Microhardness Survey Research Articles

Microstructure and mechanical properties in friction stir welded 5A06 aluminum alloy thick plate

Friction stir double-sided butt welding was carried out for the joining of 5A06 aluminum alloys with thickness of 108mm, and sound welds with full penetration were produced. Microstructure characterizations of the welding joint were investigated by optical microscopy, electron back-scattered diffraction and transmission electron microscopy (TEM). The results indicate that void defects are observed in shoulder-affected zone (SAZ) and grain size of equiaxed grains in SAZ and weld nugget zone (WNZ) decreases gradually along the thickness direction, while in SAZ the dislocation density is significantly lower and the sub-grain size is much larger than those in WNZ. Microhardness surveys reveal that on the upper part of the joint hardness profile presents a “V” shape and minimum hardness appears in SAZ whereas that in other regions exhibits a “W” shape and the lowest hardness value occurs in the heat-affected zone adjacent to the advancing side. The investigation of tensile properties on the FSW joint indicates the WNZ exhibited superior tensile properties compared with SAZ, and the analysis of fracture surface using the scanning electron microscopy (SEM) demonstrates that fractures in all tensile specimens are ductile fracture due to the presence of dimples with different sizes.

A comparative evaluation of microstructural and mechanical behavior of fiber laser beam and tungsten inert gas dissimilar ultra high strength steel welds

Abstract The influence of different welding processes on the mechanical properties and the corresponding variation in the microstructural features have been investigated for the dissimilar weldments of 18% Ni maraging steel 250 and AISI 4130 steel. The weld joints are realized through two different fusion welding processes, tungsten inert arc welding (TIG) and laser beam welding (LBW), in this study. The dissimilar steel welds were characterized through optical microstructures, microhardness survey across the weldment and evaluation of tensile properties. The fiber laser beam welds have demonstrated superior mechanical properties and reduced heat affected zone as compared to the TIG weldments.

Influence of flash trap profiles on joint properties of friction welded CP-Ti tube to 304L stainless steel tube plate using external tool

Titanium tube and stainless steel tube plate were welded by an innovative friction welding of tube to tube plate using an external tool (FWTPET). Copper was used as an interlayer for joining the dissimilar materials and also to minimize the effect of intermetallics formed at the joint interface. The process parameters that govern FWTPET process are plunge rate, rotational speed, plunge depth, axial load and flash trap profile. Among them, the flash trap profile of the tube has a significant influence on the joint integrity. Various flash trap profiles like vertical slots, holes, zig-zag holes, and petals were made on the titanium tube welded to the stainless steel tube plate. Macroscopic and microscopic studies reveal defect-free joints. The presence of copper interlayer and intermetallics was evident from X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) studies. The microhardness survey was presented across and along the interface. A novel test procedure called “plunge shear test” was developed to evaluate the joint properties of the welded joints. The highest shear fracture load of 31.58 kN was observed on the sample having petals as flash trap profile. The sheared surfaces were further characterized using SEM for fractography.

Effect of enhanced cooling on microstructure evolution of alloy 718 using the gas tungsten arc welding process

The microsegregation in alloy 718 fusion zone during gas tungsten arc welding affected the aging response and the mechanical properties. The average weld cooling rate was enhanced in gas tungsten arc (GTA) welding process by employing liquid nitrogen during the process. The combination of in-process and external cooling methods reduced the microsegregation of Nb in alloy 718 fusion zone. Detailed characterization of the weld microstructures in the as-welded and the direct aged conditions was conducted. The dendritic structure have been refined to a level of 10-20 μm dendrite arm spacing and Laves particle sizes ranging from 0.04 to 1.43 μm due to the enhanced weld cooling rate. Similarly, the average Laves volume fraction was reduced from 45 to 0.1 % when compared to the conventionally cooled GTA welding process. The microsegregation was quantified through the computed solidification time through the DSC analysis and the volume fraction of the eutectic phases. It was found that the solidification time was reduced significantly from 2.499 s to as low as 0.36 s with the liquid nitrogen-cooled GTA welding process. The microhardness survey revealed an improved aging response of the fusion zone.

