Showcase of laser welding of AHSS research

Background information

Laser weldingIn recent years advanced high strength steel (AHSS) use has increased in popularity in the automotive industry due to their excellent combination of high strength and ductility. Use of AHSS allows the parts to be formed with complex shapes and good energy absorption from thinner gauges than was possible from conventional steel. This down-gauging of parts assists with the need for vehicle weight reduction and improved safety. The unique microstructures, which generate the attractive properties of AHSS, make it a potential candidate for manufacturing energy absorbing components for vehicles. In spite of several advantages, the main issue with AHSS is the weldability due to high alloying addition, a microstructure containing metastable phases, and the presence of zinc coating, which is common to the majority of thin (< 2 mm) automotive steels. Amongst all the AHSS only dual-phase (DP), transformation induced plasticity (TRIP), and more recently, press hardenable steel (PHS) are being used for body-in-white fabrication. Martensitic steels are typically used only in applications requiring high strength and low formability, such as bumper beams.

For AHSS welding in automotive industry one of the technically important products is laser welded blanks (LWB), also known as tailor welded blank (TWB). LWBs are composed of two or more sheets of similar or dissimilar materials, thicknesses and/or coating types welded together, which are formed to fabricate the required three dimensional automotive body parts. Use of LWBs can reduce weight and minimize part cost minimization through better material utilization. Generally, LWBs are made with mild or interstitial free steels; however, in recent years the weight reduction ability of LWBs have been increased by employing high strength steels like high strength low alloy (HSLA) steel and AHSS which also improves the crash performance of the blanks. So, it is known that laser welding is an important process in automotive manufacturing which needs further understanding and research.

Current research at Centre for Advanced Materials Joining focuses broadly on characterization of microstructure and evaluation of properties (hardness, tensile and fatigue) of AHSS and HSLA LWBs. This is because LWBs represent the majority of the automotive laser welding applications. The LWBs are made using IPG YLS-6000 fiber laser and Nuvonyx ISL-4000L diode laser integrated with pneumatic Panasonic robotic arms. LWBs are almost exclusively welded as butt joints. In addition, formability of TWBs have also been explored using Hecker’s limiting dome height (LDH) test set up and compared with numerical simulation of the stretch forming process.

Objectives

  1. To characterize the microstructure and mechanical properties of the LWBs of various AHSS and HSLA steel
  2. Fundamental understanding of the softening phenomenon i.e. martensite tempering occurring in the sub-critical heat-affected zone (HAZ) of DP and martensitic steel LWBs
  3. To study the effectiveness of high speed fiber laser welding in minimizing the HAZ softening in dual-phase steel and assess the subsequent enhancement in properties
  4. Investigation of the fatigue behaviour of the LWBs as a function of the differences in properties, coating, and/or thicknesses of the AHSS
  5. To examine the effect of weld line position and geometry on fatigue and forming behaviour of LWBs

Equipment

  1. IPG YLS-6000 fibre laser
  2. ISL-4000L diode laser
  3. Optical microscope
  4. Microhardness tester
  5. JEOL 6460 Scanning electron microscope
  6. Instron tensile testing machine
  7. Limiting dome height test machine
  8. Circular grid marking (etching) system
  9. Double action MTS 866 metal form press
  10. FMTI Grid Analyzer (Model 100)
  11. FE software: Hypermesh, Hyperform and lsdyna

Pictures/figures

(Further details can be found in the Publications section)

Microstructure and hardness

An overview of DP980 steel LWB sheet weldment showing different zones

Figure 1 Overview of DP980 steel LWB sheet weldment showing different zones

The effect of welding speed on hardness profile across DP980 steel fiber laser welds

Figure 2 Effect of welding speed on hardness profile across DP980 steel fiber laser welds

Fusion zone microstructure developed in fiber laser welding of similar and dissimilar combination LWBs of DP980 and HSLA steels

Figure 3 Fusion zone microstructure developed in fiber laser welding of similar and dissimilar combination LWBs of DP980 and HSLA steels

Two graphs depicting the variation of fusion zone hardness in laser welding with distance from weld centerline (in the left graph) and the fusion zone carbon content (in the right graph)

Figure 4 Variation of fusion zone hardness in laser welding with (a) distance from weld centerline and (b) the fusion zone carbon content

Tensile and fatigue testing

A graph depicting the stress-strain curves of transverse specimens of DP980 steel LWBs prepared by fiber laser welding

Figure 5 Stress-strain curves of transverse specimens of DP980 steel LWBs prepared by fiber laser welding

Strain localisation in softened zone observed in both experiment and FE simulation

Figure 6 Strain localisation in softened zone observed in both experiment and FE simulation

The S-N curves of transverse specimens of HSLA and DP980 steel LWBs prepared by fiber laser welding (S - single linear weld, M - multiple linear welds)

Figure 7 S-N curves of transverse specimens of HSLA and DP980 steel LWBs prepared by fiber laser welding (S: single linear weld, M: multiple linear welds)

Formability

The stretch formed DP980 LWBs with different weld line positions and geometries. (Failure location is always at the soft zone i.e. sub-critical HAZ)

Figure 8 Stretch formed DP980 LWBs with different weld line positions and geometries (failure location is always at the soft zone i.e. sub-critical HAZ).

The biaxial stretch formed HSLA-DP980 (dissimilar combinations) LWBs prepared by diode laser welding

Figure 9 Biaxial stretch formed HSLA-DP980 (dissimilar combinations) LWBs prepared by diode laser welding

Predicted thickness contours by FE simulation of deformed LWBs (HSLA-DP980)

Figure 10 Predicted thickness contours by FE simulation of deformed LWBs (HSLA-DP980)