A to less heat input, heat affected zone,

A comparison of pulsed laser and pulsed TIG welding
of Ti-5Al-2.5Sn titanium alloy sheet was studied. For this purpose full
penetration bead on plate weldments of 1.6mm thick Ti-5Al-2.5Sn alpha titanium
alloy sheet was used.Microstructure,micro hardness ,tensile properties,surface
and sub-surface residual stresses distribution and deformation and distortion of
both weldments were studied. Complete ?’ martensitic transformation
in fusion zone in P-LBW and ?’ and acicular ? was formed within
equiaxed ? matrix in P-TIG weldments. Hardness in fusion zone of P-LBW was
higher than P-TIG weldments due to faster cooling rate in P-LBW. Fracture was
occurred in unwelded zone and it was ductile because fusion zone was of higher
hardness value. Distribution of residual stresses in P-LBW was narrower as
compared to P-TIG welding due to less heat affected zone in P-LBW. P-LBW
process was preferred over P-TIG welding due to superior mechanical properties.
Fusion zone of P-TIG welding was experienced wider than P-LBW. It is due to
high heat input in case of P-TIG weldments. And P-LBW has high power density as
compared to P-TIG welding. Equiaxed ? grains was observed coarser in FZ and
HAZ. Microhardness decreased from fusion zone to PM. Microhardness value is
greater for P-LBW due to fast cooling rate as compared to P-TIG welding. For
both types of weldments, fracture occurred at unwelded zone in case of tensile
test which clearly showed the ductile fracture behavior. Residual stresses were
observed more in case of P-TIG weldments due to larger heat affected zone. Non
uniformity is greater in P-LBW due to larger top to bottom width ratio. Overall
P-LBW was preferred due to less heat input, heat affected zone, less
distortions and residual stresses. That’s why P-LBW is used for thin sheets
welding.1

Pulsed Nd-YAG laser welding technique is applied 1.6
mm thick Ti-5Al-2.5Sn alloy sheet. In fusion zone, ?´ and ? phases are produced
and microhardness also increases at fusion zone by almost 55 HV as compared to
base metal. Titanium alloys are corrosion resistant, wear resistant and used
heavily in aerospace industry.Ti-5Al-2.5Sn alloy has many advantageous applications
as compared to Ti-6Al-4V at cryogenic temperature. In ? and ?+? titanium alloys
strengthening occurs by ?´ formation. While in ? titanium alloys ?´ formation
is suppressed by the presence of ? stabilizing elements. It was also concluded
that microhardness depends upon martensite formation. Shielding gas is used to
prevent the surface from oxidation. Residual stresses are produced due to
successive heating and cooling phenomena occurred during thermal cycles applied
in welding. Residual stresses reduce buckling strength, corrosion resistance
and fracture toughness. As annealed sheets of Ti-5Al-2.5Sn alloy of dimensions
100mm×80mm×1.6mm was used for study of different properties. Specimens for metallography
and tensile testing were made by using electric discharge machine (EDM). ASTM
standard E8M-04 was used to make tensile samples of gauge length 25mm. Metallography
was done by using abrasive paper of 4000 grit, then polished by 1µm diamond
suspension paste. For etching purpose kroll reagent (6% HNO3 and 2%
HF was used in balanced distilled water) then in 2% HF in balanced distilled
water was also used after using kroll reagent to more pronounce the phases. Samples
were observed by using optical microscopy of polarization technique. Scanning electron
microscope (SEM) at a vacuum of 10-6 Pascal was used for higher
magnification images. Energy dispersive X-ray (EDX) technique is used to check
the presence of oxygen (wt %). Microhardness decreases from fusion zone to base
metal. Hole drill strain measurement (HDSM) method was used to measure the
residual stresses after welding. Cooling rate directly relates with ? grain
size, as cooling rate increases, hardness increases and hardness increases due
to small ? grain size. High cooling rate is observed at HAZ/FZ boundary as
compared to HAZ/BM boundary. High cooling rate is observed in P-LBW due to very
high power of laser welding technique and also due to pulsed method. In XRD
analysis, high peaks were observed as hexagonal close packed (HCP) structure of
? phase. Hardness order was defined as ?´ martensite>acicular ?>?. They also
showed that during TIG welding, fracture occurs at fusion zone due less
hardness at fusion zone as compared to base metal. And fracture was observed as
ductile rupture due to dimples appeared at fracture surface with small
appearance of brittle fracture of cracks.2

 

1      M.
Junaid, M. N. Baig, M. Shamir, F. N. Khan, K. Rehman, and J. Haider, “A
comparative study of pulsed laser and pulsed TIG welding of Ti-5Al-2.5Sn
titanium alloy sheet,” J. Mater. Process. Technol., vol. 242, no.
November 2016, pp. 24–38, 2017.

2      M. Junaid, F. N. Khan, K.
Rahman, and M. N. Baig, “Effect of laser welding process on the microstructure,
mechanical properties and residual stresses in Ti-5Al-2.5Sn alloy,” Opt.
Laser Technol., vol. 97, pp. 405–419, 2017.