Analysis and Characterization of the Role of Ni Interlayer in the Friction Welding of Titanium and 304 Austenitic Stainless Steel

Joining of commercially pure Ti to 304 stainless steel by fusion welding processes possesses problems due to the formation of brittle intermetallic compounds in the weld metal, which degrade the mechanical properties of the joints. Solid-state welding processes are contemplated to overcome these problems. However, intermetallic compounds are likely to form even in Ti-SS joints produced with solid-state welding processes such as friction welding process. Therefore, interlayers are employed to prevent the direct contact between two base metals and thereby mainly to suppress the formation of brittle Ti-Fe intermetallic compounds. In the present study, friction-welded joints between commercially pure titanium and 304 stainless steel were obtained using a thin nickel interlayer. Then, the joints were characterized by optical microscopy, scanning electron microscopy, energy dispersive spectrometry, and X-ray diffractometry. The mechanical properties of the joints were evaluated by microhardness survey and tensile tests. Although the results showed that the tensile strength of the joints is even lower than titanium base metal, it is higher than that of the joints which were produced without nickel interlayer. The highest hardness value was observed at the interface between titanium and nickel interlayers indicating the formation of Ni-Ti intermetallic compounds. Formation these compounds was validated by XRD patterns. Moreover, in tensile tests, fracture of the joints occurred along this interface which is related to its brittle nature.

Influence of Process Parameters on Aluminium Alloy 6061-O Joints by Friction Stir Welding Process

The present work reports the influence of process parameters (tool speed, traverse speed and axial load) on the metallurgical and mechanical properties of joints fabricated by friction stir welding process. The samples were friction stir welded under tool rotational speed of 1120, 1250 & 1500 rpm and traversing speed of 0.5, 0.6 & 0.8 mm/sec with an axial load of 9000 Kg constant for all the trials. Single pass friction stir welding process was followed in this investigation. Metallographic analysis was carried out across various zones of weldments (Unaffected Base Metal, Heat Affected Zone, Thermo Mechanically Affected Zone and Nugget Zone). Mechanical tests (microhardness survey & tensile test) were evaluated and all the results were correlated with the parent base metal. For better understanding scanning electron microscope (SEM) analysis was carried out in the nugget zone to predict the flow of materials due to severe stirring process. It is observed from the results that joint fabricated with a tool speed of 1250 rpm, traverse speed of 0.8 mm/sec and axial load of 9000 Kg showed a defect free joint, with better refined grain structures and high tensile strength when compared to all other trials done.

Microstructure and mechanical properties of friction stir welded AA7075–T651 aluminum alloy thick plates

Friction stir butt welding of AA7075–T651 plates with thicknesses of 10 and 16 mm was investigated. Defect-free, full-penetration welds were obtained after careful process parameter selection. While the nuggets in both welds exhibited very fine recrystallized grains, and finer grains were observed in welds made on 10 mm thick plates. Microhardness surveys revealed that significant loss in hardness occurs in the heat-affected zone. The reduction in hardness due to the welding process is higher in the case of welds made on 16 mm thick plates. Welds made on 10 mm thick plates exhibited superior tensile properties compared with those made on 16 mm thick plates. Fracture during tensile test occurred in the heat-affected zone in both cases. TEM images of specimens revealed that the heat-affected zone consisted of widened precipitate-free zones along grain boundaries and partial dissolution of precipitates in the grain interiors. It is concluded that defect-free single pass welds can be made on AA7075–T651 thick plates using friction stir welding and the welds made on 10 mm thick plates exhibit high joint efficiency.

Characterization of an Undermatched Weld

Undermatched welds are becoming more and more common as the tensile strength of the base metal used in industry continues to increase. This paper shall discuss the metallurgical characterization of an undermatched weld in an alloy steel with approximately 170 ksi typical tensile strength. The characterization included magnetic particle and radiographic inspection of the welded plate, a metallographic examination, microhardness surveys, tensile testing, bend testing, and fractographic examination of the tensile specimens.

Characterization of Joint Developed by Fusion of Aluminum Metal Powder through Microwave Hybrid Heating

Metallurgical joining of high thermal conductivity materials like aluminum has been technically challenging. This paper illustrates a novel method for joining of bulk metallic materials through microwave heating. Joining of aluminum alloy in bulk form has been carried out using microwave energy in a multimode applicator at 2.45GHz and 900W. SiC was used at both top and bottom of the workpiece as susceptor material to facilitate microwave hybrid heating (MHH). Aluminum in plate forms have been successfully joined through microwave heating within 600 s of exposure time. A sandwich layer of aluminum powder with approximately 0.5mm thickness was introduced between the two candidate surfaces. Near complete melting of the powder particles in the sandwich layer does take place during the microwave exposure leading to metallurgical bonding of the bulk surfaces. Characterisation of the joints has been carried out through microstructure study, elemental analysis, phase analysis, microhardness survey, X-ray diffraction (XRD) pattern indicates that some aluminum powder particles got transformed into aluminum oxides. A dense uniform microstructure with good metallurgical bonds between the sandwich layer and the interface was obtained. The hardness of the joint area was observed to be 72.4 ± 10 Hv.

