Effect of grain size on heat affected zone cracking susceptibility.Study of weldability of Inconel 718 cast alloy (2nd Report)

Summary

It is generally recognised that the flux type of covered electrodes affects the profile and mechanical properties of welds and may well control the fatigue strength of fillet welded joints. Fillet welded joints, however, have seen few systematic investigations. The purpose of this investigation is to improve the fatigue strength of fillet welded joints through appropriate design of the covered electrode flux.

Ten types of covered electrode were trial‐manufactured with variation in the flux system at two levels (to improve the profile of the fillet toe region) and the added alloying element contents of each system at five levels (to vary the hardness of the weld metal). These covered electrodes were used to prepare non‐load‐carrying cruciform fillet welded joints using 570 MPa class high‐strength steel base metal. Fatigue tests (S = σ_max ‐ σ_min, σ_max = σ _Y ) were run with the fillet welded joints. The results obtained may be summarised as follows:

1. The profile of the fillet welds appears to be affected by the flux system and base metal but not by the added alloying elements.

2. The maximum stress concentration factor (K_max) is reduced to K_max ? 1.59 for covered electrodes using an MgCO₃‐MgO‐CaF₂‐iron powder system flux (M' series).

3. The fatigue strength of the fillet welded joints at 2 × 10⁶ cycles is improved to around 143 MPa in the M' series. It can safely be said that there is an inverse correlation between K_max in the toe region and the fatigue strength of the welded joints.

4. Appropriate relations are developed and used to calculate the recovery to the weld metal of added alloying elements. The recovery to the weld metal of the added alloying elements on average gives minima of around 98%Ni, 94%Mo, 85%Cr (which are good), and around 11%B.

     

Basic alloy development of low-transformation-temperature fillers for AISI 410 martensitic stainless steel

ABSTRACT

This study discusses the effect of Ni content of low-transformation-temperature (LTT) fillers on welding residual stresses of AISI 410 plates. The plates were joined by LTT fillers with 11?wt-% Cr and varying Ni from 3 to 11?wt-%. Evaluation of varying Ni content on the martensitic transformation temperature (M_s) by dilatometric tests indicates that the M_s of LTT fillers decreases from 335°C to 67°C by increasing Ni wt-%. Residual stress measurements show the highest compressive longitudinal stress (?310?MPa) in the weld bead deposited by the LTT filler contained 7?wt-% Ni and M_s?=?202°C. This result is associated with the highest final expansion strain (0.37%) in this composition. Microstructural analyses show the presence of retained austenite by increasing the Ni more than 7?wt-%.     

In situ analysis of the strain evolution during welding using low transformation temperature filler materials

ABSTRACT

Compared to conventional welding consumables using low transformation temperature (LTT) filler materials is an innovative method to mitigate tensile residual stresses due to delayed martensite transformation of the weld. For the effective usage of LTT filler materials, a deeper understanding of the complex processes that lead to the final residual stress state during multi-pass welding is necessary. Transformation kinetics and the strain evolution of multi-pass welds during welding were investigated in situ at the beamline HEMS@PETRAIII, Germany. Compared to conventional welds, the total strain was reduced and compression strain was achieved when using LTT filler materials. For an optimal use of the LTT effect in the root of multi-pass welds, the alloying concept must be adapted taking care of dilution.     

Effect of annealing and heating/cooling rate on the transformation temperatures of NiFeGa alloy

In this work, Ni₅₅Fe₁₈Ga₂₇ ferromagnetic shape memory alloy was prepared through a suck-casting method. The effects of annealing and heating/cooling rate on the martensitic transformation temperatures were investigated by differential scanning calorimetry (DSC). The results showed that the phase transformation temperatures increase with increasing the annealing temperature, upon the heating and cooling process. However, the start and the finish temperatures (M_s and M_f) of martensitic phase transformation increased firstly and then decreased upon the cooling process with the increase the annealing time at 300 °C. The start and the finish temperatures (A_s and A_f) of inverse phase transformation increased slightly upon the heating process with the increase of the annealing time. The results can be explained by the evolution of the microstructure after heat treatment. It was also found that the phase temperatures show great dependence on the heating/cooling rate of the DSC test, A_s and A_f increased and M_s and M_f decreased with the increase of the heating/cooling rate.     

