Effect of laser fluence on surface microstructure of alumina ceramic

Abstract

The present study discusses the microstructure development during surface modifications of alumina ceramic using high power continuous wave Nd:YAG laser. Laser fluence influenced the microstructure in terms of changes in morphology and (1 1 0) crystallographic texture of the surface grains. The microstructural observations can be used to establish the guidelines for optimising the laser fluence to achieve the desired morphology of the surface grains and extent of texture in the surface modified alumina ceramic.     

Solidification and microstructure of ZA27/SiCp composite fabricated by mechanical–electromagnetic combination stirring process

Abstract

The solidification behaviours and microstructural characteristics of both ZA27/SiC_p composites and monolithic ZA27 alloy were studied by using differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and X-ray diffraction. It was found that there were differences in the transformation temperature and volume fraction of the phases, although the solidification process was almost identical for the composite and the monolithic alloy. The incorporation of SiC particles in the ZA27 alloy led to slight refinement of primary grains and reduced volume fraction of eutectic-like phase. The SiC particles obstructed Zn diffusion in the residual melt during the formation of proeutectic β phase, but promoted Zn diffusion from (Al) to η (Zn) phase during eutectoid transformation. During solidification, Cu was mainly segregated in the final solidification regions; Mg was present not only in the matrix but also on SiC particles; and oxide inclusions were mainly distributed around SiC particles. The matrix microstructure for both materials mainly consisted of primary cores of Al rich +η eutectoid; β′ phase resulting from the eutectoid transformation of the proeutectic β phase; and Zn rich +η eutectoid resulting from the eutectoid transformation of the eutectic-like phase. The SiC particles were mainly distributed around the primary grains. Several new phases based on the Al–Zn–Mg–Cu system and interfacial reaction products, including Al₂₁Fe₃Si, Cu₅Zn₈, Mg₆Cu₃Al₇, MgAl₂O₄, and amorphous oxide inclusions, were identified in the final solidification regions. The nucleation of both primary phase and eutectic-like +η phase at the surface of SiC particles and their crystallographic orientation relationships were investigated theoretically and experimentally. No distinct crystallographic orientation relationship between the matrix and SiC has been identified, although the mismatch between (0001)_SiC and (111)_ was calculated to be as small as 7·6%.     

New Equipment Materials And Techniques

Abstract

Holographic interferometry has been applied to in situ measurements of vibration fields of large diameter conduits undergoing unsteady internal excitations. The ambient conditions under which the measurements were carried out were extremely difficult. The measurements, covering an area of several square meters with each holographic recording, were performed using a portable one joule pulsed ruby laser system capable of producing two sequential Q-switched pulses each with duration of ～25 ns with a variable pulse separation between 10 and 800 μs. The entire assembly of laser and holographic camera was constructed as a single unit incorporating an internal reference beam; the reference beam included a mirror with the facility to make an angular tilt between the two laser pulses with the objective of providing a facility to obtain information relating to the phase of antinodes within the recorded area of the hologram.     

Plasma channel formation for triggering arc welding

Abstract

A method of starting arc welding using a plasma channel formed between electrodes in a tungsten inert gas arc welding system was demonstrated. The plasma channel was generated by gas breakdown in the laser beam path. In a previous study by the present authors, the arc welding could be started using a laser produced plume. Results in the present study indicated that the laser energy required to start the process using the plasma channel was lower than that using the plume.     

Mechanical and fatigue behaviour of laser and resistance spot welds in advanced high strength steels

Abstract

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2˙0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at 'high load' and 'low load' levels.     