Studies on autogenous laser welding of type 304B4 borated stainless steel

304B4 Borated austenitic stainless steel is widely used in the nuclear industry due to high neutron absorption efficiency. In the present investigation, autogenous bead-on-plate (BoP) laser welding studies were carried out on 3mm thick 304B4 grade stainless steel using a 3.5kW slab CO2 laser. Influence of variables such as laser power, welding speed, shielding gas and laser beam mode on microstructure and mechanical properties were studied. Dye penetrant testing, macrostructural analysis, bead geometry measurements, microhardness survey, and microstructural analysis in both as-weld and post-weld heat treated conditions were carried out. The macrostructural and bead geometry analyses of the welds have shown that the welds were free from cracks in the fusion zone (FZ) and also in the heat affected zone (HAZ) for all the welding parameters studied. The Gaussian mode has given a very narrow weld width compared to donut mode. During welding use of helium and nitrogen has reduced the width of the FZ and HAZ. The as-weld micro hardness was more than double the base metal, and the peak hardness was shifted from the centre to the fusion boundaries with the increase in heat input. The PWHT has reduced the hardness of both the FZ and HAZ. In summary, usable laser welding parameters for welding 3mm thick 304B4 grade stainless steel have been identified.

Friction Welding of Titanium to 304 Stainless Steel with Electroplated Nickel Interlayer

Friction welding is a solid state joining process and it is best suited for joining dissimilar metals. It overcomes the problems associated with the conventional fusion welding processes. The joining of dissimilar metals using fusion welding processes produce brittle intermetallic precipitates at the interface which reduce the mechanical strength. Various aerospace, nuclear, chemical and cryogenic applications demand joints between titanium and stainless steel. Direct joining of these metals results in brittle intermetallics like FeTi and FexTiy, at the weld interface, which is to be avoided in order to achieve improved properties of the joints. Present study involves joining of two industrially important dissimilar metals such as commercially pure titanium and 304 stainless steel by friction welding with electroplated nickel coating as interlayer that can prevent the brittle intermetallic formation. Microstructural details of the interfaces of the friction welded joints were studied by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) technique and X-ray diffraction (XRD). Microhardness survey was carried out across the joints and tensile test was conducted to assess the mechanical properties of the joints. Fractography studies were carried out on the fracture surfaces of the joints to know the region of failure as well as the mode of failure. XRD patterns indicate the presence of intermetallics in the friction welded joints. These two metals were successfully joined by having electroplated nickel as interlayer. The weld interface on titanium side contained Ti-Ni intermetallics layers, in which the hardness of the weld metal showing the higher value than the base metals. Fractography study conducted on the fracture surfaces created due to pull test reveals that the failure is by brittle fracture and occurred at the intermetallics layer. The maximum strength of the joints achieved for 30 μm and 50 μm thick electroplated nickel interlayers are 242 MPa and 308 MPa, respectively.

Strain Rate Dependency of Bronze Metal Matrix Composite Mechanical Properties as a Function of Casting Technique

Strain rate dependency of mechanical properties of tungsten carbide (WC)-filled bronze castings fabricated by centrifugal and sedimentation-casting techniques are examined, in this study. Both casting techniques are an attempt to produce a functionally graded material with high wear resistance at a chosen surface. Potential applications of such materials include shaft bushings, electrical contact surfaces, and brake rotors. Knowledge of strain rate-dependent mechanical properties is recommended for predicting component response due to dynamic loading or impact events. A brief overview of the casting techniques for the materials considered in this study is followed by an explanation of the test matrix and testing techniques. Hardness testing, density measurement, and determination of the volume fraction of WC particles are performed throughout the castings using both image analysis and optical microscopy. The effects of particle filling on mechanical properties are first evaluated through a microhardness survey of the castings. The volume fraction of WC particles is validated using a thorough density survey and a rule-of-mixtures model. Split Hopkinson Pressure Bar (SHPB) testing of various volume fraction specimens is conducted to determine strain dependence of mechanical properties and to compare the process-property relationships between the two casting techniques. The baseline performances of C95400 bronze are provided for comparison. The results show that the addition of WC particles improves microhardness significantly for the centrifugally cast specimens, and, to a lesser extent, in the sedimentation-cast specimens, largely because the WC particles are more concentrated as a result of the centrifugal-casting process. Both metal matrix composites (MMCs) demonstrate strain rate dependency, with sedimentation casting having a greater, but variable, effects on material response. This difference is attributed to legacy effects from the casting process, namely, porosity and localized WC particle grouping.

