Abstract
In the automobile industry, significant efforts to reduce the weight of car bodies have been
made with fuel economy in mind. Tailored blanks (TBs) are widely used for automobile
bodies, which achieve not only weight reduction but also maintain crashworthiness and
rigidity for an auto body, hence their application for auto parts will continue increasing in
future.
Most tailored blanks consist of high strength steel sheets welded in a butt joint. The high
strength steel, of which the tensile strength is 590MPa or more, is usually strengthened by
transformation hardening. When the high strength steel sheets are welded, the weld metal
becomes harder than the base metal, due to the heat input and subsequent rapid cooling during
the welding, while the Heat affected zone (HAZ) is possibly caused by temper softening.
There is a possibility of formability changing depending on the hardness distribution in the
weld zone. The hardness distribution varies according to the welding method used for the
tailored blank production. The formability of tailored blanks is known to deteriorate with an
increase in the weld metal hardness. However, the influence of the HAZ hardness on
formability has not been discussed in as much detail.
In the present study, the relationship between the formability and the HAZ hardness change in
the laser welding was evaluated. Melt-run welding with a laser-welding machine was carried
out on high strength steel sheets with a tensile strength of 980MPa. To evaluate the
formability of TBs, the Erichsen test was examined. The height at rupture and the rupture
morphology of the TBs were investigated.
As a result, the width of the weld metal and the HAZ narrowed with the increase in the
welding speed, which led to an improvement in the formability evaluated by the Erichsen test.
In the case of the same width of the weld metal, the formability with the Erichsen test was
found to deteriorate due to the HAZ softening. CAE analysis confirmed that certain amounts
of strain were locally concentrated on the HAZ softening area, which may cause formability
deterioration in the Erichsen test.
Keywords - High Strength Steel, Tailored blanks, laser welding, HAZ softening, Erichsen value