Influence of Rotational Speed on Microstructure and Mechanical Properties of Dissimilar Metal AISI 304-AISI 4140 Continuous Drive Friction Welds

 Fundamental investigation of continuous drive friction welding of austenitic stainless steel (AISI 304) and low alloy steel (AISI 4140) is described. The emphasis is made on the influence of rotational speed on the microstructure and mechanical properties such as hardness, tensile strength, notch tensile strength and impact toughness of the dissimilar joints. Hardness profiles across the weld show the interface is harder than the respective parent metals. In general, maximum peak hardness is observed on the stainless steel side, while other peak hardness is on the low alloy steel side. A trough in hardness distribution in between the peaks is located on the low alloy steel side. Peak hardness on the stainless steel and low alloy steel side close to the interface increases with a decrease in rotational speed. All transverse tensile joints fractured on stainless steel side near the interface. Notch tensile strength and impact toughness increase with increase in rotational speed up to 1500 r/min and decrease thereafter. The mechanism of influence of rotational speed for the observed trends is discussed in the torque, displacement characteristics, heat generation, microstructure, fractography and mechanical properties.     

Influence of welding parameters on peak load and energy absorption of dissimilar resistance spot welds of DP600 and AISI 1008 steels

Abstract

This paper addresses the mechanical performance of dissimilar resistance spot welds between DP600 and AISI 1008 low carbon steels. The weldability lobe was established and proper welding conditions to produce welds with sufficient size and without expulsion were determined. Correlations among the process parameters (welding current and welding time), physical spot weld attributes (fusion zone size and electrode indentation depth) and mechanical performance (peak load and energy absorption) were analysed. It was shown that the increases in welding current and welding time result in increases in fusion zone size and electrode indentation depth. In the low heat input welding condition, welds failed in interfacial failure mode. Increasing welding heat input results in sufficient weld size and promotes pullout failure mode with improved mechanical properties in terms of peak load and failure energy. However, a further increase in heat input caused metal expulsion and the failure mode was changed to partial pullout–partial thickness mode with reduced energy absorption capability.

Cet article discute du rendement mécanique de soudures dissimilaires par points entre les alliages à faible carbone DP600 et AISI 1008. On a établi le lobe de soudabilité et l’on a déterminé les conditions appropriées de soudage afin de produire des soudures d’une taille suffisante et sans expulsion. On analyse la corrélation entre les paramètres du processus (courant de soudage et temps de soudage), les attributs physiques de la soudure par points (taille de la zone de fusion et profondeur d’indentation de l’électrode) et le rendement mécanique (charge maximale et absorption d’énergie). On a montré qu’une augmentation du courant de soudage et du temps de soudage avait pour résultat une augmentation de la taille de la zone de fusion et de la profondeur d’indentation de l’électrode. Sous condition de soudage à faible apport de chaleur, les soudures défaillaient en mode de défaillance à l’interface. L’augmentation de l’apport de chaleur de soudage avait pour résultat une taille de soudure suffisante et favorisait le mode de défaillance par arrachement avec des propriétés mécaniques améliorées quant à la charge maximale et à l’énergie de défaillance. Cependant, une augmentation additionnelle de l’apport de chaleur résultait en l’expulsion du métal et le mode de défaillance se changeait en un mode d’arrachement partiel-épaisseur partielle avec capacité réduite d’absorption d’énergie.     

Peak load and energy absorption of DP600 advanced steel resistance spot welds

The paper aims at investigating the microstructure, failure mode transition, peak load and energy absorption of DP600 dual phase steel during the tensile-shear test. It was found that the welding current has profound effect on the load–displacement characteristics. In the low welding current, welds failed in interfacial failure mode. Increasing welding current resulted in sufficient weld nugget growth to promote double-sided pullout failure mode with improved mechanical properties. Further increase in the welding current caused expulsion and failure mode was changed to single-sided pullout with reduced energy absorption capability. It was found that the fusion zone size is the key parameter controlling the mechanical properties of DP600 resistance spot welds in terms of peak load, maximum displacement and failure energy.     

