A study on the porosity of CO_2 laser welding of titanium alloy

The CO2 laser welding of BT20 titanium alloy and Ti-23Al-17Nb titanium aluminide was conducted to investigate into the porosity in titanium alloy weld. The results show that there are two sorts of porosities observed in welds of titanium alloy laser welding based on the microscopic characteristics of the porosities. One is the metallurgical porosity with round and smooth inner wall, which results from the surface contamination. The other is the processing porosity with irregular and rough inner wall that displays the trace of the pool flowing, which results from the ruffle on the keyhole wall gathering together locally and closing down the gas in the keyhole into bubbles because of the keyhole fluctuating. The CO2 laser welding could break down easily the surface oxide film and produce little metallurgical porosity, but produces easily processing porosity when partial penetration or unstable-full penetration laser welding is conducted, which always occurs in the center of weld.

Laser Welding of TC-1 Titanium Alloy

The technology of CO2 laser welding and joint properties of titanium alloy were investigated. The problem of molten pool protection was resolved by designing a shielding trailer and a special clamp. Joints with silvery appearance were obtained, which have no pore and crack. In addition, the welding speed could reach 3 m/min for the plate of 1.5 mm thickness being penetrated. The reason of the porosity formation in partial penetration joints is that the keyholes can be easily cut apart in the radial direction, which makes the gas enclosed in the molten pool. The surface oxide of specimens can not affect the porosity formation in welds directly.

Weld shape and microstructure of TC4 laser welding with activating flux of Na_(2)SiF_(6)

Na_(2)SiF_(6) was used as surface activating flux for laser welding of TC4 titanium alloy. The effect of Na_(2)SiF_(6) on TC4 titanium alloy laser welding was determined by observing the weld surface. The morphological characteristics of the high temperature plasma above the workpiece was observed and analyzed by using high-speed digital camera system. The variation of weld depth,width and microstructure were analyzed by optical microscope. The experimental results show that laser weld of TC4 titanium alloy has good appearance with activating flux of Na_(2)SiF_(6), weld penetration increases by about 0.8%–12%, while weld surface width decrease by about 10%–29%, the depth to width ratio is effectively improved. The inhomogeneity of weld microstructure was improved, and the crystallization direction of β columnar crystals on the upper part of the weld was changed, the grain size and microstructure of the weld were refined by Na_(2)SiF_(6).

Synthesis of Y_2O_3 particle enhanced Ni/TiC composite on TC4 Ti alloy by laser cladding

A Y2O3 particle enhanced Ni/TiC composite coating was fabricated in-situ on a TC4 Ti alloy by laser surface cladding. The phase component, microstructure, composition distribution and properties of the composite layer were investigated. The composite layer has graded microstructures and compositions, due to the fast melting followed by rapid solidification and cooling during laser cladding. The TiC powders are completely dissolved into the melted layer during melting and segregated as fine dendrites when solidified. The size of TiC dendrites decreases with increasing depth. Y2O3 fine particles distribute in the whole clad layer. The Y2O3 particle enhanced Ni/TiC composite layer has a quite uniform hardness along depth with a maximum value of HV1380, which is 4 times higher than the initial hardness. The wear resistance of the Ti alloy is significantly improved after laser cladding due to the high hardness of the composite coating.

Improving pulsed laser weldability of duplex stainless steel to 5456 aluminum alloy via friction stir process reinforcing of aluminum by BNi-2 brazing alloy

Effects of laser pulse distance and reinforcing of 5456 aluminum alloy were investigated on laser weldability of Al alloy to duplex stainless steel (DSS) plates. The aluminum alloy plate was reinforced by nickel-base BNi-2 brazing powder via friction stir processing. The DSS plates were laser welded to the Al5456/BNi-2 composite and also to the Al5456 alloy plates. The welding zones were studied by scanning electron microscopy, X-ray diffractometry, micro-hardness and shear tests. The weld interface layer became thinner from 23 to 5 μm, as the laser pulse distance was increased from 0.2 to 0.5 mm. Reinforcing of the Al alloy modified the phases at interface layer from Al-Fe intermetallic compounds (IMCs) in the DSS/Al alloy weld, to Al-Ni-Fe IMCs in the DSS/Al composite one, since more nickel was injected in the weld pool by BNi-2 reinforcements. This led to a remarkable reduction in crack tendency of the welds and decreased the hardness of the interface layer from ~950 HV to ~600 HV. Shear strengths of the DSS/Al composite welds were significantly increased by ~150%, from 46 to 114 MPa, in comparison to the DSS/Al alloy ones.

