Introduction – PHS Technology Trend

? Tailored properties of the Press Harden Steel (PHS) can maximize the crashworthiness of the BIW structure.

Competitive Processes for PHS Tailored Properties

? Auto industry is developing various processes to achieve the PHS tailored properties.

? Laser tempering (LT) is a laser- based heat treating process to alter mechanical properties.
? LT produces local heating below the melting temperature.

? Three key parameters of LT are spot shape, power density, and travel speed.
? LT can lower the hardness and increase ductility with proper heating conditions.

Objectives

? Improve the local ductility of the PHS structural components using laser tempering.
? Evaluate the effectiveness of laser tempering to prevent brittle cracking of PHS materials.
? Predict the failures in buckling and trimmed-hole area during crash testing.

1. Characterize the LT process parameters.
2. Coupon-level mechanical testing.
3. Design effective laser tempering patterns for the PHS component.
4. Component-level mechanical testing.

1. CHARACTERIZING THE LT PROCESS PARAMETERS (SPOT SHAPE, POWER DENSITY AND TRAVEL SPEED)

? IPG 20-kW Yb-Fiber Laser
? 30- ? 30-mm square integrator optic
? Six-axis Fanuc robot

? Power density was controlled to obtain target temperatures of 400, 600, and 800oC.
? Travel speeds of 20, 60, and 100 in./min.
? Temperatures recorded using an infrared (IR) camera.

? An IR camera was used to monitor the temperature history of the part profile before, during, and after the laser beam crossed the targeted tempered area.

2. COUPON LEVEL MECHANICAL TESTING  (Hardness and Tensile Testing)

? Laser heating produced a relatively uniform decrease in hardness across the width of the coupon.
? 600oC showed the greatest effect on reducing hardness.
? 800oC may result in partially reforming martensite during cooling if the peak temperature caused re-austenization.

? During the tensile testing, the Digital Image Correlation (DIC) measurement was conducted to measure the strain field on the tensile specimen.

? Average value of failure strain was calculated from multiple measurements for the CAE.

3. DESIGN THE EFFECTIVE LT PATTERNS FOR THE PHS COMPONENTS.

? The Laser Tempering (LT) pattern was designed by comparing the performance of the structure in bending and axial crush deformations.

? The most desirable LT pattern was found for the maximum displacement without any cracking.

? Heat treat areas carry most of the crash energy (desirable).
? PHS hat section showed a minimum crash energy absorption.

4.  COMPONENT LEVEL MECHANICAL TESTING.

? All the PHS structures showed severe cracking while the LT- PHS structures showed no cracking during the test.

? Axial crush testing was conducted at a low speed in a hydraulic compression machine.
? Transition size between soft and hard zones is <10 mm.

Findings

? LT gave uniform temperature distribution on the heated metal surface.
? Hardness measurement and tensile tests confirmed the effective improvement of ductility of the LT area.
? Laser heating pattern can be optimized for two different deformations such as bending and axial crush.
? The LT-PHS samples showed no cracking in both 3-point bend and axial crush tests.

Conclusions

? LT is a simple and fast process to obtain tailored PHS properties.
? Desirable tailored properties of the PHS part can be obtained by controlling the laser beam power and travel speed.
? LT process unlocks infinite patterns that are very difficult to obtain by alternative processes for PHS- tailored properties.
? LT can be implemented by integrating with the existing laser cutting cell of the PHS production.

Future Work
? Develop a reliable prediction model of the LT-PHS behavior in high-speed axial crush test to optimize the laser tempering pattern.
? Transfer the developed LT process with interested industry partners.
? Investigate the effectiveness of the LT process with 980 and 1180 grade Advanced High Strength Steels (AHSS).