Si-Mn系超高强度热成形淬火钢板的点焊性能

 采用不同的点焊工艺参数对研发的1700MPa级Si-Mn系热成形淬火钢板与低碳钢板DC04进行异种材料之间点焊,并对焊接接头的拉伸性能、显微硬度分布及微观组织特征等进行了分析。结果表明,焊接电流对点焊接头熔核直径和抗剪强度具有显著的影响,而焊接时间的影响相对较小。超高强度钢板侧的热影响区存在两个明显的软化区和硬化区,即在靠近母材处存在一个硬度明显低于母材的软化区,其组织为回火马氏体;在靠近熔核处存在一个硬度明显高于母材的硬化区,其组织为细小的马氏体。点焊接头熔核部位为柱状粗大马氏体组织,其硬度明显低于超高强度钢板母材且远高于低碳钢板母材。低碳钢板热影响区低的硬度和明显粗化的铁素体组织,使得点焊接头单向拉伸时均从低碳钢板的热影响区一侧破断。     

Effect of Welding Processes on Tensile and Impact Properties, Hardness and Microstructure of AISI 409M Ferritic Stainless Joints Fabricated by Duplex Stainless Steel Filler Metal

The effect of welding processes such as shielded metal arc welding, gas metal arc welding and gas tungsten arc welding on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade is studied. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure and fracture surface morphology of the welded joints have been evaluated and the results are compared. From this investigatio.n, it is found that gas tungsten arc welded joints of ferritic stainless steel have superior tensile and impact properties compared with shielded metal are and gas metal arc welded joints and this is mainly due to the presence of finer grains in fusion zone and heat affected zone.     

Mechanical Properties and Metallurgical Characterization of Dissimilar Welded Joints between AISI 316 and AISI 4340

In the present study, the influence of six different process parameters and three interactions on joint tensile strength, toughness, fusion zone microhardness variation are studied during dissimilar tungsten inert gas welding between austenitic stainless steel AISI 316 and alloy steel AISI 4340. Detailed experimental study using fractional factorial experimental design and subsequent statistical analysis show that higher tensile strength, toughness can be achieved using ER 309 filler material and suitably selecting the other process parameters and heating conditions. Addition of small proportion of hydrogen in shielding gas increases the heat transfer efficiency, melting and subsequent penetration. Preheating of AISI 4340 material reduces the chance of solidification cracking and post-heating helps to improve the joint mechanical property. Microstructural observations show that improper selection of process parameters may lead to micro-pores and degrade the joint quality. Successful joining of the dissimilar materials greatly depends on the selection of optimum process parameters, filler material and shielding gas.     

Microstructure and mechanical properties of a welded (α + β) ti alloy

The microstructure and the mechanical properties were studied in bead-on-plate welds in a Ti-6Al-2V-1Mo alloy. The heat affected zone (HAZ) and the fusion zone (FZ) consisted of very large primaryβ grains with theβ-phase transformed to martensite. A special bead-on-plate welding technique allowed independent measurement of the mechanical properties of the HAZ and the FZ. Compared to the as-received (AR) material, the strength and ductility decreased in the weld. The highest fatigue strength was found for the AR material followed by the HAZ and the FZ, whereas the ranking for fatigue crack growth was opposite.     

The effects of process variables on pulsed Nd:YAG laser spot welds: Part I. AISI 409 stainless steel

The weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of Nd:YAG laser welding parameters, including laser pulse time and power intensity, and material-dependent variables, such as absorptivity and thermophysical properties, on laser spot-weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and sur-face cratering, have been studied experimentally. The results of this work are reported in two parts. In Part I, the weldability of AISI 409 stainless steel by the pulse laser welding process is reported. In Part II, the weldability of A A 1100 aluminum under the same operating con-ditions is reported and compared to those of the stainless steel. When welding AISI 409 stainless steel, weld pool shapes were found to be influenced most by the power intensity of the laser beam and to a lesser extent by the pulse duration. Conduction mode welding, keyhole mode welding, and drilling were observed. Conduction mode welds were produced when power in-tensities between 0.7 and 4 GW/m² were used. The initial transient in weld pool development occurred in the first 4 ms of the laser pulse. Following this, steady-state conditions existed and conduction mode welds with aspect ratios (depth/width) of about 0.4 were produced. Keyhole mode welds were observed at power intensities greater than 4 GW/m². Penetration of these keyhole mode welds increased with increases in both power intensity and pulse time. The major weld defects observed in the stainless steel spot welds were cratering and large-occluded gas pores. Significant metal loss due to spatter was measured during the initial 2 ms of keyhole mode welds. With increasing power intensity, there was an increased propensity for occluded gas pores near the bottom of the keyhole mode welds. Formerly Graduate Student.     

