Resolidified Zone Research Articles

Study on the microstructure and mechanical performance for integrated resistance element welded aluminum alloy/press hardened steel joints

Resistance element welding (REW) is a recently developed hybrid joining process for aluminum (Al)/steel dissimilar materials. In this study, the microstructure and mechanical performance of the Al/press hardened steel (PHS) joints generated using an integrated REW process were investigated. The microstructure of fusion zone (FZ) between rivet and PHS was lath martensite, which was transformed from the fast cooling of austenite. The Al could be divided into four zones according to the microstructure and microhardness distribution: re-solidified zone (RZ), softening zone (SZ), transition zone (TZ) and base metal (BM). The dissolution and coarsening of the precipitates are responsible for the hardness reduction of Al sheet. The variation of mechanical performance was explained in light of the increasing FZ size and the softening Al sheet as heat input rising. Moreover, four block shear models were introduced to predict the peak load of REW joints using the average hardness of SZ in Al sheet, among which the models from Architectural Institute of Japan (AIJ) provided a relatively good prediction. Considering both the average hardness of SZ and sheet thickness, an analytical model was established to predict the variation of critical nugget sizes and explain the failure mode transition.

Ring-like laser spot welding of Ti grade2 to AAl3105-O using AlSiMg filler metal

A pulsed Nd:YAG laser welding of 1 mm thick Ti (grade 2) to 0.5 mm thick AA3105-O was performed by a ring-like spot joining. A 0.15 mm thick Al-12Si-2.5 Mg alloy was used as a filler metal. Intermetallic compounds of Ti3Al, Ti2Al, TiAl, Ti(Al,Si)3 and Mg2Si were observed respectively from Ti towards Al sides at the fusion zone. Si and Mg increased the strength of the Al re-solidified zone near the Ti-Al interface because of their easy dissolution in the Al. Exothermic effect of the Mg addition resulted in porosity in the welding zone. Joint strength of up to 98% of the Al base metal was obtained, which was much 14% higher than that achieved by autogenous welding.

Experimental investigation on electrical discharge diamond grinding of RB-SiC ceramics

The reaction-bonded silicon carbide (RB-SiC) ceramics are very difficult to be machined by conventional techniques. In this work, a hybrid process termed electrical discharge diamond grinding (EDDG) was applied to the precision grinding of RB-SiC ceramics considering its weak electrical conductivity. Focus of investigation was especially placed on the effects of grit sizes and polarities of grinding wheel on the surface quality. The machined surface texture and roughness were measured by confocal scanning laser microscope. The surface texture topography and cross-section profile at different grit sizes and polarities were analyzed. The results show that the dominant state of grinding or EDM decides surface texture of machined surface and consequently affects the surface roughness. The surface morphology and subsurface damage were measured by scanning electron microscopy and energy dispersive spectrometer (EDS). The influence of grit sizes and polarities of grinding wheel was focused on the resolidified zone of the machined surface. The material migration phenomenon was found. The subsurface damage was also analyzed by detecting the polished cross-sections, and the depth of subsurface was determined by analyzing the element distribution with EDS spectrums. At last, the formation mechanism of machined surface and the effects of grit sizes and polarities of grinding wheel on machined surface quality were discussed from the viewpoint of discharge energy based on EDM theory.

Microstructural development of electron beam processed Al-3Ti-1Sc alloy under different electron beam scanning speeds

A systematic study has been made of the microstructural development in as-cast 30mm thick Al-3Ti-1Sc (wt%) plate samples that were processed by electron beam (EB) melting at different scanning speeds of 3, 5, 8, 12, 15 and 20mm/s. The composition of the alloy is designed to capitalize on the potential for significantly extended solubility values of both Sc and Ti in Al when cooled at high cooling rates. The resulting microstructures are characterised using transmission electron microscopy (TEM) and other analytical means, assisted with Thermo-Calc predictions and Vicker's microhardness measurements. EB scanning speed plays a key role in determining the phase formation and microstructural development in the Al-3Ti-1Sc alloy when the beam is applied an accelerating voltage of 50kV and a current of 30μA. The major microstructural features in the re-solidified zone include (i) the formation of the primary tetragonal Al3(Ti,Sc) phases and their retention or subsequent transformation to stable cubic Al3(Ti,Sc) phases; (ii) the complete suppression of the primary tetragonal Al3(Ti,Sc) phases at the scanning speed of 20mm/s; and (iii) the formation of cubic Al3(Ti,Sc) precipitates in the α(Al) matrix supersaturated with both Sc and Ti. These experimental findings are informative for both EB processing of Al-Ti-Sc alloys and the design of new Al alloys for additive manufacturing processes.