Autogenous laser welding investigations on modified 9Cr–1Mo (P91) steel

Modified 9Cr–1Mo steel plates of 6 mm thickness have been laser welded using CO2 laser. The effects of beam intensity and overall heat input (168-1500J/mm) on the bead characteristics, microstructure and mechanical properties of the welds have been investigated by varying the laser welding parameters such as laser beam mode, power and welding speed. The microhardness survey carried out on the welds after post-weld heat treatment did not reveal any soft zones in the intercritical heat affected zone for welds made with a heat input of up to 420 J mm−1. The tensile strengths of the welds were comparable to that of the base material. Charpy impact tests on subsize specimens revealed that the welds have good toughness. δ-Ferrite was observed in the fusion zone of the welds made at heat input of 700 J mm−1 and above, the content of which increased with the increased heat input.

A new approach to joining of bulk copper using microwave energy

Metallurgical joining of high thermal conductivity materials like copper has been technically challenging. This paper illustrates a novel method for joining of bulk metallic materials through microwave heating. Joining of copper in bulk form has been carried out using microwave energy in a multimode applicator at 2.45 GHz and 900 W. Charcoal was used as susceptor material to facilitate microwave hybrid heating (MHH). Copper in coin and plate forms have been successfully joined through microwave heating within 900 s of exposure time. A sandwich layer of copper powder with approximately 0.5 mm thickness was introduced between the two candidate surfaces. Near complete melting of the powder particles in the sandwich layer does take place during the microwave exposure leading to metallurgical bonding of the bulk surfaces. Characterisation of the joints has been carried out through microstructure study, elemental analysis, phase analysis, microhardness survey, porosity measurement and tensile strength testing. X-ray diffraction (XRD) pattern indicates that some copper powder particles got transformed into copper oxides. XRD analysis also reveals that the dominant orientation (3 1 1) in starting copper powder got transformed into a preferential orientation (1 1 1) in the joint. A dense uniform microstructure with good metallurgical bonds between the sandwich layer and the interface was obtained. The hardness of the joint area was observed to be 78 ± 7 Hv, while the porosity in the joint was observed to be 1.92%. Strength character of the copper joints shows approximately 29.21% elongation with an average ultimate tensile strength of 164.4 MPa.

Microstructure, hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.

Study on Chemical Conversion Treatment and Electroless Nickel Plating on AZ91D Magnesium Alloy

AZ91D magnesium alloy were used as the materials in this experiment. The directly electroless nickel plating, the stannate conversion processing and the electroless nickel plating on the surface of the magnesium alloys that have formed the stannate chemical conversion film on their surface were researched. The morphologies, anticorrosion ability and the hardness of plating of the magnesium alloys after chemical conversion processing and plating nickel were analysed by scanning electron microscope, salt droplet experiment, microhardness survey etc. The result of the experiment showed that, the conversion film unions with the substrate well, the conversion coating can avoid intense galvanic corrosion after the expiration of the plating, and enhanced the corrosion resistance of the plating. The nickel plating deposited on the conversion coating has high microhardness which could provide good protection for the magnesium alloys.

Joining of Aluminium Metal Matrix Composites

ABSTRACT Aluminium metal matrix composites (AlMMCs) offer several advantages relative to monolithic aluminium alloys such as high stiffness, strength, wear resistance, low thermal expansion coefficient, etc. However, despite considerable improvements in developing AlMMCs, the lack of reliable joining methods restrict their greater application. Fusion welding of AlMMCs has not proved successful because high temperature nature of the process normally causes unfavorable reactions between the reinforcement and the matrix, leading to the formation of a variety of defects. On the other hand, solid-state welding and diffusion bonding may not be suitable due to the presence of chemically stable surface layer of aluminium oxide, which, being insoluble in aluminium, inhibits metal-to-metal contact during diffusion bonding. Furthermore, diffusion bonding requires a very smooth and clean contact surface, which is difficult to obtain in industrial applications. As an alternative, transient liquid phase (TLP) diffusion bonding, which operates at a lower temperature, can be used to circumvent the problems associated with the oxide layer. The formation of liquid phase (eutectic) can assist the disruption of the oxide layer and promote metallic contact. The composite material used in the present study consisted of 6061 alloy containing 15 volume % of SiC particulates of 23 μm diameter. TLP bonding was carried out at 560 ˚C in argon atmosphere using copper as an interlayer with different pressures and holding times. TLP-bonded AlMMCs were characterized by optical and scanning electron microscopy, microhardness survey, and shear tests. The results indicated that adequate bond strength could be achieved with suitable bonding parameters such as holding time and initial pressure.

