1 research outputs found
Monitoring and characterization of abnormal process conditions in resistance spot welding
Resistance spot welding (RSW) is extensively used for sheet metal
joining of body-in-white (BIW) structure in the automobile
industry. Key parameters, such as welding current, electrode
force and welding time, are involved in the RSW process.
Appropriate welding parameters are vital for producing good
welds; otherwise, undersized weld and expulsion are likely to be
caused. For a specific type of sheet metal, an acceptable nugget
is produced when an appropriate combination of welding parameters
is used. However, undersized welds and expulsion are still
commonly seen in the plant environment, where some abnormal
process conditions could account for the production of the poor
quality welds.
Understanding the influence of abnormal process conditions on
spot weld quality and other RSW related issues is crucial. A
range of online signals, strongly related to the nugget
development history, have attracted keen interest from the
research community. Recent monitoring systems established the
applied dynamic resistance (DR) signal, and good prediction of
nugget diameter was made based on signal values. However, the DR
curves with abnormal process conditions did not agree well with
those under normal condition, making them less useful in
detecting abnormal process conditions. More importantly, none of
the existing monitoring systems have taken these abnormal process
conditions into account. In addition, electrode degradation is
one of the most important issues in the plant environment. Two
major electrode degradation mechanisms, softening and
intermetallic compound (IMC) formation, are strongly related to
the characteristics of welding parameters and sheet metals.
Electrode misalignment creates a very distinct temperature
history of the electrode tip face, and is believed to affect the
electrode degradation mechanism. Though previous studies have
shown that electrode misalignment can shorten electrode life, the
detailed mechanism is still not understood.
In this study, an online-monitoring system based on DR curve was
first established via a random forest (RF) model. The samples
included individual welds on the tensile shear test sample and
welds on the same sheet, considering the airgap and shunting
effect. It was found that the RF model achieved a high
classification accuracy between good and poor welds. However, the
DR signals were affected by the shunting distance, and they
displayed opposite trends against individual welds made without
any shunting effect. Furthermore, a suitable online signal,
electrode displacement (ED), was proposed for monitoring abnormal
process conditions such as shunting, air gap and close edged
welds. Related to the thermal expansion of sheet metal, ED showed
good consistency of profile features and actual nugget diameters
between abnormal and normal welds. Next, the influence of
electrode misalignment on electrode degradation of galvannealed
steel was qualitatively and quantitatively investigated. A
much-reduced electrode life was found under the angular
misalignment of 5°. Pitting and electrode softening were
accelerated on the misaligned electrodes. δ Fe-Zn phase from the
galvannealed layer that extends electrodes was found
non-uniformly distributed on the worn electrode. Furthermore,
electron backscatter diffraction (EBSD) analysis was implemented
on the worn electrode, showing marked reduction in grain diameter
and aspect ratio. The grain deformation capacity was estimated by
the distribution of the Taylor factor, where the portion of
pore grain was substantially weakened in the recrystallized
region compared to the base metal region