Effects of Electricity Parameters on Surface Quality of Reaction Bonded Silicon Carbide Ceramic with Electrical Discharge Diamond Grinding

With some conductivity and low grinding affectivity, a hybrid machining process termed electrical discharge diamond grinding (EDDG) is applied to the precision machining of reaction bonded silicon carbide (RB-SiC) ceramic. As there is electrical spark in the hybrid machining process, the electrical parameters are varied to explore their effects on the surface quality of RB-SiC ceramic with EDDG. In this paper, the experiments of different polarity and gap voltage with EEDG were investigated, and the microstructure and surface roughness on the machined surface of RB-SiC ceramic were analyzed. The surface morphology and micro-cracks were examined with a scanning electron microscope, and the surface roughness was measured with a confocal scanning laser microscope. It is found that surface roughness initially increases and then decreases with increase of the gap voltages and is higher with negative polarity than that with positive polarity. The micromorphology Micro-cracks were observed on the surface machined and are outstanding in re-solidified zone with EDDG.

Microstructure change in the interface of co2 laser welded zirconium alloys

Welding is a joining procedure that offers some benefits over mechanical fasteners such as weight reduction and absence of notches induced by machining operations.CO2 laser beam welding with a continuous wave is a high energy density and low heat input process. The result of this is a small heat-affected zone (HAZ), which cools very rapidly with very little distortion, and a high depth-to-width ratio for the Welding is a necessary process during fabricating fuel rods and fuel assemblies with Zircaloy-4 cladding, and electron beam welding is one of the commonly- used method.In this work, the joining of zirconium alloys was attempted by laser beam welding. A 2 kW CO2 laser is used and the joints are obtained from similar materials, which are plates of Zircaloy-4 (2 mm thick).A series of zirconium alloys were welded and investigated in a tow-fold approach: (1) process optimisation: the laser processing parameters are optimized to obtain welds with minimum defects, and (2) material characterisation: weld microstructures were evaluated.The microstructure and the phases present in the resolidified zone of the laser -welded specimens were analyzed by optical and scanning electron microscopy, X-ray diffraction, and also by the realization of micro hardness diagrams. A particular attention was made to study the correlation between surface structure and mechanical behaviour.

Pitting and galvanic corrosion behavior of laser-welded stainless steels

Autogenous welded specimens of austenitic (S30400 and S31603), duplex (S31803) and super duplex (S32760) stainless steels were fabricated by laser penetration welding (LPW) with a CW Nd:YAG laser in an argon atmosphere. The microstructure and the phases present in the resolidified zone of the laser-welded specimens were analyzed by optical microscopy and X-ray diffractometry, respectively. The pitting and galvanic corrosion behavior of the stainless steels in the laser-welded and unwelded conditions in 3.5% NaCl solution at 23 °C was studied by means of electrochemical measurements. X-ray diffraction analysis showed that the phases present in the weld metal depended on the composition of the base metal. While the laser weld for S31603 retained the original austenitic structure, the laser weld of S30400 contained austenite as the major phase and δ-ferrite as the minor phase. On the other hand, a slight change of δ-ferrite to austenite ratio was found in both the laser welds of S31803 and S32760, with austenite present at the δ-ferrite grain boundaries. The welds exhibited passivity but their pitting corrosion resistance was in general deteriorated as evidenced by a lower pitting potential and a higher corrosion current density compared with those of the unwelded specimens. The decrease in pitting corrosion resistance of the welds was attributed to microsegregation in the weld zone of S31603, and to the presence of δ-ferrite in S30400. For the duplex grades S31803 and S32760, the disturbance of the ferrite/austenite phase balance in the weld zone could be the cause of the decrease in corrosion resistance. The initial free corrosion potentials of the unwelded specimens were considerably higher than those of the corresponding laser welds, indicating that the welds were more active and were expected to act as anodes in the weldment. The ranking of galvanic current densities ( I G) of the couples formed between the laser-welds (LW) and the as-received (AR) specimens with area ratio 1:1, in ascending order, is: AR S31603/LW S31603 < AR S31803/LW S31803 < AR S32760/LW S32760 < AR S30400/LW S30400. The recorded I G in all couples was low (in the range of nA/cm 2).

