Fatigue life assessment of weld joints manufactured by GMAW and CW-GMAW processes

The fatigue strength of weld joints manufactured using gas metal arc welding and cold wire GMAW (CW-GMAW) was evaluated under stress-controlled cyclic loading. The material used in this study was class ASTM 131 grade A steel, joined using ER70S wire filler metal. The addition of cold wire led to a decrease in the amount of intergranular ferrite and an increase in hardness in the heat-affected zone. The assessment of fatigue life was performed by using the Weibull distribution and the results revealed that with a 99% reliability the joints produced using the CW-GMAW process have a longer fatigue life, especially as the stress amplitude increases.     

Fatigue behavior of welded joint spray fused by nickel-base alloy powder

Modification of spray fusing (MSF), which is a new method, was used to improve the fatigue strength of welded joints. Cruciform welded joint of Q235B steel was processed by MSF with nickel-base alloy powder of Ni60A and Ni25A. The bonding layer between coating and weld was studied by optical microscopy, scanning electron microscopy (SEM) and energy spectrum analysis. The results show that flame spray fusing can produce a coating layer with finer microstructure, lower impurities content, less air hole and higher bonding strength. Stress concentration at weld toe was significantly reduced due to smooth coating of high curvature after MSF. High cycle fatigue test results indicate that fatigue strength of welded joint spray fused by Ni60A is higher than that of welded joint spray fused by Ni25A owing to the good effect of MSF. Low cycle fatigue test results show that fatigue life of welded joint spray fused by Ni25A dose not affected by the poor effect of MSF compared with welded joint spray fused by Ni60A.     

Fatigue crack growth of bainitic rail steel welds

Abstract

The objective of the present study is to develop and evaluate novel weld repairs of bainitic rail steel defects, such as detail fracture, induced by in service loading. Slots were machined in the bainitic rail steel to simulate the removal of service defects. Multipass gas metal arc welding was used to fill and repair the slots. Three-point bend tests of the parent and welded steels revealed that the flexural weld efficiency was 75%. Fatigue crack propagation (FCP) tests were performed on specimens from the welded joints and compared with the parent materials to determine the mechanical integrity of the slot repairs. The fatigue lifetime and FCP kinetics were similar for the parent and slot welded bainitic steels, indicating similar resistance to FCP. The fatigue fracture surface morphology of the parent and welded bainitic rail steels both revealed mostly a ductile fracture mechanism in all the three FCP stages.     

Comparative study of fatigue behaviour in dissimilar Al alloy/steel and Mg alloy/steel friction stir spot welds fabricated by scroll grooved tool without probe

Abstract

Aluminium alloy A6061-T6 or magnesium alloy AZ31 sheet was welded to steel sheet by a friction stir spot welding technique using a scroll grooved tool without a probe. The material flow in the nugget of the Mg/steel weld was less than that in the Al/steel one. The Al/steel weld exhibited higher static tensile–shear strength than the Al/Al weld, while the strengths of Mg/steel and Mg/Mg welds were comparable. Tensile–shear fatigue tests were performed using lap shear specimens of both dissimilar and similar welds. The dissimilar welds exhibited nearly the same fatigue strengths as the similar ones. The effective nugget size in the dissimilar welds was defined as the area where Al or Mg alloy remained on the steel side after static fracture. When the fatigue strengths of dissimilar welds were evaluated based on the effective nugget size, the normalised fatigue strengths of Al/steel and Mg/steel welds were comparable.     

Fatigue strength of tungsten inert gas-repaired weld joints in airplane critical structures

In this work the effect of Gas Tungsten Arc Welding (GTAW) repairs on the axial fatigue strength of an AISI 4130 steel welded joint used in airframe critical to the flight-safety was investigated. Fatigue tests were performed at room temperature on 0.89 mm thick hot-rolled plates with constant amplitude and load ratio of R = 0.1, at 20 Hz frequency. Monotonic tensile tests, optical metallography and microhardness, residual stress and weld geometric factors measurements were also performed. The fatigue strength decreased with the number of GTAW repairs, and was related to microstructural and microhardness changes, as well as residual stress field and weld profile geometry factors, which gave origin to high stress concentration at the weld toe.     

7020铝合金型材的疲劳行为及微观机制

采用光学显微镜、扫描电镜、扫描透射电镜和电子背散射衍射等分析方法研究了 7020铝合金型材疲劳行为及微观机制.结果表明在应力比R为0、疲劳极限寿命取107周次时,合金疲劳强度为232.9 MPa.疲劳裂纹尖端应力强度因子△K=8 MPa·m1/2时,裂纹扩展速率约为6.44×10-5 mm/cycle.合金中尺寸在3～...     

