Investigation mechanical properties and microstructure of pulsed Nd

International Journal of Materials and Mechanics Engineering, Vol. 2, No. 3
Investigation mechanical properties and microstructure of pulsed Nd:YAG
laser welding titanium
Gh. R. Razavi1a, J. Razavi2b, M. Taheri3c, M. Saboktakin1d
1
Department of Materials Engineering, Najafabad Branch, Islamic Azad University, Isfahan, P.O.
Box 517, Iran
2
Noore Hedayat University, Shahre-Kord, Iran.
3
Master of Materials engineering, nonprofitable university of kar Qazvin branch, Iran
a
[email protected], [email protected], [email protected],
d
[email protected]
Keywords:Ti alloys, laser welding, mechanical properties.
Abstract. Titanium
alloys are widely used in many fields such as the automotive, aerospace
and chemical industries. In some applications, Titanium alloys are needed for use with
particular welding methods. This is laser welding. In thisstudy,Titanium Grade-2 (Cp-Ti)
plates were welded using Lase rwelding. Tensile and flexural tests were applied to the
welded samples.The microstructure and SEM images of main material and welded regions
were studied and microhardness measurements were performed. Tensile and flexural
strengths of laser welded samples were higher than Pure Ti welded samples. The
microhardness values of the weld-zone of laser welded samples were higher than the Pure
Ti welded samples welding zone.
Introduction
Titanium has become an attractive material because of its low density and higher mechanical
properties/density ratio. Because of its high strength and high corrosion resistance, titanium
and titanium alloys have become the preferred engineering material in recent years.
However, because of its lower wear resistance it can not be preferred in engineering
applications which include friction and wear[1-5].
Titanium-based materials gain preference by their low density, high strength, high corrosion
resistance and high fracture strength. They are also preferred in theair craft and aerospace
industry, chemica lindustry, medicine, marine, automotive industry, and the biomedical
industry[6,7]. All traditional machining processes are applicable to these materials, but
because of their particular properties these materials have some difficulties in machining
processes. Because of its high corrosion resistance, titanium and its alloys are widely used in
dental implants, chemical industry apparatus, oil industry pipes, ship building, fasteners and
exhaust systems in the automotive industry, jet engines in aircraft, and in the aerospace
industry and construction industries.
Although titanium and alloys have the feature of high corrosion resistance at low
temperatures, it is undesirable to weld the min atmospheric air as they have an increasing
tendency to react with oxygene, hydrogen, carbon and nitrogen[2,8,9]. Titanium is
chemically reactive at high temperatures. While welding, titanium alloys pick up oxygen and
nitrogen from the atmosphere. Choiand Choi investigated the effect of GTA welding
condition according to mechanical properties of pure titanium and they reported that if
oxygen or nitrogen in the air infiltratesin to the WMZ (weldingmetalzone), the hardness of
the titanium welding will increase. Therefore, the welding of titanium requires complete gas
shielding[2]. The welding processes recommended for use when welding titanium and its
International Journal of Materials and Mechanics Engineering, Vol. 2, No. 3
alloys are; tungsten inert gas (GTA) welding, metallic inert gas (MIG) welding, diffusion
welding,(both spot and seam) electron beam welding, friction stir welding and laser
welding[5,10]. Before the welding of titanium and alloys, it is necessary to clean the oxide
layer of the surface, and the welding zone must be prevented from making contact with
atmospheric air. Intitanium built constructions, the material properties, weld abilityand
mechanical behavior ofthe different welding methods must bee vident.
Some research about welding of titanium alloys has been carried out. Akman et al, used
pulsed Nd-YAG laser welding technique to join 3-mm thick Ti6Al4V plane sheets. And they
determined that the ratio between the pulse energy and pulse durationis the most important
parameter in defining the penetration depth[7]. Kahraman et al. investigated the effect of
welding current on the plasma arc welding of pure titanium[8].
In this study, 2.5 mm thick commercially pure titanium sheets (Cp-Ti) were welded by laser
welding and TIG welding methods.The mechanical properties of the titanium sheets which
were welded by different welding methods were investigated.
Experimental
1000x450x2.5 mm sized Titanium Grade 2 sheet which is th ereferred to material in the
studies 250x450x2.5 mm sized piece,which are going to be used in laser welding
125x450x2.5 mm sized pieces, The chemical composition of the titanium sheet (Grade-2) is
shown in Table 1.
Table 1. Chemical composition of Titanium used in the study.
Element
Fe
C
N
O
H
Ti
0.15
0.02
0.02
0.13
0.02
Bal.
The sheets which were used in the laser welding were cuts harply with a wire erosion
cutting machine suitable for DIN EN ISO4136 (2011-05) as tensile and bending samples.
The materials urfaces were cleaned with a solvent and then fixed. The parameters shown in
Table 2 with the pulsed Nd:YAG laser butt-welding without the welding wire were used.