Effect of variant transformations in fusion zones of gas metal arc welds

Abstract

The macro- and microtextures of gas metal arc welds fabricated using both conventional ferritic and low transformation temperature (LTT) filler metals were examined. Both welds were largely composed of acicular ferrite. The weld textures were found to be distinct, which is observed to be due to variant selection during the austenite to ferrite transformation. In situ electron backscatter diffraction performed during thermal cycling of the LTT fusion zone into the austenite phase field confirmed that these transformations satisfy the expected crystallographic relationships. Implications for welding consumable design and finite element process modelling are drawn.     

Microstructure and mechanical properties of dissimilar welded Mg–Al joints by ultrasonic spot welding technique

Abstract

Dissimilar spot welds of magnesium–aluminium alloy were produced via a solid state welding process, i.e. ultrasonic spot welding, and a sound joint was obtained under most of the welding conditions. It was observed that a layer of intermetallic compound (IMC) consisting of Al₁₂M₁₇ formed at the weld centre where the hardness became higher. The lap shear strength and failure energy of the welds first increased and then decreased with increasing welding energy, with the maximum lap shear strength and failure energy occurring at ～1250 J. This was a consequence of the competition between the increasing diffusion bonding arising from higher temperatures and the deterioration effect of the intermetallic layer of increasing thicknesses. Failure predominantly occurred in between the aluminium alloy and the intermetallic layer, which normally stayed at the magnesium side or from the cracks of the IMCs in the reaction layer.     

The influence of bending on stress fields and fatigue strength of cruciform welds

Summary

This paper deals with stress fields in the vicinity of weld toes of cruciform non‐load carrying fillet welds.

An approach recently formulated for welded joints under remote tensile loading is extended to pure bending as well as to bending and shear conditions. The approach is based on two notch stress intensity factors that independently quantify Mode I and Mode II stress distributions.

The new method provides an analytical framework to a stress field criterion, already proposed by Atzori some years ago and more recently by Pluvinage and Verreman, to determine the fatigue strength of a welded joint.     

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.     

Study of the phase transformation of cobalt

The allotropic transformation of cobalt was studied with a differential scanning calorimeter. The temperatures of the fcc to hep transformation, A_s, and fcc to hcp transformation, M_s, exhibit a linear dependence on the heating and cooling rates: A_s(K) = 723.2 + 0.28R and M_s(K) = 671.4-0.28R whereR is the rate in K/min. A_s, M_s, and the associated enthalpies of the phase changes were observed to increase with increasing number of cycles through the transformation. The M_s reaction was observed to proceed slowly isothermally while exhibiting athermal behavior when cooled rapidly.     

Effect of thermocycling on the temperatures of phase transformations,structure, and properties of the equiatomic alloy Ti<Subscript>50.0</Subscript>Ni<Subscript>50.0</Subscript>

This article is devoted to studying the influence of thermocycling in the range of temperatures of the thermoelastic martensitic transformation B2–B19' on the microstructure, the temperatures of the martensitic transformations, and the mechanical properties of the equiatomic alloy Ti₅₀Ni₅₀ in the coarse-grained (CG) and ultrafine-grained (UFG) states, the latter obtained by equal-channel angular pressing (ECAP). One hundred cycles of thermocycling and the related increase in the dislocation density in the CG alloy led to a decrease in the temperatures of martensitic transformations. In the UFG alloy, the temperatures of the forward transformation (M_s, M_f) decrease by 2–3 K, and the temperatures of the reverse transformation (A_s, A_f) increase by 6 K. The ultimate strength remains almost unaltered upon the thermocycling, but the yield stress increases substantially from 430 to 550 MPa and from 935 to 1120 MPa for the CG and UFG states, respectively.     

Observations on welding of  2+O+&bgr; titanium aluminide

Abstract

The weldability characteristics of an  ₂+O+&bgr; titanium aluminide of nominal composition Ti–24Al–15.5Nb (at.-%) have been investigated. Conventional gas tungsten arc (GTA) and electron beam (EB) welds exhibited columnar fusion zone grains. Pulsed current and arc oscillated GTA welds exhibited predominantly equiaxed fusion zone grains. The microstructure of GTA welds and pulsed current GTA welds exhibited  ₂+O phases, whereas arc oscillated GTA welds and EB welds contained  ₂+O+&bgr; ₀/&bgr; ₂; however, &bgr; ₀/&bgr; ₂ is predominant in EB welds. The EB welds, which contained  ₂+&bgr; ₀/&bgr; ₂ microstructure, exhibited high strength and ductility compared with GTA welds. The observed microstructural variations are explained on the basis of possible weld thermal cycles and convective currents in the weld pool.     