起重机钢丝绳的疲劳可靠性设计

指出了两种传统钢丝绳设计计算方法的弊端。根据起重机钢丝绳失效主要是金属疲劳的原理 ,采用应力 -强度干涉模型 ,对起重机钢丝绳进行了疲劳可靠性设计。通过实例计算 ,得出了钢丝绳所需直径大小 ,并与两种传统设计方法得出的结果进行比较 ,可以看出 ,疲劳可靠性设计方法更为符合实际     

Fatigue behaviour of dissimilar friction stir welds between wrought and cast aluminium alloys

Wrought aluminium alloy, A6016-T4, was welded to cast one ADC12 by a friction stir welding technique. A6016 was set on the advancing side, while ADC12 was on the retreating side based on the pre-tensile test results. The dissimilar welds exhibited lower fatigue strengths than A6016 but higher than that of ADC12. Fatigue strengths increased when the tool was offset into ADC12, which could be explained based on the risk volume concept. When ADC12 was set on the advancing side, the fatigue strengths decreased. Microstructural observation revealed that the macroscopic structure in the stir zone drastically changed by putting ADC12 on the advancing side, and insufficient mixing of dissimilar alloys resulted in the lower fatigue strengths.     

Gerber抛物线在疲劳可靠性设计的应用研究

本文介绍了国际上五种简化疲劳极限应力(σm-σn)图,分析了Good man图、简化疲劳极限折线图、Soderberg直线图、Биргер抛物线图和Gerber抛物线图应用特点,针对其中之一的Gerber抛物线在可靠性强度计算中的疲劳极限的均值和标准差的计算,与实验数据比较符合,并对比折线方程的优点加以分析说明,为可靠性设计研究奠定了理论基础。     

电刷镀Fe-Re对20CrMnTi钢疲劳性能的影响

通过20CrMnTi钢经电刷镀Fe-Re、电刷镀Fe和离子硫碳氮共渗处理的对比试验,分析了其疲劳前后的硬度变化、疲劳断口形貌、疲劳源的位置和过渡层的强度等问题.结果表明,20CrMnTi钢经Fe-Re电刷镀可显著提高弯曲疲劳强度,比电刷镀Fe和离子硫碳氮共渗分别提高了9.9%和21.9%,增加了刷镀层的厚度,提高了过渡层强度,并使疲劳源位置向心部推移,从而有效地增强了20CrMnTi钢的疲劳强度和超负荷持久性.     

Comparative study on laser welding characteristics of aluminium alloy under atmospheric and subatmospheric pressures

Laser bead on plate welding of 10 mm thick aluminium alloy under atmospheric and subatmospheric pressures were comparatively investigated. With the decrease of ambient pressure, the penetration depth increased sharply at first and then gradually levelled off. The largest penetration depth could reach 8·7 mm when welded under the pressures of 10¹ Pa, while only 4·9 mm under atmospheric pressure. Weld bead without any porosity was produced under ambient pressures of 10^?1 Pa. The average tensile strength of joints welded under the pressure of 10¹ Pa was 300·2 MPa. The tensile strength remained constant as the ambient pressure decreased further. The shielding effect of plasma plume on laser beam was suppressed as the ambient pressure decreased. Therefore, the laser power deposition inside the keyhole was enhanced effectively. Under subatmospheric pressure, the porosity defects were eliminated effectively due to the keyhole stability and the change of liquid flow, i.e. moving upward along the rear wall of keyhole.     

调质对钴基碳化钨复合涂层疲劳强度的影响

由于真空烧结涂层基体的硬度较低,采用对基体调质处理的方法来提高涂层基体的硬度.然后测量涂层和基体的硬度,并通过采用旋转弯曲疲劳试验比较涂层试样与未涂层试样的疲劳强度.试验结果表明:调质处理提高基体的硬度,而涂层硬度基本不变;涂层与未涂层试样的S-N曲线相互交叉,在N＜3×105时,涂层试样的疲劳强度大于未涂层试样,但在N＞3×105时,则结果相反.     

防喷器壳体材料疲劳试验的声发射信号分析

根据新防喷器声发射检测遇到的数据难分析、难评定等问题,针对防喷器壳体材料进行多种疲劳试验,在试验过程中全程采集声发射信号数据。通过对比不同的试验数据,分析出防喷器壳体材料中缺陷扩展和典型干扰的信号特征,以及材料在疲劳过程中声发射信号特征的变化趋势。并且把这些分析结果应用到新防喷器的检测中,检测证明整个分析非常正确。     

提高压力机摇摆轴疲劳强度的技术措施

在压力机动力传动中承受动载荷的摇摆轴，因其恶劣的受力状态，对其疲劳强度提出了更高的要求。文章提出了提高压力机的摇摆轴焊缝的疲劳强度的技术措施，即将传统的在光轴上的“T”形接头改为台阶轴上的对接接头，可显著提高压力机的摇摆轴焊缝的疲劳强度及压力机的安全使用寿命，此方法值得有关人员借鉴。     

