Welding Current (ka) Welding Current (ka) 2 nd Pulse Comparison of heat generation pattern by 2 nd pulse 4kA 25cy 5.5kA 10cy 2m m 2m m Welding Time(Cy) Welding Time(Cy) 16
2 nd Pulse Comparison of nugget diameter and contact diameter by 2 nd a pulse b 17
Diameter (mm) 2 nd Pulse - Results Comparison of nugget diameter and contact diameter by 2 nd pulse 8 a 6 4 2 b a b 18 0 4 6 8 10 12 14 16 18 20 22 Welding time (cycle)
3 rd Pulse 19
Welding Current (ka) Button Diameter (mm) 3 rd Pulse Optimized welding conditions using three pulsed current steps 6.5 6.0 5.5 5.0 8kA-1Cy 4.5 4.0 1.5 ka 2Cy 5.5kA- 12Cy 2Cy 5.0-6.5 ka 15Cy Welding Time(Cy) 3.5 4.5 5.0 5.5 6.0 6.5 7.0 Weld Current (ka) 20
Welding Current (ka) Optimized Conditions 8kA-1Cy 5.5kA- 12Cy 5.0-6.5 ka 15Cy Welding Time(Cy) 21
Hot Stamped Boron Steel Failure Behavior Case Study 2 Initial Development of Criterion for Crash Simulation Andrea Peer, Ying Lu, Tim Abke, Menachem Kimchi, and Wei Zhang "Deformation Behaviors of Subcritical Heat Affected Zone of Ultra-high Strength Steel Resistance Spot Welds." in 9th International Seminar & Conference on Advances in Resistance Spot Welding. Miami, (3 2016). Paper No. 12 Ying Lu, Andrea Peer, Tim Abke, Menachem Kimchi, and Wei Zhang "Heat-Affected Zone Microstructure and Local Constitutive Behaviors of Resistance Spot Welded Hot-Stamped Steel." in Sheet Metal Welding Conference XVII. Livonia, (10 2016). 22
UHSS Performance Testing Approach Objective: Develop an understanding of the deformation and failure behavior of resistance spot welds of ultra-high strength steels 23
Metallographic Characterization The spot weld microstructure is highly inhomogeneous 24
Temperature Profile Martensitic Microstructure Base Metal Weld Nugget Coarse Grain HAZ Fine Grain HAZ Unique Microstructure Subcritical HAZ 25
Hardness Profile A ring of softened material surrounds the weld nugget Subcritical Heat Affected Zone (SCHAZ) 0.7 mm Usibor BM & WM: > 500 HV SCHAZ: ~300 HV 26
Ncorr Post-Processing Extended Stress Strain Curve Constitutive Behavior Development Example of SCHAZ tensile test. Virtual extensometer of 2 mm used to extract data. 27 YS (MPa) UTS (MPa) Base Metal 1179 1464 CGHAZ 1342 1811 SCHAZ 618 866
Single-Sided Wedge Testing Observe the localized deformation with the aid of DIC 28
Weld Size Results Interfacial Weld Metal WM/CGHAZ SCHAZ 4 t minimum nugget diameter 29 5 t minimum nugget diameter No interfacial failure when weld diameter > 5.8 mm No defined trend for non-if failure mode
Incorporating softened SCHAZ properties is essential to predict the localized deformation FEA Comparison Wedge Test FEA simulation with SCHAZ flow FEA Small simulation Weld with SCHAZ Large Weld flow DIC results 30 No strain localization without incorporating SCHAZ flow stress Effect of nugget size on local deformation Small weld (5.5 mm): Concentrated on notch Large weld (7.00 mm): Concentrated on SCHAZ
As-welded Lap Shear Difficult to observe localized deformation Failure in Hot Stamped Boron Steel Microstructurespecific properties needed to simulate actual failure behavior 31
Three common problems associated with welding AHSS: Narrow Current Range: Summary By implementing the proper welding schedule, the weldability window can be broadened. More complex welding schedules can allow for more control over heat generation and current density, thus minimizing the possibility of expulsion to grow the nugget further. Hardness Values: The increased hardness values seen in AHSS weld nuggets can induce interfacial failure. Acceptable strengths can be achieved with interfacial failure. Nonhomogeneous Microstructures: The nonhomogeneous microstructures seen in AHSS welds cannot be universally classified as a beneficial or detrimental effect of welding. Solutions need to be made on a case by case study and cannot be blanketed over all advanced high-strength steels. Different solutions will need to be taken for different grades as well as different stack-up configurations and sheet thicknesses. 32
For More Information Menachem Kimchi The Ohio State University 614-270-4296 kimchi.4@osu.edu Andrea Peer The Ohio State University 614-716-9692 peer.22@osu.edu 33
Base Metal Characterization Unaffected Base Metal: Fully martensitic microstructure with fine martensite laths Hot stamping heat treatment prior to welding Heated above the austenitization temperature (Ac3) Austenite completely transformed into martensite, which is supersaturated in carbon 34
Weld Metal Characterization Solidified Weld Metal: Fully martensitic microstructure with fine martensite laths Molten weld nugget solidifies rapidly during welding Austenite completely transformed into martensite, which is supersaturated in carbon 35
CGHAZ Characterization Coarse-Grained Heat Affected Zone: Fully martensitic microstructure with fine martensite laths Heated above the austenitization temperature (Ac3) for an extended period of time Large austenite grains completely transform into martensite 36
SCHAZ Characterization Subcritical Heat Affected Zone: Ferrite grains and cementite precipitates Ferrite Cementit e Cementite precipitates decorate along the prior austenite grain boundaries and along the inter-lath regions of martensite Over-tempering of martensite decomposition of metastable martensite into ferrite and cementite. Heated to peak temperatures below 37
INPUT Y. Adonyi. Heat-Affected Zone Characterization by Physical Simulations, Welding Journal, 2006 38 Microstructure Simulation SCHAZ span < 1 mm CGHAZ span < 0.6 mm Very difficult to measure local mechanical properties of this region Gleeble Simulation: Creates a bulk homogeneous SCHAZ microstructure to extract mechanical property data Heating Rate Holding Time Holding Temperature Cooling Rate CGHAZ SCHAZ
Weld Size Study Objective: Determine effect of weld size on failure mode. 39 Weld Sizes: Below Min 4.0-4.1 ka Small 4.8-4.9 ka Medium 5.6-5.7 ka Large 6.3-6.4 ka Near Exp 6.7-6.8 ka
Interfacial Failure Weld Metal SCHAZ Peak Load Failure Peak Load End of Test Peak Load End of Test 0.13 0 -.13 40
Cooling Rate (C o /s) Key Challenges with AHSS - Joining Selection of Joining Technique Body-in-White Joining Processes Resistance spot Projection welding GMAW MIG brazing Laser welding (TWB) Mechanical fastening Magnetic pulse welding Deformation resistance welding 100000 10000 1000 100 10 IF AKDQ M1400 TRIP 600 DP 980 DP 600 TRIP 800 DP 780 GMAW LBW RMSW RSW 0 0.5 1 1.5 2 2.5 Sheet Thickness (mm)
Cooling Rate (C o /s) Key Challenges with AHSS - Joining Selection of Joining Technique Body-in-White Joining Processes Resistance spot Projection welding GMAW MIG brazing Laser welding (TWB) Mechanical fastening Magnetic pulse welding Deformation resistance welding 100000 10000 1000 100 10 IF AKDQ M1400 TRIP 600 DP 980 DP 600 TRIP 800 DP 780 GMAW LBW RMSW RSW 0 0.5 1 1.5 2 2.5 Sheet Thickness (mm)