Shing-Hoa Wang

Tensile properties of LBW welds in Ti-6Al-4V alloy at evaluated temperatures below 450 °C

Abstract The influence of temperatures below 450 °C on the tensile properties of laser beam (LB) welds in dual phase Ti–6Al–4V titanium alloy was investigated. The ultimate tensile strength of the weldment is slightly superior than that of the as-received parent materials. Conversely, the yield stress of the weldment is inferior to that of the parent metal, especially in the 150–450 °C range. The elongation of the weldment was about 5% lower than that of the parent metal for the entire temperature range. The prominent dislocation gliding on the pyramidal planes giving (1011) and (1122) type slip with low critical resolved shear stress leads to both the weldment and parent metal exhibiting the lowest ductility at a temperature range from 200 to 350 °C. The maximum hardness in the fusion zone (FZ) corresponds to the needle-like martensite α′ formed after the postsolidification phase transformation. As the temperature increases, the dimple dimension becomes larger and deeper.

Hydrogen permeation in a submerged arc weldment of TMCP steel

Abstract The effects of the heterogeneous microstructure at the base metal, the heat affected zone (HAZ) and weld metal on hydrogen permeation in thermo-mechanical controlled rolling (TMCP) steel weldments have been investigated. The base metal with equiaxed refined ferrite and scattered fine grain pearlite has the highest permeation rate and effective diffusivity. The HAZ with bainite shows the lowest values of the permeation rate and effective diffusivity. Weld metal yields a higher permeation rate coupled with an intermediate diffusivity value. The hydrogen apparent solubility is low for the base metal, intermediate for the HAZ and high for the weld metal. The mechanisms of hydrogen diffusion path and hydrogen traps are discussed and experimentally confirmed using the hydrogen microprint technique. The high diffusivity paths and the hydrogen trapping site are the grain boundary and the ferrite/carbide interfaces for both the base metal with refined ferrite and the HAZ with bainitic microstructure. The spaces among the basket-weave acicular ferrite, where the martensite and the retained austenite (M/A) constituents present, are the main hydrogen trapping sites for the weld metal.

Cyclic deformation and phase transformation of 6Mo superaustenitic stainless steel

A fatigue behavior analysis was performed on superaustenitic stainless steel UNS S31254 (Avesta Sheffield 254 SMO), which contains about 6wt.% molybdenum, to examine the cyclic hardening/softening trend, hysteresis loops, the degree of hardening, and fatigue life during cyclic straining in the total strain amplitude range from 0.2 to 1.5%. Independent of strain rate, hardening occurs first, followed by softening. The degree of hardening is dependent on the magnitude of strain amplitude. The cyclic stress-strain curve shows material softening. The lower slope of the degree of hardening versus the strain amplitude curve at a high strain rate is attributed to the fast development of dislocation structures and quick saturation. The ε martensite formation, either in band or sheath form, depending on the strain rate, leads to secondary hardening at the high strain amplitude of 1.5%.

Structure and hardness of explosively consolidated molybdenum

Abstract The structure of explosively consolidated molybdenum powder was observed to consist of inhomogeneous mixtures of both very fine grains and coarser grains, ranging from 1 to 25 μm in diameter. Both the fine and the coarse grains contain subgrain structures characterized by grain boundary misorientations averaging 6°; the average subgrain size ranges from 0.1 to 1 μm. The hardness of the explosively consolidated molybdenum powder, about 380 HV 10, is similar to that observed for shock-loaded molybdenum sheet, but the microstructures of the two materials differ. The hardness of the explosively consolidated powder and its change as a function of 1 h annealing temperature were observed to be considerably different from the hardness levels and changes in hardness observed for unsintered molybdenum extrusions and conventionally processed P/M molybdenum bars.

Explosive shock deformation of metallic glasses

Abstract Thin sheets and small wires of Fe80B20 and thin sheets of Fe38Ni40Mo4B18 glassy metals were explosively shock loaded together with thin 304 stainless steel sheets in density-compatible stainless steel assemblies to peak pressures of 15, 25 and 35 GPa at a shock pulse duration of 2 μs. An electron microscopy examination of the shock-loaded sheet materials after shock loading did not show any detectable alteration in the structure for the amorphous metals; this is consistent with a negligible change in the initial hardness. Examination of the shock-loaded Fe80B20 wires in the field ion microscope confirmed this response. By comparison the shockloaded stainless steel exhibited increased hardening after shock loading as a result of the production of dislocations and other crystal lattice defects. Simple isochronal annealing experiments suggested that shock loading of amorphous metals might only have an effect when the associated shock heating is sufficient to cause crystallization to occur, and this might also require a sufficiently long shock pulse duration. For the glassy metals studied, the requisite peak pressure would probably be in excess of 90 GPa. An attempt is made in this paper to provide a crude phenomenological comparison between the shock loading of an amorphous material and a crystalline (or polycrystalline) material.