含焊接缺陷的30CrMnSiNi2A钢焊接件的疲劳寿命估算

本文分析了焊接缺陷对超高强度钢３０ＣｒＭｎＳｉＮｉ２Ａ焊接件疲劳寿命的影响，给出了定理评估含焊接缺陷的焊接件疲劳性能的方法，焊接缺陷可分成类裂纹型和非裂纹型两类，前者的作用与裂纹相当，含这类缺陷的焊接件寿命近似等于裂纹扩展寿命；后者的作用则类似缺口，含这类缺陷的焊接件寿命为裂纹起始与扩展寿命之和，裂纹扩展寿命可用断裂力学的方法计算，始裂寿命则可用材料的始裂寿命公式计算，但在计算中要考虑表面状态修正     

Fatigue strength of chromium coated elements and possibility of its improvement with slide diamond burnishing

Surface layer condition and fatigue strength of chromium coated elements were tested. Special shape samples cut out of chromium coated shafts were symmetrically deflected on electrodynamic vibrator at sample frequency oscillations in the 1000 Hz order. They were 42CrMo4 and 41Cr4 steel samples with a 25 and 50 μm chromium coat finished by slide diamond burnishing and polishing. Surface topology parametres, surface microhardness and residual stresses in the surface layer were checked and fatigue strength was tested. Before the coating was applied, the roller surfaces of the samples were ground and band polished. It was found that chromium electroplating causes detrimental, tensile stresses in the surface layer and worsens the fatigue strength limit. Slide burnishing of chromium coatings produces advantageous, compressive stress in the surface layer and the oscillatory bending fatigue strength of elements with burnished coatings can be improved up to 40%, which completely reduces the detrimental chromium plating effects. It can also be stated that slide burnishing of coatings does not create bigger technical problems and gives better results than band polishing.     

Fatigue strength of chromium coated elements and possibility of its improvement with ball peening

The paper presents the results of the investigation of fatigue strength improvement possibilities making use of ball peening. Made of 41Cr4 steel samples with 25 µm thin chromium coatings were tested. Surface stereometric parametres and fatigue strength limit at oscillatory bending were measured. The correlation between the Sa parametre of the ball-peened surface and fatigue strength limit was proved. It was stated that ball peening of chromium coatings produces advantageous, compressive stress in the surface layer and the oscillatory bending fatigue strength of elements with ball-peened coatings can be improved up to 34%, which almost completely reduces the detrimental chromium plating effects. As a result it was found that ball peening can find its application in restoring fatigue strength impaired by chromium electroplating.     

Performance of activated TIG process in austenitic stainless steel welds

Five kinds of oxide fluxes, MnO₂, TiO₂, MoO₃, SiO₂, and Al₂O₃, were used to investigate the effect of activated tungsten inert gas (activated TIG) process on weld morphology, angular distortion, delta-ferrite content, and hardness of Type 316L stainless steels. An autogenous TIG welding was applied to 6 mm thick stainless steel plates through a thin layer of flux to produce a bead-on-plate welded joint. The oxide fluxes used were packed in powdered form. The experimental results indicated that the SiO₂ flux facilitated root pass joint penetration, but Al₂O₃ flux led to the deterioration in the weld depth and bead width compared with conventional TIG process. Activated TIG welding can increase the joint penetration and weld depth-to-width ratio, thereby reducing angular distortion of the weldment. On the basis of the present results, it is considered that the centripetal Marangoni convection and constricted arc plasma as a mechanism in increasing the penetration of activated TIG joint.     

Effect of tool geometry on friction stir processing and fatigue strength of MIG T welds on Al alloys

Several tool geometries were developed and their effect on weld morphology, material flow, microstructure and hardness of processed regions was analyzed. Their effect on fatigue strength of welds was also examined for the most promising tools. The feasibility of FSP MIG T welds was proved. Quality of processed regions is very influenced by tool geometry. FSP removes defects in the MIG weld toe and increases its radius of curvature. Also promotes significant grain refinement in processed regions, reduces hardness in welds on AA 6082-T651 and hardens AA 5083-H111 welds. Only tools with concave and rounded edge shoulder and cylindrical threaded pin allow the improvement of fatigue strength of MIG welds on both alloys.     

真空熔烧Ni基合金-WC复合涂层疲劳强度的研究

采用旋转弯曲疲劳试验的方法，比较真空熔烧Ni基合金-WC复合涂层试样与未涂层试样在正火处理后的疲劳强度。测量正火前后涂层与基体的洛氏硬度，还测试涂层与基体的显微硬度变化规律。用扫描电镜观测涂层的微观组织，用X射线能谱仪测量熔烧后涂层中各相化学成分，用X衍射仪分析涂层的相结构。借助这些微观测试来分析疲劳试验结果。