Significance of melt-fraction in HVOF sprayed hydroxyapatite particles, splats and coatings

Microstructure characterization and property evaluation of high velocity oxy-fuel (HVOF) sprayed hydroxyapatite (HA) splats and coatings were conducted in the present study as a function of the proportion of melting that occurred in HA particles during HVOF spray. In vitro behavior of single and folded HA splats in simulated body fluid was also investigated. Results showed that phase composition of as-sprayed HA coatings was influenced significantly by the melt fraction in HVOF sprayed particles. Melt fraction of the HA powders were experimentally determined from particle morphology analysis. It was found that the spray parameters and starting powder size influenced the melt fraction of the particles. In vitro investigation of individual HA splats made from different HA particles revealed decisive role of local phase composition in influencing their dissolution/precipitation behavior during the test. Furthermore, Raman spectroscopy qualitative inspection on the sprayed HA particles (partial melted) revealed that thermal decomposition occurred within the melted part rather than the unmelted zone. Young's modulus and micro-hardness of the as-sprayed particles and coatings were determined using nano-indentation technique. The resolidified zone of the sprayed HA particles exhibited an average Young's modulus value of 41.25 GPa. The measured values ranged from 23.1 to 65.3 GPa. The unmelted part of the HA powders showed a markedly narrower range. Young's modulus value of 83.9 GPa (±9.4 GPa) was recorded for this region. This succinctly highlight the difference between the unmelted region and melted regions of a HA particle. Young's moduli values measured on HVOF coatings were found to mirror the trend found in the spheroidised particles and splats with apt fidelity.

Mechanism of surface influence on dislocation loop yield in copper andCu3Au

A recently developed hybrid binary-collision/continuum model [V. G. Kapinos and D. J. Bacon, Phys. Rev. B 52, 4029 (1995)] of temperature and defect evolution in displacement cascades has been used to investigate the influence of a nearby surface on vacancy defect formation in cascade events. The shape, vacancy, and thermal conditions in cascade zones that melt and intersect the surface during the thermal spike phase have been analyzed. Simplified geometries of such zones have been simulated by molecular dynamics on the basis of these data and the mechanisms of vacancy generation in the melt zone analyzed. It has been demonstrated that additional vacancies are formed in the resolidified zone under the surface through a combination of thermal sputtering and viscous flow of atoms from the cascade core. Quantitative relationships between the geometry and size of the melt zone and number of additional vacancies have been considered, and used in the hybrid model to calculate the depth, yield, and size distribution of vacancy dislocation loops in Cu and ${\mathrm{Cu}}_{3}\mathrm{Au}$ irradiated with 50-keV ${\mathrm{Ni}}^{+}$ ions. The results are in good agreement with experimental data obtained by transmission electron microscopy of ion-irradiated thin foils.

The effect of laser surface nitriding with a spinning laser beam on the wear resistance of commercial purity titanium

Laser nitriding of commercial purity titanium using various concentrations of helium and nitrogen has been carried out. The surface appearance and microstructure of a treated layer were found to be dependent on the beam power density, interaction time, velocity and concentration of nitrogen. X-ray diffraction analyses have led to the conclusion that the dendrite layer in the resolidified zone of the nitrided specimens consisted mainly of TiN. The surface roughness of specimens after various laser treatments was investigated by SEM and a surface profilemeter. Using optical microscopy, the dendrite TiN and needle-like structure in the melt zone, and the large grain structure in the heat affected zone, were investigated. The surface wear resistance of nitriding CPTi was significantly improved compared to the untreated or laser glazed material, and the wear data were found to correlate with scanning electron microscopy observations. Two layers, having different microstructures, thickness and abrasive wear resistance were identified. Further, 100% overlapping considerably improved the wear resistance of the nitrided specimens.

Microstructure, chemical composition and properties of the surface layer of M2 steel after laser melting under different conditions

Metallographical (optical, SEM, TEM), microprobe, wear resistance and microhardness investigations of M2 high-speed tool steel surface melted by continuous CO 2 (with different power) and Nd:YAG pulsed lasers are described. It became apparent that there was a considerable influence of the laser type, scanning velocity, gas jet atmosphere and the antireflective graphite coating on the melted zone dimensions and its as-resolidified structure. The notable differences between the microstructures of the melted zones were related to the laser type and laser power. In comparison to the uncoated specimens, laser melting of the specimens coated with a colloidal graphite layer resulted in significantly increased dimensions of the laser-affected layer and a coarser as-resolidified structure. Depletion of the Cr, V, Mn and C in the resolidified zone after treatment under oxygen atmosphere was related with the high reaction efficiency of these elements to oxygen. Hardness of the melted zone in the uncoated samples was higher than that of both the graphite-coated samples and the conventionally hardened matrix. The lathe tools treated by the Nd:YAG pulsed-laser radiation showed the longest cutting life-time.

High heat flux test on first wall materials

The plasma facing components in a tokamak type fusion reactor are subjected to intense heat load during a plasma disruption. The effects of the resulting high heat flux on the wall materials must be evaluated. The authors performed high heat flux tests with a high-frequency rastering beam as the heat source and thermal and stress analyses to investigate the thermo-mechanical behaviors of candidate wall materials. Based on the results of the studies, melting, evaporation and crack initiation, propagation behaviors of metallic and carbon materials are discussed. The heat transfer enhancement of melted metal motion of metallic materials, and the particle emission of carbon materials is important to the disagreement between experimental and analytical results on melting and evaporation. For stainless steel, a lot of micro-cracks with a depth of about 0.1 mm along dendrites initiated in the resolidified zone. They did not propagate in the non-melted zone. For tungsten, cracks with a depth of a few millimeters initiated from the periphery zone of the heated surface and propagated into the non-melted zone with repeated heating.