Effect of initial microstructure on the creep behavior of TMCP EH36 and SM490C steels

Abstract Two steels, TMCP EH36 and SM490C, were prepared with the same composition but different rolling processes. The equiaxed grain of TMCP EH36 steel was produced by thermo-mechanical control rolling (TMCP) with an accelerated cooling process. The banded structure of SM490C steel was produced using a conventional hot rolling process. After creep, the results show that the apparent activation energy and apparent stress exponent in the band structure of SM490C steel are much higher than that in the equiaxed grain of TMCP EH36 steel. The creep stress exponent value from the threshold stress, σ 0 , concept in the dispersion strengthening mechanism can explain how the second phase distribution and morphology have an important effect on the creep behavior in this study.

Tensile properties of gas tungsten arc weldments in commercially pure titanium, Ti-6Al-4V and Ti-15V-3Al-3Sn-3Cr alloys at different strain rates

Abstract The influence of microstructure and strain rate on the mechanical behaviour of three titanium alloys having applications in aerospace, namely, commercially pure titanium (α phase), Ti–6Al–4V (α + β phases) and Ti– 15V–3Cr–3Sn–3Al (β phase) is investigated for both the parent metals and their gas tungsten arc weldments. The results indicate that the tensile strengths of the three as received titanium alloys and their weldments increase with increasing strain rate. However, their elongations decrease with increasing strain rate. The as received Ti–6Al–4V alloy and its weldment, with a mixed α and β phase microstructure, have the maximum strength and microhardness. Commercial purity titanium metal and its weldment exhibit the minimum strength and microhardness. The tough Ti–15V–3Cr–3Sn–3Al alloy and its weldment, having a fully β phase microstructure, appear to have optimum strength and microhardness. The tensile properties of all three titanium alloy weldments are inferior to those of the as received metals.

Sub-micron-structure machining on silicon by femtosecond laser

Sub-micron-structure machining on silicon substrate was studied by direct writing system of femtosecond laser with the central wavelength of 800 nm, pulse duration of 120 fs and repetition rate of 1 kHz. Three kinds of experiments were conducted: 1) the effect of photoresist on silicon; 2) machinability of different orientations of silicon; and 3) the size of micro-structure and the cross-section shape. Photoresist SU8 was coated onto silicon substrates in thicknesses of 100 μm. SU8 remained on the silicon substrate while the silicon under the machined SU8 was removed after laser machining. Orientations of (100), (110), and (111) silicon substrates were machined with the laser power of 60 μW and the scanning speed of 3 mm/min. Spike morphologies were observed on all three orientations of silicon substrates without obvious directional difference of these spikes on different silicon substrates. In addition, the ablation threshold energies were also similar. In the consideration of ablation energy, one numerical model of the machining parameters has been proposed to simulate the cross-section of the micro-structure. The predicted shape by simulation can fit the profile of the cross-section shape well.

Martensite nucleation site and grain refinement in duplex titanium alloy weldment by active flux with nanoparticle addition

Abstract The effects of activating flux containing nanoparticles on the welding microstructure and hardness distribution of titanium alloy were investigated. The fluxes, consisting of NaF with 1 wt-% of either nano-TiO2 or nanodiamond, effectively yielded refined grain size and deep weld penetration. The hardness of the weldment was close to that of the base metal for 99 wt-% NaF plus 1 wt-% diamond, leading to a desirable uniform mechanical property in the weld design. In the meantime, the transmission electron microscopy images illustrate that the nanoparticles promote β phase coarsening and martensitic α′ phase formation at the β veins.

The influence of Sc addition on the welding microstructure of Zr-based bulk metallic glass: The stability of the amorphous phase

Pulsed direct current autogeneous tungsten inert gas arc welding was conducted on rods of bulk metallic glasses (BMGs) Zr55Cu30Ni5Al10 and (Zr55Cu30Ni5Al10)99.98Sc0.02 under two different cooling conditions. The crystalline precipitates in the fusion zone of BMG Zr55Cu30Ni5Al10 were confirmed by microfocused x-ray diffraction pattern analysis as Zr2Ni and Zr2(Cu,Al) intermetallic compounds. In contrast, BMG with Sc addition (Zr55Cu30Ni5Al10)99.98Sc0.02 shows an excellent stable glass forming ability. The fusion zone of BMG (Zr55Cu30Ni5Al10)99.98Sc0.02 remains in the same amorphous state as that of the amorphous base metal when the weld is cooled with accelerated cooling.