Effect of laser characteristics on the weld shape and properties of penetration laser weld of BT20 titanium alloy

The laser beam welding of BT20 titanium alloy was conducted to investigate the weld shape, microstructures and properties. The full penetration weld characteristics produced by CO_2 laser and by YAG laser were compared. The results show that the full penetration weld of YAG laser welding closes to “X” shape, and weld of CO_2 laser welding is “nail-head” shape. Those result from special heating mode of laser deep penetration welding. The tension strength of CO_2 laser and YAG laser joints equal to that of the base metal, but the former has better ductility. All welds consist mainly of the acicular α phase and a few β phase in microstructure. The dendritic crystal of CO_2 laser weld is a little finer than YAG laser weld. According the research CO_2 laser is better than YAG laser for welding of BT20 titanium alloy.

Effect of undercut defect on deformation behavior TC4 titanium alloy laser welded butt joint under static tensile loading

By measuring and analyzing infrared thermal image of the specimen in static load tensile test process, it was studied that the influence of the undercut defects and double-sided dressing method on the deformation behavior of the laser welded joint specimens of TC4 titanium alloy. The results showed that for the unmodified specimens, the yield phenomenon occurs first in the region of the joint, but the undercut value has an effect on the stress and strain of starting to yield phenomenon, and a great effect on the plastic deformation behavior.When the undercut is less than a certain value, the large plastic deformation occurs in the base metal region and the plasticity of the specimen is comparable to that of the base metal, but the larger undercut defect results in a concentrated plastic deformation in the joint region and rapidly failed in this region. But the double-sided dressing specimen is significantly different. The physical yield is no longer concentrated in the joint region, but at the same time occurs in the several regions including joint and the base metal. And the plastic deformation mainly occurs in the base material area, similar to that of the base material.

Study on Microstructure and Properties of TIG Welded Joint of TC2 Titanium Alloy and Quality Inspection

TIG welding experiments of TC2 titanium alloy sheet was carried out,and the well-formed weld was obtained.After welding process,the cross-section microstructure,mechanical properties,fracture morphology and quality inspection of the joint were studied.The results show that the microstructure of the weld consists of a large number of acicularα′andβblock.The microhardness curve shows that the microhardness value in the fusion zone(FZ)of the joint is significantly higher than that in the heat affected zone(HAZ)and the base metal(BM),and the microhardness of the base metal is the lowest.The tensile strength of the joint is equivalent to that of the base metal,and the fracture morphology shows that the fracture mechanism of the joint is mixed ductile-brittle fracture mode.The weld quality is excellent through chemical inspection,penetrant inspection and X-ray inspection.

High-temperature Tensile Behavior of Laser Welded Ti-22Al-25Nb Joints at Different Temperatures

The high-temperature tensile behavior of laser welded Ti-22Al-25Nb (at%) joints was investigated at 500,650,800,and 1 000 ℃.The temperatures for tensile tests were selected according to the phase transformation sequence of Ti2AlNb-based alloys.At temperatures lower than the B2+O phase field (500 ℃) and higher than the B2+O phase field (1 000 ℃),the joints fracture in the base metal in ductile fracture mode.By contrast,the joints exhibit obvious high-temperature brittleness in the B2+O phase field (650 °C and 800 ℃).Heat treatments were conducted with respect to the thermal history of tensile specimens.Intergranular microcracks along the grain boundary of B2 phase are found in the fusion zone after the heat treatments at 650 ℃ and 800 ℃.The high-temperature brittleness at 650 ℃ and 800 ℃ is attributed to the B2→O transformation along the grain boundary.The stress concentration caused by the volume change of B2→O transformation also contributes to the high-temperature brittleness of laser welded Ti-22Al-25Nb joints.

添加稀土元素的TC4钛合金激光焊接头纵向超塑性能研究

针对钛合金激光焊接接头超塑性能低的难题,提出在焊缝中以Yb_(2)O_(3)的方式添加稀土Yb元素,分析Yb_(2)O_(3)含量对TC4钛合金激光焊接头高温拉伸峰值流变应力、断后伸长率及组织的影响.结果表明,Yb_(2)O_(3)能够促进变形过程的组织等轴化进程,减小焊缝β晶粒尺寸和接头超塑性变形流变应力,并提高断后伸长率.随着Yb_(2)O_(3)含量增加,接头应力应变曲线下移,峰值流变应力呈先降低后升高的趋势,而断后伸长率则为先增大后减小.在超塑性变形过程中,Yb_(2)O_(3)含量6%时峰值流变应力最低为11.9 MPa,断后伸长率为572.3%;焊缝中心β晶粒尺寸最小为127μm,相较于未添加Yb_(2)O_(3)时β晶粒尺寸的337μm细化了62.3%.此时,马氏体组织发生相变,形态转变成片层,焊缝区域组织等轴化程度最高,焊缝的超塑性能最好.