The effects of strain rate and welding current mode on the dynamic impact behavior of plasma-arc-welded 304L stainless steel weldments

This article uses a split-Hopkinson pressure bar to investigate the effects of strain rate in the range of 10³ s⁻¹ to 8 × 10³ s⁻¹ and welding current mode upon the dynamic impact behavior of plasma-arc-welded (PAW) 304L stainless steel (SS) weldments. Stress-strain curves are plotted for different strain rates and welding parameters, and optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques are used to analyze the microstructure and fracture characteristics of the weldments. The results confirm that the strain rate and the welding current mode have a significant influence upon the dynamic impact behavior and microstructure evolution of 304L SS weldments. It is shown that for a constant strain, the flow stress increases with strain rate for both welding current modes, and that the pulsed current (PC) mode results in a higher weldment strength than the continuous current (CC) mode. Weldments fabricated using the PC mode exhibit an improved resistance to thermal softening, a greater strain-rate sensitivity, and a lower activation volume. The OM and SEM observations reveals that an adiabatic shear band dominates the fracture characteristics of both weldment types under impact loading. Microstructural analysis reveals that for both welding current modes, the dislocation density and volume fraction of α′ martensite increase with an increasing strain rate, while the twin formations reduce under the same conditions. Comparing the evolution of the microstructure in the base metal and the fusion zone, it is found that for both welding current modes, a higher dislocation density exists in the fusion zone, and that a larger volume fraction of α′ martensite and a greater twin density are present in the base metal. Furthermore, the dislocation density and volume fraction of α′ martensite is greater in PC weldments than in their CC counterparts. Finally, the present results indicate that the PC welding mode produces a weldment with superior dynamic impact response and improved weldment fracture characteristics.     

Effect of welding processes on tensile, impact, hardness and microstructure of AISI 409M ferritic stainless joints fabricated using duplex stainless steel filler metal

 The present investigation is aimed at to study the effect of welding processes such as shielded metal arc welding, gas metal arc welding and gas tungsten arc welding on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, micro hardness, microstructure and fracture surface morphology of the welded joints have been evaluated and the results are compared. From this investigation, it is found that gas tungsten arc welded joints of ferritic stainless steel showed superior tensile and impact properties compared with shielded metal arc and gas metal arc welded joints and this is mainly due to the presence of finer grains in fusion zone and heat affected zone.     

Evaluation of Microstructure and Mechanical Properties of Laser Beam Welded AISI 409M Grade Ferritic Stainless Steel

 The microstructure analysis and mechanical properties evaluation of laser beam welded AISI 409M ferritic stainless steel joints are investigated. Single pass autogeneous welds free of volumetric defects were produced at a welding speed of 3000 mm/min. The joints were subjected to optical microscope, scanning electron fractographe, microhardness, transverse and longitudinal tensile, bend and charpy impact toughness testing. The coarse ferrite grains in the base metal were changed into dendritic grains as a result of rapid solidification of laser beam welds. Tensile testing indicates overmatching of the weld metal is relative to the base metal. The joints also exhibited acceptable impact toughness and bend strength properties.     

On failure mode of resistance spot welded DP980 advanced high strength steel

Abstract

This paper aims at investigating the correlation between metallurgical and mechanical characteristics of DP980 dual phase steel resistance spot welds with attention focused on the transition in failure mode from interfacial to pullout mode during the tensile shear test. It was studied whether the conventional/industrial weld size criteria can produce pullout failure modes for DP980 spot welds. Based on theoretical analysis and experimental results, there is a minimum fusion zone size which beyond that the interfacial failure mode is avoided. Results showed that by increasing welding current, pullout failure mode was promoted due to increasing the fusion zone size and encouraging martensite tempering in subcritical heat affected zone (HAZ). Fusion zone size was proved to be key factor controlling mechanical properties of DP980 welds in terms of peak load, ductility and failure energy.

Ce document vise à examiner la corrélation entre les caractéristiques métallurgiques et mécaniques de soudures par points par résistance de l’acier biphasé DP980, en mettant l’accent sur la transition du mode de défaillance de mode d’interface à mode par arrachement lors de l’essai de traction-cisaillement. On voulait savoir si les critères conventionnels ou industriels de taille de la soudure pouvaient produire des modes de défaillance par arrachement des soudures par points de DP980. L’analyse théorique et les résultats expérimentaux indiquent qu’il y a une taille minimale de la zone de fusion au-delà de laquelle on évite le mode de défaillance d’interface. Les résultats ont montré qu’en augmentant le courant de soudage, le mode de défaillance par arrachement était favorisé, à cause de l’augmentation de la taille de la zone de fusion et de l’encouragement du revenu de la martensite dans la HAZ sous critique. On a démontré que la taille de la zone de fusion constituait une composante clé contrôlant les propriétés mécaniques des soudures de DP980 en ce qui a trait à la charge de pointe, à la ductilité et à l’énergie de défaillance.     

