超声冲击处理MB8镁合金十字接头的表层组织及疲劳性能

采用超声冲击方法对MB8镁合金十字接头焊趾处进行超声冲击处理,对比测试焊态及冲击处理态接头的表层组织和疲劳性能。结果表明:超声冲击处理可以在MB8镁合金焊接接头表面获得纳米晶组织。在循环寿命为2×106条件下,焊态试样的条件疲劳强度为32.07MPa,冲击处理态试样的条件疲劳强度为41.88MPa,提高了30.59%。未冲击接头疲劳断裂大多发生在焊缝缺陷处,冲击处理后的接头则发生在热影响区。超声冲击处理不仅可以大幅提高MB8镁合金十字接头的疲劳寿命,还可以改变接头疲劳断裂位置。此外,热影响区也是MB8镁合金十字接头疲劳断裂的薄弱区域,这与热影响区晶粒粗大有很大的关系。     

超声冲击处理改善焊接接头力学特性研究进展

焊接接头处往往存在较高的残余应力，使得焊接接头成为焊接结构的薄弱区域。而超声冲击处理可以有效改善焊接接头力学特性，在各领域得到了广泛应用。本研究主要介绍超声冲击技术在降低残余应力和提高接头疲劳性能方面的研究情况，以及随焊超声冲击技术的最新相关研究和应用。     

超声捶击提高超细晶粒钢焊接接头的疲劳性能

焊接接头疲劳强度是其最重要的服役性能，由于焊接残余应力的作用和焊接接头处的几何不连续性，焊接接头的疲劳强度一般大大低于母材，采用超声捶击方法提高超细晶粒钢焊接接头的疲劳强度，通过对对接接头焊践处进行超声波冲击处理，对比超声波冲击处理后焊接接头的疲劳强度，实验结果表明：超声捶击使得S-N曲线右移，FAT（循环寿命为10^6时的疲劳强度）提高幅度达到66%，在应力范围为200MPa的疲劳寿命提高58倍，研究表明，经超声捶击处理，焊趾处的应力集中系数相应减小，焊接残余应力由拉应力转换为压应力，这是超声捶击提高焊接接头疲劳强度的主要机制。     

超声冲击对纯钛材焊接接头疲劳性能的影响

目前,有关超声冲击法对工业纯钛焊接接头疲劳性能改善效果的影响研究报道不多。采用超声冲击处理方法对工业纯钛TA2焊接接头进行全覆盖强化处理,通过疲劳对比试验分析了不同超声冲击处理时间对TA2焊接接头疲劳性能的影响;通过接头微观组织、断口形貌观察分析了超声冲击处理提高TA2焊接接头抗疲劳断裂性能的微观机理。结果表明,在200 MPa应力条件下,原始焊态试样平均历经11 645次循环就已断裂,经超声冲击处理3 min的焊接试样平均历经23 424次循环后才断裂,疲劳寿命提高约1倍;在超声冲击过程中,试样表层受到大的应变量、高应变速率和重复载荷的作用,使得形变孪晶不断增加;重复载荷的作用,可以再次在内部产生更加细小的孪晶,孪晶与孪晶间重复交割导致晶粒进一步细化。     

镁合金焊接接头表面自纳米化及其疲劳性能研究现状与展望

焊接接头表面纳米化解决了决定焊接接头疲劳裂纹萌生和扩展的内在因素之间的矛盾,在细化接头表面晶粒的同时,使基体保持粗晶状态,因而是提高焊接接头疲劳性能的一种有效方法。综述了镁合金表面自纳米化机理及研究现状,分析了表面自纳米化技术在改善镁合金焊接接头疲劳性能方面的研究概况,指出从镁合金焊接接头表面自纳米化的角度开展镁合金焊接接头疲劳性能改善研究具有重要意义。最后,对今后应用表面自纳米化技术改善镁合金焊接接头疲劳性能的研究方向做了展望。     

超声冲击对铝合金搅拌摩擦焊接头组织及性能的影响

目的分析超声冲击对铝合金搅拌摩擦焊成形后接头的组织及耐蚀性的作用效果。方法采用超声冲击设备对2A12铝合金搅拌摩擦焊接头表面进行超声冲击处理,并对超声冲击前后接头的显微组织、显微硬度以及耐腐蚀性能进行了分析。结果经过超声冲击处理后的铝合金接头上表面会产生一层塑性变形层,并且塑性层内位错密度增大,使表层金属得到一定程度的加工硬化,促使冲击后接头各区域的表面硬度明显提高,冲击后接头热机械影响区和热影响区硬度提高达60%以上;腐蚀浸泡试验发现,超声冲击后接头的点腐蚀程度较超声冲击前明显减缓,腐蚀速率约是冲击前的1/2。结论超声冲击有效改善了铝合金搅拌摩擦焊接头区域材料过时效的软化现象,并且有效改善了接头的抗腐蚀性能。     