Influence of heating source position and dilution rate in achieving overmatched dissimilar welded joints

Abstract

The investigation addresses the overall performance of black and white joints (BWJ) of low carbon steel (LCS) and stainless steel thin sheets achieved by laser hybrid welding. Assuming that the structural integrity is directly influenced by the processing temperature, a thermal simulation of BWJ of thin sheets was developed. Afterwards, the base metals apportionment at joint formation, namely their distinct dilution rate, was originally estimated from the top surface temperature variation. Defect-free laser hybrid dissimilar welds were experimentally obtained using the selected filler metal and the energetic input from the process simulation, even for a critical analysed case of heat source displacement from the weld gap centreline towards LCS. Detailed macro and microstructural examination of the BWJ and related microhardness analysis results are presented. The tensile tests results indicate that in the case of transversally loaded BWJ, the positive difference in yield between the weld metal and the base materials protects the weld metal from being plastically deformed; the flat transverse tensile specimens loading up to failure reveals large strains in LCS, far away from the weld.     

Keyhole stability in disc laser welding of AZ31B and AZ61A magnesium alloys and weld metal properties

Abstract

To confirm the process potential and limitations of laser welding of magnesium alloys, a newly developed disc laser of 16 kW in maximum power was used to weld thin plates of AZ31B and AZ61A alloys. Melting characteristics and weld metal properties were studied under different process parameters. The very high power density of the applied beam caused the keyhole and the molten pool to be unstable. Underfill was the main welding defect limiting the process window and its formation was closely related to the keyhole instability. A process diagram indicating areas of sound and defected welds could be constructed. Sound welded joints could be obtained for AZ31B alloy with similar mechanical properties as that of the base material. The brittle intermetallics network present along the grain boundaries in AZ61A weld metals required a higher energy input to alleviate its detrimental effect on the mechanical properties of the welded joints.     

Fluxless laser beam joining of aluminium with zinc coated steel

Abstract

A laser welding–brazing (LWB) process to join zinc coated steel and aluminium sheets in two different flange geometries is reported. The deep drawing steel sheets are covered by a zinc layer of maximum thickness 10 μm, and a zinc based filler wire was used in the welding experiments with a Nd–YAG laser. Because of the differences in melting temperatures between iron (1808 K), aluminium (933 K), and zinc (693 K), it is possible to weld the aluminium alloy only. Owing to the zinc coating on the steel side, a Zn–Al alloy can be brazed onto the steel without any flux agent. The inevitable formation of a Fe–Al intermetallic phase at the bondline of the weld seam and the steel can be limited to a thickness of less than 5 μm and to a proportion of the contact area only. Mechanical as well as dynamic tests show results comparable to those obtained via other joining techniques. Salt chamber corrosion tests of varnished specimens display minor damage and no decline in tensile strength.     

Hybrid laser/arc welding process for controlling bead profile

Abstract

A laser hybrid welding process in which a defocused laser beam is applied beside a gas metal arc weld (GMAW) pool to modify the bead shape was studied. The present paper aims to produce welds with improved toe geometry and better fatigue life than those made with GMAW alone and to apply a numerical simulation to help configure the hybrid process. First, stationary hybrid welds were made to validate weld bead shape predictions and to characterise the spreading of the arc weld deposit to the laser heated spot. Next, the travelling hybrid process was configured with the aid of simulations and fatigue test specimens were welded. Proper application of the laser heat input induced molten metal to spread to the laser heated area, increasing the fillet weld leg length. This produced a larger weld toe angle that decreased the stress concentration and increased the fatigue life of the welds relative to standard mean values.     

Laser pressure welding of Al alloy and low C steel

Abstract

This study was performed to obtain fundamental knowledge concerning the development of laser pressure welding technology for the joining of dissimilar metals. Laser pressure welding of Al alloy A6061 and low C steel SPCC sheets was carried out to investigate the effects of the roller pressure, laser beam scanning speed and irradiation position on the tensile shear and peel strength of welded joints. The interfaces of the joints were observed and analysed by SEM and EDX, and the formation phases on the peeled surfaces were identified with XRD. It was revealed that prevention and suppression of oxidation during welding was extremely important to the production of a sound joint with good mechanical properties. The highest tensile strength and the highest peel strength of joints were obtained at a laser power of 1·8 kW, laser scanning speed of 30 Hz, laser irradiation position at the centreline, roller pressure of more than 245 MPa and welding speed of 0·5 m min^?1 in an Ar atmosphere. The fracture occurred not in the welded zone but in the A6061 base alloy specimen.