Experimental investigation into microstructure,metallurgy and mechanical properties of AISI 422 stainless steel weldments produced using fibre laser welding direct forming

In this study, the Taguchi design method was used to determine the optimal parameters for the fibre laser welding direct forming (FLWDF) of AISI 422 stainless steel (SS) weldments for use in the repair of the tenons in steam turbine blades. Experiments were then conducted to investigate the microstructural characteristics and mechanical properties of the weldments. Our results show that the fusion zone of the AISI 422 SS weldments possesses a grain structure of significantly higher refinement than does the original AISI 422 SS. Moreover, tempering at 700°C for 2?h was found to enhance the hardness as well as the impact toughness of the weldments. Finally, mock-up trials of tenon repair were performed using the optimal FLWDF parameters. Our results reveal that the repaired tenons are able to withstand a tensile load of 30?kN in as-welded condition and up to 55?kN following tempering at 700°C for 2?h.     

Effect of Shielding Gas on the Microstructural Evolution in Laser Beam Welded AISI 410 Martensitic Stainless Steel

Laser welding of AISI 410 martensitic stainless steel was attempted in a diffusion cooled RF excited CO₂ slab laser under Gaussian mode with argon and nitrogen as shielding gas. The effect of shielding gas and energy density on the resultant weld bead geometry, microstructure and hardness were assessed and discussed. It has been observed that welds obtained under nitrogen shielding conditions had higher and uniform hardness across the weld metal on account of reduced ferrite content.     

A thermal and microstructure evolution model of direct-drive friction welding of plain carbon steel

A model of direct-drive friction welding has been developed, which can be used to predict the time-temperature histories, the resultant microstructure, and the microhardness distribution across the weld interface of direct-drive friction-welded AISI/SAE 1045 steel bars. Experimentally measured power and axial displacement data were used in conjunction with a finite-element transient thermal model to predict the time-temperature history within the heat-affected zone (HAZ) of the weld. This was then used with a microstructure evolution model to predict the volume fraction of the subsequent microconstituents and the microhardness distribution across the weld interface of welds produced using three significantly different welding conditions: one with optimal conditions, one with a long burn-off time, and one with high axial pressure and rotational speed but short burn-off time. There was generally good agreement between the predicted and the measured time-temperature histories, volume fraction of the resultant microstructures, and microhardness distribution in the HAZ of AISI/SAE 1045 steel friction welds produced using these three significantly different welding conditions.     

Effect of Welding Current and Time on the Microstructure,Mechanical Characterizations,and Fracture Studies of Resistance Spot Welding Joints of AISI 316L Austenitic Stainless Steel

This article aims at investigating the effect of welding parameters, namely, welding current and welding time, on resistance spot welding (RSW) of the AISI 316L austenitic stainless steel sheets. The influence of welding current and welding time on the weld properties including the weld nugget diameter or fusion zone, tensile-shear load-bearing capacity of welded materials, failure modes, energy absorption, and microstructure of welded nuggets was precisely considered. Microstructural studies and mechanical properties showed that the region between interfacial to pullout mode transition and expulsion limit is defined as the optimum welding condition. Electron microscopic studies indicated different types of delta ferrite in welded nuggets including skeletal, acicular, and lathy delta ferrite morphologies as a result of nonequilibrium phases, which can be attributed to a fast cooling rate in the RSW process. These morphologies were explained based on Shaeffler, WRC-1992, and pseudo-binary phase diagrams. The optimum microstructure and mechanical properties were achieved with 8-kA welding current and 4-cycle welding time in which maximum tensile-shear load-bearing capacity or peak load of the welded materials was obtained at 8070 N, and the failure mode took place as button pullout with tearing from the base metal. Finally, fracture surface studies indicated that elongated dimples appeared on the surface as a result of ductile fracture in the sample welded in the optimum welding condition.     