超声冲击处理钛合金焊接接头的性能研究

采用超声冲击工艺,处理了钛合金两种型式的焊接接头。试验结果表明:超声冲击处理可显著降低接头焊接残余应力,并可提高接头的疲劳强度和疲劳寿命。其中对于Ti80合金的对接接头和十字接头,疲劳极限分别提高22.7%和64.5%,疲劳寿命分别提高5.4倍和13.5倍;对于TA2和Ti75合金的对接接头,焊接残余应力分别降低68%和65%。     

超声冲击对高速列车转向架焊接十字接头超高周疲劳性能的影响

针对高速列车转向架在实际运用中出现的疲劳问题,利用HJ-Ⅲ型超声冲击处理装置,对高速列车转向架用SAM490BW钢十字接头焊趾进行超声冲击处理,并对处理后接头的超高周疲劳性能进行研究。结果表明,原始焊态试样和超声冲击态试样的S-N曲线均出现了传统意义上的水平段。超声冲击处理试样的疲劳强度相比于未处理试样提高了25%。经过超声冲击处理后,疲劳裂纹源的数量由多个变为单个,焊接接头焊趾处的应力集中系数下降了32.4%,降低了裂纹萌生概率;超声冲击降低了焊趾区域的残余拉应力,并诱发产生残余压应力,高达-255.5 MPa;焊趾表层晶粒得到细化,晶粒平均尺寸小于100nm;裂纹在塑性变形区扩展路径变长。     

氧化膜对6082铝合金搅拌摩擦焊接头疲劳性能的影响

对6 mm厚的6082-T6铝合金进行两种表面处理然后实施搅拌摩擦焊接,研究了对接面氧化膜对接头组织和疲劳性能的影响。结果表明,进行速度为1000 mm/min的高速焊接时,对接面未打磨和打磨的接头焊接质量都良好,接头强度系数达到81%;两种接头的疲劳性能基本相同,疲劳强度均为100 MPa;少数样品在焊核区外断裂,大部分样品在热影响区断裂。与接头相比,两种接头焊核区的疲劳性能有所提高,均为110 MPa,在疲劳测试中裂纹并未沿“S”线萌生和扩展。     

Comparison of fatigue properties between friction stir welds and TIG welds

对比分析了搅拌摩擦和氩弧焊两种工艺方法对铝合金焊接接头疲劳性能的影响,建立了焊接接头的S-N曲线,结果表明：在相同的载荷条件下,搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头。搅拌摩擦焊接头疲劳寿命N=106次的疲劳强度值约为59~65MPa之间。对焊接接头显微组织的分析表明：搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区,阻碍了滑移带的形成和裂纹的扩展,从而提高了接头的疲劳性能。TIG焊接接头疲劳端口分析显示,焊接缺陷是主要的疲劳裂纹源。     

Effects of ultrasonic impact treatment on pre-fatigue loaded high-strength steel welded joints

Comparative fatigue tests were conducted on as-welded, weld toe peened specimens before and after fatigue loading. Fracture surface, residual stress, microstructure and hardness were determined. The test results showed that as the pre-fatigue loading period extended, deeper cracks may have initiated and propagated and the fatigue life improvement decreased. The processes of ultrasonic peening on welded joints with existing cracks were modeled by finite element analysis. The numerical results indicated that the mechanism of UIT improving fatigue performance included two factors: compressive residual stress and the change of crack orientation. Both effects reduced as the crack became deeper.     

Experimental evaluation of fatigue behaviour of thin Al5456 welded joints

Because of wide applications of welded structures in different industries, using design codes and standards such as IIW recommendations is known as a safe and common method to design welded joints. The weld geometry and thickness of welded joint are the most important parameters that affect the fatigue strength of welded joints. In the present study, the fatigue behaviour of thin Al5456 butt‐welded joints has been investigated, and the effect of thickness on fatigue strength has been evaluated. Contrary to the above‐mentioned recommendations about thin welded joints, it was shown that the thickness of welded joints affects the fatigue strength. Moreover, the fatigue test results have been compared with the IIW design recommendations for three well‐known approaches in order to analyse the reliability of the codes. According to the design stress‐life diagrams, it was found that in some cases, the fatigue strength has much larger values than the IIW predictions, and IIW‐based design causes an over conservative design. While in some other cases, the fatigue strength is lower than IIW recommendations, and it leads to a non‐conservative design. Based on the experimental results, the new values for slope of S‐N curve and FAT have been proposed in order to improve the design diagrams.     

Fatigue strength improvement of longitudinal fillet welded out-of-plane gusset joints using air blast cleaning treatment

Blast cleaning treatments are used widely in newly built steel structures to clean forged surfaces and increase the adhesive properties of subsequent coatings. On the other hand, the beneficial effects of a blast cleaning treatment, which are similar to shot peening, on the fatigue strength of welded steel structures are not considered in the fatigue design procedure. In this study, fatigue tests were carried out on as-welded and blast-treated longitudinal fillet welded out-of-plane gusset joints subjected to three different cyclic loading conditions: uniaxial tension, out-of-plane bending and in-plane bending stress cycles. The effect of the blast cleaning treatment on the fatigue strength of the gusset joints was studied. The fatigue tests showed that the blast cleaning treatment increased the fatigue strength of the gusset welded joints, particularly at the lower stress range. A 19% increase in fatigue strength at 2 million cycles and 66% increase in fatigue limit could be realized using the blast cleaning treatment.     