Microstructural development during solidification of stainless steel alloys

The microstructures that develop during the solidification of stainless steel alloys are related to the solidification conditions and the specific alloy composition. The solidification conditions are determined by the processing method,i.e., casting, welding, or rapid solidification, and by parametric variations within each of these techniques. One variable that has been used to characterize the effects of different processing conditions is the cooling rate. This factor and the chemical composition of the alloy both influence (1) the primary mode of solidification, (2) solute redistribution and second-phase formation during solidification, and (3) the nucleation and growth behavior of the ferrite-to-austenite phase transformation during cooling. Consequently, the residual ferrite content and the microstructural morphology depend on the cooling rate and are governed by the solidification process. This paper investigates the influence of cooling rate on the microstructure of stainless steel alloys and describes the conditions that lead to the many microstructural morphologies that develop during solidification. Experiments were performed on a series of seven high-purity Fe-Ni-Cr alloys that spanned the line of twofold saturation along the 59 wt pct Fe isopleth of the ternary alloy system. High-speed electron-beam surface-glazing was used to melt and resolidify these alloys at scan speeds up to 5 m/s. The resulting cooling rates were shown to vary from 7°C/s to 7.5×106°C/s, and the resolidified melts were analyzed by optical metallographic methods. Five primary modes of solidification and 12 microstructural morphologies were characterized in the resolidified alloys, and these features appear to be a complete “set” of the possible microstructures for 300-series stainless steel alloys. The results of this study were used to create electron-beam scan speedvs composition diagrams, which can be used to predict the primary mode of solidification and the microstructural morphology for different processing conditions. Furthermore, changes in the primary solidification mode were observed in alloys that lie on the chromium-rich side of the line of twofold saturation when they are cooled at high rates. These changes were explained by the presence of metastable austenite, which grows epitaxially and can dominate the solidification microstructure throughout the resolidified zone at high cooling rates.

Effect of Laser Processing on the Electrochemical Behavior of Fe‐Cr Alloys

Fe‐Cr substrates electroplated with 1–3 μm of chromium were processed with an Nd‐YAG pulsed laser to produce surface alloys with 25–35 μm melt depths. The corrosion resistance of laser‐processed alloys in deaerated solution improved with increasing chromium concentration. The corrosion properties of laser‐processed and bulk alloys were comparable only after oxide was removed from the laser‐processed surface. The resolidified zone consisted of a columnar morphology. Although electron microprobe analysis indicated that compositional uniformity was obtained throughout the melt depth, scanning transmission electron microscopy‐energy dispersive x‐ray spectroscopy (STEM‐EDS) revealed solute concentration inhomogeneities in the upper portion of the resolidified region.

Modelling of the nucleation temperature of the f.c.c. phase competing with glass formation in Cu-Ti alloys

The observed occurrence of a metastable f.c.c. phase competing with glass formation, and also occurring partially with metallic glass, in Cu-Ti alloys subjected to pulsed laser irradiation has been modelled in terms of both thermodynamic and kinetic parameters. The driving free energy ΔGv for the formation of the f.c.c. phase from supercooled liquid alloys can be shown to be the result of a shallow T0 curve extending from the Cu side of the phase diagram and laying substantially above the glass transition temperature (Tg) at all compositions. The dependence of the nucleation temperature Tn upon the cooling rate Ṫ has been modelled for conditions related to substrate epitaxy and also for nucleation in the bulk supercooled liquid. In the former case, when suitable assumptions are made about the values of interfacial energy (γ) and activation energy for diffusion (Q), it has been shown that Tn is profoundly depressed with increasing Ṫ for Ṫ values exceeding 109K/s, the effect being m the middle of the phase diagram, as observed experimentally. When Ṫ is about 1010K/s, Tn, is depressed below Tg over a wide range of compositions, approximately in agreement with the observed glass formation range in the Cn-Ti system. Considerations of possible bulk nucleation (homogeneous or heterogeneous) indicate that the nucleation temperature associated with this type of nucleation must be above the nucleation temperature for substrate-related nucleation. The estimated growth times of the f.c.c. microcrystals competing with glass formation are of the order of microseconds. Combined with the growth velocities (ifνc), which are of the order of meters/sec, they account for the very small average size (100 Å) of the experimentally observed microcrystals. The actual growth velocities may be smaller than the speed (ντ) with which the temperature isotherms traverse the resolidifying zone. A model of growth is proposed which accounts for the observed mixture of f.c.c. crystals and metallic glass in the re-solidified zone.