正面角焊缝的静载强度及其计算公式的探讨

本文通过分析角焊缝中应力应变分布及角焊缝的静载强度试验结果,对角焊缝的强度及其计算公式作了初步探讨。结果表明:角焊缝形状对焊缝中的应力应变分布影响很大,增加沿加载方向的焊脚尺寸,能改善焊缝中的应力应变集中程度,从而提高正面角焊缝的静载强度。同时,评定了目前经常使用的几个角焊缝强度计算公式的安全系数。最后,根据本文的试验结果及分析,提出了正面角焊缝强度的简化计算公式。     

Effects of ageing on bainitic and thermally induced martensitic transformations in ductile Cu–Al–Mn-based shape memory alloys

The effects of ageing at 473–573 K on the hardness, microstructure and thermoelastic martensitic transformation in Cu–Al–Mn-based shape memory alloys were investigated. It was found that hardness was dramatically increased by ageing due to the formation of fine bainitic plates and that the volume fraction of the bainite phase with ageing time can be described by the Austin–Rickett equation. The martensitic transformation temperatures decreased with the formation of bainite plates, mainly due to the composition change of the β-matrix. Moreover, the growth of thermally induced martensite plates was disturbed by the existence of bainite plates. Consequently, the transformation intervals (M_s–M_f and A_f–A_s) and transformation hystereses (A_f–M_s and A_s–M_f) increased with the progress of bainitic transformation.     

Welding of corrosion-resisting steels to titanium alloys

Summary

This paper describes a new method for measurement of the three‐dimensional residual stresses in fillet welds using inherent strains, which are considered to be a source of residual stresses. The new method implies first the inherent strains being determined from a few measured elastic strains with the aid of a proposed inherent strain distribution function, and second the residual stresses being calculated through an elastic analysis when the inherent strains are applied to welded joints.

The measurement method is called the TL_yL_z method when three types of T, L_y and L_z specimen are cut from the fillet weld, and the T method when only one specimen T is cut from the weld.

The TL_yL_z and T methods are respectively used to measure three‐dimensional residual stress distributions on the transverse sections of single‐pass and multipass T‐type fillet welds. The residual stresses measured by the TL_yL₂ and T methods show good agreement with the ones directly measured on the surfaces of welded joints, thus demonstrating the validity and practical applicability of the proposed method.     

The effect of electronic and magnetic valences on the martensitic transformation of CoNiGa shape memory alloys

Martensite start temperatures (M_s) and magnetic properties of several Co–Ni–Ga shape memory alloys (SMAs) with B2 austenite structure were investigated as a function of composition. It was found that, in addition to the well-known effect of the valence electron concentration (e/a ratio) in increasing M_s, the magnetic character of the alloy also plays a decisive role in determining M_s. These results were further corroborated through electronic structure calculations based on density functional theory. Experiments and calculations suggest that, for a given composition of Ga, the higher e/a ratios result in the higher M_s. Moreover, at a constant e/a ratio, the higher the magnetic valence number (Z_m) of the alloy is, the lower the M_s becomes. It was concluded that Z_m can be used as an indicator for the compositional trends in M_s in the present ferromagnetic SMAs in addition to the e/a ratio. Statistical analysis of the experimental results suggests that an increase in e/a of 0.1 yields an increase in M_s of about 190 K, while a change in Z_m of +0.1 results in a decrease in M_s of about 160 K. The calculations also confirmed that ferromagnetism may ultimately result in entropic and/or energetic stabilization of the austenite (cubic) phase, yielding lower M_s. These findings seem to be valid for other ferromagnetic SMA systems, where different M_s temperatures were reported for a given e/a ratio.     

Peak temperatures and microstructures in aluminium and magnesium alloy friction stir spot welds

Abstract

The peak temperatures during friction stir spot welding of similar and dissimilar aluminium and magnesium alloys are investigated. The peak temperatures attained during friction stir spot welding of Al 6111, Al 2024, and AZ91 are within 6% of their solidus temperatures. In dissimilar AZ91/Al 6111 spot welds the peak temperature corresponds with the α-Mg solid solution and Mg₁₇Al₁₂ eutectic temperature of 437°C. An a-Mg plus Mg₁₇Al₁₂ eutectic microstructure is produced in dissimilar friction stir spot welds when material displaced during pin penetration into the lower sheet material contacts the upper sheet material at the eutectic temperature.     