Influence of stress concentrators on the fatigue and fracture characteristics of steels and welded joints

We study the effect of enhancement of the fatigue strength of welded joints in steel structures caused by the removal of the concentration of welding stresses. Main attention is given to the so-called method of contact treatment of the surface of welded joints close to the technology of arc welding. It is shown that the application of this method not only removes the defects of welded joints but also significantly increases their fatigue strength (by 20–100%). Welding Research Institute, VúZ, Bratislava, Slovakia. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 5, pp. 87–90, September–October, 1998.     

Strength enhancement of the welded structures by ultrasonic peening

Residual stresses exist in all manufacturing processes which use heat and/or force such as casting, forming, machining and welding. Sometimes they cause decrease in strength and life of components especially under dynamic loads and vibration conditions. To improve fatigue strength, a number of post treatment operations are being used such as grinding, shot peening, re-melting and heat treatment. Ultrasonic peening is a newly developed method for the improvement of fatigue strength of, mainly, welded joints and structures. By employing this process, geometry of weld toe can be modified for reducing the stress concentration. In addition, elimination of tensile residual stresses, exertion of compressive residual stresses and closing of cracks, voids and cavities are expected, too. The extra advantage of this technology is its application on massive and large structures which cannot be treated by other procedures. For investigating the effect of ultrasonic peening on stainless steel-304 welded parts, a series of experiments were designed and implemented. Ultrasonic peening is mostly used as a mechanical surface treatment method in the automotive and aerospace industries. However, this paper comprises the results of experimental fatigue strength tests along with metallography, micro hardness and corrosion resistance tests of welded pieces with and without processing by ultrasonic peening. Experiments proved that under post treatment by ultrasonic peening, a better mechanical and corrosion resistance is achieved.     

Fatigue strength of fillet‐welded joints at subzero temperatures

Ships and offshore structures may be operated in areas with seasonal freezing temperatures and extreme environmental conditions. While current standards state that attention should be given to the validity of fatigue design curves at subzero temperatures, studies on fatigue strength of structural steel at subzero temperatures are scarce. This study addresses the issue by analysing the fatigue strength of welded steel joints under subzero temperatures. Although critical weld details in large welded structures are mostly fillet‐welded joints, most published data are based on fatigue crack growth rate specimens cut out of butt‐welded joints. This study analyses fillet‐welded specimens at ?20°C and ?50°C against controls at room temperature. Significantly higher fatigue strength was measured in comparison to estimates based on international standards and data from design codes even at temperatures far below the allowed service temperature based on fracture toughness results.     

基于失效评定图(FAD)研究含疲劳裂纹T型圆钢管节点的安全性

失效评定曲线(FAD)常用来评价焊接结构在出现裂纹后的安全性,为了验证这种曲线在评价焊接管结构在节点部位出现疲劳裂纹后安全性的适用性,采用实验测试和有限元分析的方法研究了3个含疲劳裂纹的T型管节点试件在静力作用下的极限承载能力及破坏过程。3个T型管节点试件首先进行疲劳实验在焊趾处产生表面裂纹,然后通过在支管端部施加轴向拉力作用检测节点的破坏过程。基于自行开发的含表面裂纹T型管节点的有限元网格自动产生程序以及ABAQUS分析软件,研究了在管节点破坏过程中表面裂纹最深点的应力强度因子大小,并通过实验的荷载-位移曲线确定了T节点试件的塑性极限承载力。在这些结果的基础上,验证了FAD在评价含疲劳裂纹的焊接管节点安全性方面的适用性。研究结果标明:FAD在评价含疲劳裂纹管节点的安全性方面是安全可靠的,但偏于保守。     

Analysis of the S–N curves of welded joints enhanced by ultrasonic peening treatment

Contrast fatigue tests were carried out on T-shape tubular joints of 20 steel in three conditions: as welded, treated by ultrasonic peening treatment (UPT) before loading and UPT under loading. Results are: (1) Dispersity of test results measured by nominal stress is much larger than that measured by hot spot stress. After UPT before loading, fatigue strength of 20 steel tubular joints measured by hot spot stress increases by 67% and fatigue life is prolonged by 22–45 times. (2) Under low stress ratio R, UPT before loading can improve the fatigue performance of welded tubular joints significantly. (3) Under high stress ratio R, UPT under loading (static load) is recommended to improve the fatigue performance of welded tubular joints. UPT under loading not only enhances the fatigue properties at low stress level, but also at high stress level. (4) The general rule of S–N curves of welded joints treated by UPT is commonly effected by external load (static load) and self release of residual stress.