Plates of 125x450x2.5 mmin size were welded with GTA butt-welding.
Table 2.
Laser welding parameters
Pulse Duration
Pulse Repetition Rate
Pulse Energy
Peak Power
Focal Location
Speed
Shielding Gas
5 ms
30 Hz
9 Joule
1,8 kW
1mm
4 mm/sec
Argon
International Journal of Materials and Mechanics Engineering, Vol. 2, No. 3
All tensile tests were carried out by using Shimadzu AG-IS (100kN) tensile testing device
and extensometer. Experiments were conducted at room temperature and 1 mm/min pulling
speed. The bending tests of the samples were done by using a Shimadzu AG-IS100 kN
device, according to support distances of BSEN ISO 5173. The bending speed of the
machine was 1 mm/min.Vickers micro-hardness measurements of the samples are done by
using 300 gf force for 10 seconds 500 µm pause from the welding zone to the main
material. Microscophic studies are done by using Nikon stereomicro scope. Samples are
sanded with 220, 400, 600, 800, 1000 and 1200 grid SiC abrasives. Then they are polished
with 3µ diamond paste. They are etched by using 50 ml H2O, 40 ml HNO3 ve 10 ml HF
solution.
Each tensile test was repeated three times depending on the type of each sample, and then
the average value was taken as the tensile strength.The tensile test data is given in Table 3
& 4. The test results of the samples are shown in Figure 1 & 2. The comperative
microhardness measurement results of the samples are shown in Figure 3.
Specimen
Base Material
Laser Welded
Table 3. Tensile test data’s of specimens
Yield Strength (N/mm2)
Tensile Strength (N/mm2)
368.7±12.5
410.3±7.9
359.6±5.4
388.7±9.6
Figure 4 shows that titanium grade 2 has a fine equiaxed microstructure and Figure 5
shows the joint area structures by the laser welding methods. The surfaces of the welded
joints indicate structures composed of dendritic grains in the center of the welded seam.
SEM studies of laser-welded samples are shown in Figure 6-7. Studies are performed from
welding zone to main metal zone. As a result of the tensile test obtained from SEM studies
of fracture surfaces of samples taken from the exampleis shown in Figure 8. At the end of
the tensile and bending tests. According to the literature, laser welded samples of titanium
alloys have higher tensile strength than Gas Tungsten Arc welded and Electron Beam
welded samples because the welding area of the lase rwelding of titanium alloys has a
narrow range and during welding low heat input is provided [9].
Figure 1.View of specimen after the tensile tests
International Journal of Materials and Mechanics Engineering, Vol. 2, No. 3
Figure 2. Welded seams
Table 4. Bending test data of samples
Flexural Strength
(N/mm2)
Base Material
Laser Welded
759±16.3
747±22.5
The comperative microhardness measurement results of the samples are shown in Figure 3.
The welding zone of Laser welded specimen's hardness is higher than the pure Ti
specimen's hardness. Li et al. Found that the hardness of the weldmetal of Fiber Laser GMA
hybrid welding of commercially pure titaniumis higher than the hardness of the heat
affected zone and the hardness of the base metal part, and he stated that it is related to the
oxygene contact during the welding[1]. Figure4 shows that titanium grade2 has a fine
equiaxed microstructure.
Figure 3.Microhardness measurements graphic
International Journal of Materials and Mechanics Engineering, Vol. 2, No. 3
Figure 4. Microstructure of main material
The micro structure of the laser welded joint consists of fine lamellar and acicular alpha
structure (Figure5).
Figure 5.A) Microstructure of laser welded sample’s main material zone
B) Microstructure of laser welded sample’s welding zone
Akbari mousavi and Gohari Kia investigated the mechanical properties and microstructure
of dissimilar cp-titanium and AISI 316 L austenitic stainless steel continuous friction welds,
and they reported that if oxides on the material are not cleaned before welding, these oxides
penerate in to the welding zone and it causes a reduction in the strength of the joint[6].
International Journal of Materials and Mechanics Engineering, Vol. 2, No. 3
Figure 6. A) Laser welded sample’s welding zone B) Laser welded sample’s heat affected zonemain material zone
HAZ has a much narrower range in laser welded samples (Figure7) and Ductile fracture
occur after the tensile testson the specimens (Figure8).
Figure 7. Laser welded sample’s width of the welding
Figure 8. Fracture surface of main material
Conclusions
In this researchthe pulsed Nd–YAG laser welding technique has beenemployed to join pure
titaniun sheets. The laser-welded samples had a higher number of welding flaws such as lack
of penetration and microcavities. Fracture analysis of the laser-welded regions indicated
brittle fracture, and the control group exhibited a flat fracture surface owing to the ductile
behavior of titanium. The results show that the LBW welded joints have good combination of
International Journal of Materials and Mechanics Engineering, Vol. 2, No. 3
strength and ductility. The process of the LBW welding is proved to be much more feasible
for the production of titanium plate joints.
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