Influences of post-weld heat treatment on fatigue crack growth behaviour of strength mismatched HSLA steel welds

Abstract

Welding of high strength low alloy steels (HSLA) involves usage of low, even and high strength filler materials (electrodes) compared with the parent material depending on the application of the welded structures and the availability of the filler material. In the present investigation, the influences of post-weld heat treatment (PWHT) on fatigue crack growth behaviour of under matched (UM), equal matched (EM) and over matched (OM) weld metals has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. The Shielded Metal Arc Welding (SMAW) process has been used to fabricate the single 'V' butt joints. Centre Cracked Tension (CCT) specimens have been used to evaluate the fatigue crack growth behaviour of the welded joints. Fatigue crack growth experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading (R = 0). From this investigation, it has been found that the fatigue performance of over matched joints is superior compared to under matched and equal matched joints. Moreover, PWHT reduced the magnitude of the tensile residual stress field in the weld region and subsequently enhanced the fatigue performance of the joints irrespective of weld metal strength mismatch.     

Friction welding of Al–Mg–Si alloy to Ni–Cr–Mo low alloy steel

Abstract

It is difficult to weld the dissimilar material combination of aluminium alloys and low alloy steels using fusion welding processes, on account of the formation of a brittle interlayer composed of intermetallic compound phases and the significant difference in physical and mechanical properties. In the present work an attempt has been made to join these materials via the friction welding method, i.e. one of the solid phase joining processes. In particular, the present paper describes the optimisation of friction welding parameters so that the intermetallic layer is narrow and joints of acceptable quality can be produced for a dissimilar joint between Al-Mg-Si alloy (AA6061) and Ni-Cr-Mo low alloy steel, using a design of experiment method. The effect of post-weld heat treatment on the tensile strength of the joints was then clarified. It was concluded that the friction time strongly affected the joint tensile strength, the latter decreasing rapidly with increasing friction time. The highest strength was achieved using the shortest friction time. The highest joint strength was greater than that of the AA6061 substrate in the as welded condition. This is due to the narrow width of the brittle intermetallic layer generated, which progressed from the peripheral (outer surface) region to the centreline region of the joint with increasing friction time. The joints in the as welded condition could be bent without cracking in a bend test. The joint tensile strength in the as welded condition was increased by heat treatment at 423 K (150° C), and then it decreased when the heat treatment temperature exceeded 423 K. All joints fractured in the AA6061 substrate adjacent to the interface except for the joints heated at 773 K (500° C). The joints fractured at the interface because of the occurrence of a brittle intermetallic compound phase.     

Welding of titanium alloys with activating flux

Abstract

In the present paper, the effects of an activating flux on Ti–6Al–4V alloy welding were investigated. Tungsten inert gas welding was used to weld 8.0 mm thickness Ti–6Al–4V alloy plates. Results show that applying the activating flux on the Ti–6Al–4V alloy surface leads to an increase in weld penetration depth, whereas the corresponding weld bead width is reduced. It was also found that various welding conditions, particularly flux thickness, influence the effectiveness of the activating flux. Furthermore, a data acquisition system was used to monitor the current and voltage signals during welding. Results from monitoring of the welding current and voltage signals reveal that there is a clear correlation between the signals and the weld penetration when the welding arc is steady. Analysis of the acquired signals can be used to identify inconsistencies in weld penetration. In summary, to take advantage of the use of activating flux in Ti alloy welding, it is important that a uniform flux layer is present at the alloy plate surface and suitable welding parameters are selected.     

Secondary austenite and chromium nitride precipitation in simulated heat affected zones of duplex stainless steels

Abstract

Primary and secondary intragranular austenite precipitation and its relationship with chromium nitride (Cr₂N) were studied in a simulated multipass heat affected zone (HAZ) of five duplex stainless steel alloys (UNS S32304, S32205, S32550, S32750, and S32760). The Gleeble thermal-mechanical simulator was used to perform short duration and high cooling rate ferritisation and reheating heat treatments. TEM and FEG-SEM analysis, coupled with a specially developed electrolytic etching technique, revealed the cooperative growth of secondary austenite and Cr₂N precipitation along the ferrite/austenite (α/γ) interfaces. Additionally, the observed close coexistence of intragranular nitride (Cr₂N) and intragranular secondary austenite suggests the heterogeneous nucleation of secondary austenite from the nitrides as supported by previously reported low energy nitride/austenite (Cr₂N/γ) interfaces for the observed orientation relationship between both phases. Based on these observations, a new mechanism is proposed for intragranular secondary austenite nucleation related to the intragranular nitride precipitates.     