Microwave glazing of alumina–titania ceramic composite coatings

The application envelop of atmospheric plasma sprayed ceramic composites can be widened considerably by reducing/eliminating inherent surface defects by treating them through techniques like microwave irradiation. In microwave processing, microwave energy is directly applied to the material. High-frequency microwaves (>1 GHz usually) penetrate into the bulk of the material and the volumetric interaction of the electromagnetic fields with the material results in dielectric (volumetric) heating. This leads to higher heating efficiency with faster processing. Ceramics are transparent to microwaves at low temperatures, however, start absorbing microwaves at higher temperatures resulting change in microstructure and material characteristics. This paper presents microwave processing of atmospheric plasma sprayed alumina–titania ceramic composite coatings in conventional microwave heating system and evaluation of the processed materials through XRD, SEM, microhardness survey and surface finish with illustrations. Results indicate microwave irradiation induces densification of the material and possible flow of dominant gamma-alumina phase that leads to glazing of coated surface. Glazed surfaces exhibit enhanced microhardness as well as surface finish.

Enhanced Wear Resistant Nickel-based Alloy Coatings Produced by High Velocity Oxy-Fuel Spraying

SUMMARYThree Ni-Cr based powders (Meteo 700, Armacor C and PSI-1) have been thermally sprayed using the HVOF process. The resultant deposits were then characterised using SEM, TEM and XRD. Microhardness surveys were carried out on the coatings together with abrasive wear tests. Coatings 250 μm thick, were produced with low oxides and porosity levels. The most consistent coatings were prepared from powder sources with a minimum particle size of 25 μm. Coating microstructures were either nano-crystalline or mixtures of amorphous and microcrystalline phases. Boron additions to the powder encouraged the formation of amorphous coatings and higher microhardness values i.e. > 675 Hv. The formation of boride precipitates in the coatings does not appear to raise their hardness as the precipitates were either of larger diameter > 500 nm or present in tow volume fractions. Limited amounts of oxides were produced in the coatings, e.g. as α-Cr2O3 or spinel oxide NiCr2O4 particularly at intersplat boundaries. Abrasive wear resistance of the PSI-1 coating was superior to the other two coatings because it contains more amorphous material. The combination of wear and corrosion resistance associated with these coatings may prove to be attractive in many applications where both these properties are needed, carbides and borides.

Investigation into properties of laser welded similar and dissimilar steel joints

Laser beam welding is currently used in the welding of steels, aluminium alloys, thin sheets, and dissimilar materials. This high power density welding process has unique advantages of cost effectiveness, deep penetration, narrow bead and heat affected zone (HAZ) widths, and low distortion compared to other conventional welding processes. However, the metallurgical and mechanical properties of laser welds and the response of conventional materials to this new process are not yet fully established. The welding process may lead to drastic changes in the microstructure with accompanying effects on the mechanical properties and, hence, on the performance of the joint. The thermal cycles associated with laser beam welding are generally much faster than those involved in the conventional arc welding processes. This leads to the formation of a rather small weld zone that exhibits locally a high hardness in the case of C–Mn structural steels owing to the formation of martensite. It is currently difficult to determine the tensile properties (full stress–strain curves) of the laser welded joint area owing to the small size(∼V 2·3 mm) of the fusion zone. Complete information on the tensile and fracture toughness properties of the fusion zone is essential for prequalification and complete understanding of the joint performance in service, as well as for conducting a defect assessment procedure on such welded joints. Therefore, an experimental investigation into the mechanical properties of laser welded joints was carried out to establish a testing procedure using fiat micro tensile specimens (0·5 mm in thickness, 2 mm in width) for determination of the tensile properties of the weld metal and H AZ of the laser beam welds. Three similar joints, namely St 37–St 37, St 52–St 52, and austenitic–austenitic, and two dissimilar ferritic–austenitic joints were produced by CO2 laser, using 6 mm thickness plates. The mechanical properties have been examined by microhardness survey and testing of conventional transverse tensile, round tensile, and fiat microtensile specimens. The results for the micro tensile specimens were compared with those for standard round tensile specimens and this clearly showed the suitability of the microtensile specimen technique for such joints.