Fusion zone microstructure and porosity in electron beam welds of an α+β titanium alloy

The effect of electron beam welding parameters on fusion zone (FZ) microstructure and porosity in a Ti −6.8 Al −3.42 Mo −1.9 Zr −0.21 Si alloy (Russian designation VT 9) has been investigated. It has been observed that the FZ grain width increased continuously with increase in heat input when the base metal was in the β heat-treated condition, while in the α+β heat-treated base metal welds, the FZ grain width increased only after a threshold energy input. The difference is attributed to both the weld thermal cycle and the pinning effect of equiaxed primary alpha on grain growth in the heat-affected zone (HAZ) of α+β heat-treated base metal. Postweld heat treatment (PWHT) in the subtransus and supertransus regions did not alter the columnar grain morphology in the FZ, possibly due to the lack of enough driving force for the formation of new grains by the breaking up of the columnar grains and grain boundary movement for grain growth. As the PWHTs were conducted in a furnace, the role of thermal gradients can be ruled out. Intragranular microstructure in the aswelded condition consisted of hexagonal martensite. The scale of the martensite laths depended on welding speed. The highest porosity was observed at intermediate welding speeds. At low speeds, a majority of pores formed at the fusion boundary, while at high speeds, occurrence of porosity was maximum at the weld center. The trends on porosity can be explained on the basis of solubility of hydrogen in titanium as a function of temperature and the influence of weld thermal cycle on nucleation, growth, and escape of hydrogen gas bubbles. The porosity at slow welding speeds is low because sufficient time exists for the nucleation, growth, and escape of hydrogen gas bubbles, while insufficient time exists for the nucleation of gas bubbles at high welding speeds. The effect of pickling of joint surface, vacuum annealing of the base metal, and successive remelting of the weld metal has also been investigated.     

金刚石锯片焊接部位的组织结构与力学性能

采集金刚石圆锯片激光焊和高频钎焊部位作试样,分析研究了钢基体与粉末烧结刀头过渡层对焊焊缝横向的区段划分、焊缝形貌、金相组织、成分与物相、硬度及抗弯强度,并阐述了激光焊气孔的成因。结果表明:激光焊熔化区柱晶组织排布细腻,有残留气孔,硬度呈线性变化;钎焊熔化区为两相组织,枝晶明显,无裂纹气孔,硬度较低。激光焊钢热影响区产生针状马氏体;钎焊不存在相变引起的热影响区。激光焊和钎焊的刀头过渡层,在特定的配料体系下,也不存在固态相变引发的热影响区,基本保持了粉末烧结组织。激光焊和钎焊焊缝的抗弯强度均值分别为1 175 MPa、983 MPa,都大于国外安全强度要求,但激光焊焊缝强度波动相对较大,这与其残留气孔密切相关。     

Effect of Autogenous Arc Welding Processes on Tensile and Impact Properties of Ferritic Stainless Steel Joints

 The effect of autogeneous arc welding processes on tensile and impact properties of ferritic stainless steel conformed to AISI 409M grade is studied. Rolled plates of 4 mm thickness have been used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure, and fracture surface morphology of continuous current gas tungsten arc welding (CCGTAW), pulsed current gas tungsten arc welding (PCGTAW), and plasma arc welding (PAW) joints are evaluated and the results are compared. It is found that the PAW joints of ferritic stainless steel show superior tensile and impact properties when compared with CCGTAW and PCGTAW joints, and this is mainly due to lower heat input, finer fusion zone grain diameter, and higher fusion zone hardness.     

On the Failure Behavior of Highly Cold Worked Low Carbon Steel Resistance Spot Welds

Highly cold worked (HCW) low carbon steel sheets with cellular structure in the range of 200 to 300 nm are fabricated via constrained groove pressing process. Joining of the sheets is carried out by resistance spot welding process at different welding currents and times. Thereafter, failure behavior of these welded samples during tensile-shear test is investigated. Considered concepts include; failure load, fusion zone size, failure mode, ultimate shear stress, failure absorbed energy, and fracture surface. The results show that HCW low carbon steel spot welds have higher failure peak load with respect to the as-received one at different welding currents and times. Also, current limits for failure mode transition from interfacial to pullout or from pullout to tearing are reduced for HCW samples. Fusion zone size is the main geometrical factor which affects the failure load variations. Ultimate shear stress of spot welds is increased with decreasing the heat input and for HCW samples at a specific welding current and time, it is lower than that of the as-received ones. Before pullout mode, failure absorbed energy (FAE) for HCW low carbon steel spot welds is higher than that of the as-received one, but after failure mode transition, trend would be vice versa and FAE of the as-received spot welds is extremely higher (about 3 times). In addition, spot welds fracture surface (in interfacial failure mode) for HCW sample is more dimpled which indicates higher energy absorption than that of the as-received one.