Weldability of X100 linepipe

Abstract

Research has been carried out to identify weld metal compositions and microstructures capable of meeting high strength and toughness requirements for X100 seam welded linepipe. Single pass, multiwire submerged arc welds were made in experimental, high strength low alloy steel plates using consumables to give a wide range of weld metal alloying. Work has shown that the optimum strength and toughness are obtained in Mo–B–Ti alloyed weld metals with P _cm values between 0.218 and 0.250. Weld metal microstructures were almost fully acicular ferrite with an ultrafine grain size (1–2 µm). Dilatometric studies demonstrated that at typical weld cooling rates the optimised welds transformed at significantly lower temperatures than those reported for X65 plate deposits, which contain acicular ferrite in the form of idiomorphic primary ferrite and intragranular Widmanstätten ferrite. The maximum rate of transformation in the optimised welds occurred between 515 and 570°C, which indicates that the acicular ferrite in this case consisted of intragranular Widmanstätten ferrite and/or bainite. The ferrite would appear to have a fine plate morphology growing from large as well as small inclusions, but not very far before the onset of hard impingement, thereby ensuring an ultrafine grain size. Tensile strengths of 708–784 MPa were achieved with an 80 J Charpy impact transition temperature toughness between -68 and -115°C. More highly alloyed weld metals containing 2–3%Mn and 1.5%Si transformed at lower temperatures and showed increased strength, but there was a substantial loss of toughness, attributed to the relatively unimpeded growth of large ferrite plates from larger inclusions, and the replacement of ultrafine acicular ferrite between these plates by blocks of martensite–austenite. One pass per side, multiwire submerged arc welds manufactured to the optimum weld metal chemistry confirmed their applicability for thin section X100 linepipe.     

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.     

中低强度钢超声冲击处理焊接接头热点应力设计曲线

用ANSYS计算了对接接头、横向非承载十字接头和纵向非承载角接板的热点应力集中系数K_hs,根据Q235B和Q345B的超声冲击处理焊接接头的相关疲劳试验数据得出经过应力比、板厚和几何不平度修正后的超声冲击处理焊接接头的热点应力设计S-N曲线. 结果表明,经过超声冲击处理后,中低强钢(R_m≤355 MPa)对接接头、横向非承载十字接头和纵向非承载角接板的热点应力S-N曲线的分散性降低,与接头形式无关,可以用一条热点应力设计S-N曲线表达. 在采用热点应力法进行疲劳设计时,选取m=10.0,上述三种类型焊接接头在2×106次疲劳强度级别可统一采用FAT135来表示.     

Effect of magnetic field on the morphology and fine structure of low-temperature martensite phase in a ferromagnetic Ni<Subscript>2.08</Subscript>Mn<Subscript>0.96</Subscript>Ga<Subscript>0.96</Subscript> alloy

The morphology and fine structure of high-temperature austenitic and low-temperature martensitic phases in a ferromagnetic Ni_2.08Mn_0.96Ga_0.96 alloy and the effect of magnetic field on the peculiarities of the martensite formation have been studied. The Ni_2.08Mn_0.96Ga_0.96 alloy in the initial cast and annealed states was found to undergo martensitic and magnetic phase transformations at temperatures M _s = 280 K, M _f = 265 K, A _s = 280 K, A _f = 295 K, and T _C = 375 K, respectively. Above M _s, the alloy is in a metastable premartensitic state, which leads to a characteristic diffuse scattering and tweed contrast when studying by electron microscopy. An applied magnetic field of up to 600 kA/m was found to affect the martensitic transformation in the alloy. The field application leads to changes in the morphology and fine structure of martensite due to orientation along the field direction of the magnetizations of the energetically advantageous (in terms of the directions of the magnetization vectors) c domains of the existing orientation variants of the martensite crystals having a packet pairwise-twinned morphology. The martensitic structure of the Ni_2.08Mn_0.96Ga_0.96 alloy already formed previously during cooling is not affected by an external magnetic field with a strength of up to 600 kA/m.