BT20钛合金CO_2激光焊接工艺

通过对BT2 0钛合金CO2 激光焊技术开展试验研究 ,对焊前清理、焊接过程中气体保护、离焦量、激光功率、焊接速度等工艺参数进行了优化选择 ,确定了合理的焊接工艺参数。

Effect of Ti-Hf-Zr-Cu-Ni high entropy alloy addition on laser beam welded joint of Ti2AlNb based intermetallic alloy

TiAlNb based intermetallic alloys,a potential competitor for next-generation super alloys,are susceptible to high-temperature embrittlement due to nucleation of a metastable single B2 phase in the fusion zone(FZ)during laser beam welding(LBW).In this study,a high entropy alloy(HEA),Ti-Hf-Zr-Cu-Ni,was self-developed and introduced as an interlayer into laser beam welded joint(LBWJ)of Ti-22 Al-27 Nb to analyze its impact on the evolution of microstructure in the weld zone(WZ)and subsequently on joint performance.Microstructural examination was carried out through electron probe micro analysis(EPMA),electron backscattered diffraction(EBSD)analysis,high-resolution scanning transmission electron microscopy(HRSTEM)comprising bright field(BF),selective area electron diffraction(SAED)and high angle annular dark-field(HAADF)imaging.Addition of the HEA into FZ of LBWJ triggered heterogenous nucleation during solidification,resultantly,fine-grained B2 with a greater proportion of high angle grain boundaries(HAGBs)was developed.FZ of Ti-22 Al-27 Nb LBWJ,prepared with an interlayer of HEA,was composed of planar,cellular,columnar and equiaxed dendritic grains;a solidification mode which was different from that observed in LBWJ prepared without adulteration of the HEA.The impact of heterogenous nucleation during epitaxial solidification on mechanical properties was established through micro vickers hardness mapping and tensile test,conducted at room temperature.The average hardness,343.5 HV,in the FZ of LBWJ prepared with an interlayer of HEA,was compatible with that of base material(BM),345 HV.The ultimate tensile strength(UTS),1062 MPa,and percentage elongation,11.2%,of the HEA tempered LBWJ were found in close approximation with that of BM,1060 MPa and 13.4%,respectively.A ductile mode of failure was observed during tensile test of the Ti-Hf-Zr-Cu-Ni supplemented LBWJ of Ti-22 Al-27 Nb,while quasi-cleavage mode of fracture was apparent in the joint of Ti-22 Al-27 Nb welded without addition of the HEA.

Microstructural evolution and numerical simulation of laser-welded Ti_(2)AlNb joints under different heat inputs

The influence of heat input on the microstructural evolution of laser-welded Ti_(2)AlNb joints was investigated in this study.The thermal cycles during welding process were analyzed by numerical simulation.In the heat affected zone(HAZ),the amount ofα_(2)and O phases decreased with laser power increasing.During the heating period,α_(2)→B2and O→B2transformations occurred,but the decomposition of the B2phase intoα_(2)and O phases was suppressed during the cooling period.The heat transfer in the HAZ generated more equiaxed B2grains,fewer LAGBs and a weaker{001}<110>texture due to recovery,recrystallization and grain growth.The phase composition of the fusion zone remained single with only the B2phase with the increase in heat input,but the mode of grain growth transformed from cellular growth into cellular dendritic growth.A finite element model was established to simulate the thermal cycles during the welding process.Higher heat input induced higher peak temperature,leading to higher temperatures in the HAZ for longer periods of time,which was beneficial for theα_(2)→B2and O→B2transformations.The calculated cooling rates in both the HAZ and in the fusion zone were faster than the critical cooling rate for B2→α_(2)and B2→O transformations.

稀土Yb_(2)O_(3)对TC4钛合金激光焊接头超塑变形行为的影响

通过添加稀土Yb_(2)O_(3)提高TC4钛合金激光焊缝的超塑性变形能力,改善接头的超塑性变形均匀性。结果表明,Yb_(2)O_(3)的加入能够降低焊缝超塑性变形流变应力,提高延伸率,增加接头变形均匀系数值。随着Yb_(2)O_(3)含量的增加,焊缝纵向峰值流变应力先降低再升高,延伸率先升高再降低。在Yb_(2)O_(3)含量为6%(质量分数,下同)时峰值流变应力最低为11.9 MPa,延伸率最高为592.3%,此时焊缝区域组织等轴化程度最高,焊缝的超塑性最好。横向变形时随着Yb_(2)O_(3)含量的增加,接头变形均匀系数K值呈现先增加后减小的趋势,Yb_(2)O_(3)含量为6%时的试样K值取得最大值为0.209。

CO_2激光焊接铝合金工艺的研究

在自己研制的5kW级横流CO2激光器上采用2～3kW低阶模功率输出,在20～120cm/min焊速下实现了1～4mm厚的6063型号铝合金的熔焊及对焊和1.5～4mm厚的LY12铝合金的对焊。