New structure of automatic torch and new approach for controlling drop transfer in rotating spray welding arc

Abstract

Using computer aided design/computer aided manufacturing technology, an automatic welding system is designed and manufactured for high current, high speed, and high deposition metal arc active gas (MAG) welding. The welding torch structure is evaluated and optimised via the finite element method and practical welding technology experiments. Finally, the process of high deposition MAG welding is investigated, and it is highlighted that on application of a longitudinal magnetic field to the high deposition MAG welding process, a steady metal transfer and drop deposition condition can be achieved, which is suited to the demands of high current, high speed welding technology.     

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.     

Experimental study on distortion of Al-6013 plate after friction stir welding

Abstract

Friction stir welding (FSW) experiments with different panel dimensions and welding parameters have been designed to study the distortion of FSW. The FSW experiments were carried out with a load control facility to make the welding parameters reliable. The distortion of FSW is much smaller than that of arc welding, but it is still very significant. Three-dimensional distortion measuring system was applied to further study distortion trends. The results show that the distortion after FSW is in saddle shape, with convex bending in longitudinal direction and concave bending in transverse direction. This distortion pattern is in contrary with that of traditional arc welding. It is also found that increasing the panel length increases the longitudinal distortion but almost do not influence the transverse distortion. Increasing the rotation speed increases both longitudinal distortion and transverse distortion. The influence of welding speed on distortion is not very clear.     

Effect of oxygen content in He–O2 shielding gas on weld shape in ultra deep penetration TIG

Abstract

The effect of the shielding gas concentration on the weld shape was studied for the moving bead on plate TIG welding of SUS304 stainless steel under He–O₂ mixed shielding. The small addition of oxygen to the helium base shielding gas can precisely control the oxygen content in a liquid pool and the weld shape. Oxygen is a surface active element for stainless steel. When the oxygen content in the liquid pool is above the critical value of ～ 70 ppm, the weld shape suddenly changes from a wide shallow type to a deep narrow one due to the change in the Marangoni convection from the outward to inward direction on the liquid pool surface. Weld shape variations influenced by the welding parameters including welding speed, welding current and electrode tip work distance under pure He and He–0.4%O₂ mixed gas shielding were systematically investigated. The investigation results showed that the final shape of the TIG weld depends to a large extent on the pattern and magnitude of the Marangoni convection on the pool surface, which is governed by the combined effect of the oxygen content in a liquid pool, temperature coefficient of the surface tension (dσ s/dT) and the temperature gradient on the pool surface (dT/dr, r is the radius of the weld pool surface). It is considered that the change in welding parameters alters the temperature distribution and gradient on the pool surface, and thus, affects the magnitude of the Marangoni convection and final weld shape.     

Numerical simulation of spot welding for galvanised sheet steels

Abstract

Galvanised sheet steels are now widely used to be the substrate for body in white (BIW) construction in the automotive industry. Weldability of galvanised sheet steels much worsened compared to spot welding of low carbon steels. The present paper develops a 2D axisymmetric model and employs an incremental coupled thermal–electrical–mechanical analysis to predict the nugget development during resistance spot welding (RSW) of galvanised sheet steels. Temperature dependent contact resistance for faying surfaces was determined to take into account of the influence of zinc coat for spot welding galvanised sheet steels. The effect of dynamic contact radii on temperature distribution was studied and compared with results under constant contact area assumption. The predicted nugget shape and size agreed well with the experimental data. Higher current and longer welding time should be applied for galvanised sheet steels compared to low carbon steel spot welding. The proposed model can be applied to predict weld quality and choose optimal welding conditions for spot welding galvanised sheet steels.     

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.