When butt-welding metal sheets with high power lasers the gap
distance between the sheets determine the final quality of the
seam. In a number of systematic laboratory experiments the
critical gap distance that results in sound beads is identified.
By grinding the edges of the sheets, a number of "reference" welds
are made and compared to sheets with the edges shear cut. The gap
distance is precisely controlled by inserting spacers between the
sheets. In the tests the gap is set at 0.00, 0.02, 0.05, 0.08 and
0.10 mm. Mild steel (St 1203) with thickness? of 0.75 and 1.25 mm
with and without zinc coating were analysed. A total of 120 welds
are made at different welding speeds.As quality norm DIN 8563 is
used to divide the welds into quality classes. Since this norm
only deals with surface defects a number of welds are also x-ray
photographed.According to DIN 8563 the welds have classes of
either B, C or D where B is best. If a weld falls outside of these
classes it is termed class XX.Of the weld combinations analysed 80
% are of class B and 17 % of class XX. Thus the welds are either
good or bad. Of the bad welds identified it is evident that the
gap distance plays a major role. 90 % of the bad welds have a gap
distance larger than 0.05 mm. The results also show that 85 % of
the bad welds are shear cut and only 15 % ground. Furthermore the
results show that two third of the bad welds are zinc coated.X-ray
pictures revealed that welding at "low" welding speeds (2 m/min.
for 0.75 mm and 1 m/min. for 1.25 mm sheets) results in a
characteristic "Christmas tree" structure, useless for production.
Thus, these welding speeds should be avoided.
|Period||01/01/1998 → …|