Improved productivity through failure analysis: Case studies in precision forging of aerospace components

In this study, usefulness of failure analysis in improving productivity through selection of better materials and heat treatments that delay or prevent the on set of fracture in precision forging of aerospace components is illustrated. Examples have been taken from failed cold working dies, hot forging dies and broaches. Detailed metallographic analysis of the microstructures and fractures has pinpointed the sources and mechanisms of failure. This, in turn, has indicated the means of preventing such failures in service and, in improving die life and performance, by using advanced materials and optimizing heat treatments and surface engineering treatments.     

闭塞冷锻行星齿轮齿形修正方法研究

针对闭塞冷锻行星齿轮齿形修正方法进行了研究.利用弹塑性有限元法模拟了行星齿轮闭塞冷锻过程,采集成形终了前两步工件表面各点处的接触应力作为模具弹性变形分析的边界条件,对模具进行弹性有限元分析.获得了模具弹性变形分布图,提取了齿形凹模齿长平分截面由齿根至齿顶的齿厚弹性变形量.以变位修正法为指导思想,提出了齿轮闭塞冷锻齿形凹模新的修正设计方法,并获得了试验验证.     

Metallurgical failure analysis of cold cracking in a structural steel weldment: Revisiting a classic failure mechanism

Cold cracking of structural steel weldments is a well-characterized, well-documented, and well-understood failure mechanism. Extensive effort has been put forth to recognize the welding and materials selection parameters that are conducive to cold cracking; however, these engineering efforts have not fully eliminated the occurrence of such failures. This article describes cold cracking failure specifically related to the construction industry. This particular failure was successfully identified prior to final erection of the structural member, and the weld was successfully reworked. These actions potentially prevented a serious catastrophic event that could have occurred have occurred either later in the construction process or possibly during the use of the building. Individual welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure, and a number of human factors relating to the actual fabrication practices also contributed to the failure process.     

Premature failure analysis of forged cold back-up roll in a continuous tandem mill

In this paper, premature failure of a forged back-up roll from a continuous tandem mill was investigated. Microstructural evolutions of the spalled specimen and surface of the roll were characterized by optical microscopy, X-ray diffraction, scanning electron microscopy and ferritscopy, while hardness value of the specimen was measured by Vickers hardness testing. The results revealed that the presence of pore and MnS inclusion with spherical and oval morphologies were the main contributing factors responsible for the poor life of the back-up roll. In addition, metal pick up and subsequently strip welding on the surface of the work roll were found as the major causes of failure in work roll which led to spalling occurrence in the back-up roll. Furthermore, relatively high percentage of retained austenite, say 9%, in outer surface of the back-up roll contributed spalling due to conversion of this meta-stable phase to martensite and creation of volume expansion on the outer surface through work hardening during mill campaign.     

Process design of cold forging with thick plate for seat recliner parts

A seat recliner is a device that connects the seat back with the seat frame. It can be used to adjust the lean angle. An important component of a seat recliner is the sector tooth, which has thus far been manufactured by the fine blanking process. In this study, we propose the use of the cold-forging process as an alternative in order to achieve improved mechanical stiffness and reduced weight. To apply the cold-forging process, the entire forming process is divided into two stages in order to improve the dimensional accuracy of the tooth part of the sector tooth. Uniform plastic deformation is induced by the design of the preform on the contacting area in the sector tooth by simultaneously using a bending and a forging mechanism. By the proposed process design for the two-step cold forging process, the mechanical stiffness and surface roughness of the test product can be improved. Furthermore, the weight can be reduced by 10% by applying a 3.6-mm-thick plate instead of a 4.0-mm-thick one. Finally, the number of forming stages can be reduced without sacrificing the dimensional accuracy of the sector tooth and increasing the punch load.     

Effect of friction factor on barrelling in elliptical shaped billets during cold upset forging

This work deals with the effect of friction factor of different lubricants on barrelling in elliptical shaped billets during cold upset forging. Experiments were carried out to generate data on cold upset forging of commercially pure aluminium solid of elliptical billets with different lubricants applied on both sides in order to evaluate the effects of friction factor on the forming behavior of barrelling phenomenon under plane stress state condition. The radius of curvature of both major and minor bulge measured conformed to the calculated one using experimental data and the calculations were made with the assumption that the curvature of the bulge followed the form of a circular arc. Three different b/a ratios (ratio of minor to major diameter) namely, 0.6, 0.7 and 0.8 and aspect ratio (ratio of height to diameter) of 0.75 were prepared and cold upset forged. The relationship was also established between the various bulge parameters like the hoop strain, the axial strain, the new geometric shape factor, the stress ratio parameters, the major and minor radius of curvature of the bulge and the friction factor ‘m’ of different lubricants.     

汽车六角球头冷锻工艺优化与数值仿真

冷锻成形工艺是一种少无切削的净近成形工艺,以其精度高、生产效率高、低耗节能等优点,大量使用在汽车零配件的生产。六角球头销是汽车转向系统中的关键零件,其六角成形的质量直接影响到产品的使用性能。本文针对六角一次成形发生镦锻失稳问题,基于金属塑性成形理论,分析在镦锻过程中的金属流动,提出采用预成形工艺。根据产品结构,提出了大V锥形、锥形、锥六角形三种预成形工艺方案,采用DEFORM-3D对六角头成形方案进行数值仿真。为了得到最优参数,通过建立优化数学模型,对数值仿真获取的应力应变、损伤值分布、载荷进行优化得到最优方案。结合有限元数值分析对比分析三种方案,得到了预成形为锥形的成形效果最好,材料流动更为合理。经生产试验证明,通过预成形为锥形生产的六角头充填饱满,无不良缺陷,对生产同类型的成形工艺具有一定的参考价值。     

Fatigue and mechanical characteristics of nano-structured tool steel by ultrasonic cold forging technology

Ultrasonic cold forging technology (UCFT) utilizing ultrasonic vibration energy is a method to induce severe plastic deformation to a material surface, therefore, the structure of the material surface becomes a nano-crystal structure from the surface to a certain depth. It improves the mechanical properties; hardness, compressive residual stress, wear and fatigue characteristics. Applying UCFT to a rolling process in the steel industry is introduced in this study. First, the UCFT specimens of a tool steel (SKD-61/equivalent H13) are prepared and tested to verify the effects of the UCFT in a variety of mechanical properties, the UCFT is applied to the trimming knives in a cold rolling process. It has been determined that UCFT improves the mechanical properties effectively and becomes a practical method to improve productivity and reliability by about two times compared with the conventionally treated tooling in the trimming process in a cold rolling line.     

Thermo-mechanically coupled FE analysis and sensitivity study of simultaneous hot/cold forging process with local inductive heating and cooling

The numerical investigation of the production of a stub shaft is presented, where the highly innovative metal forming technology of the simultaneous hot and cold forging is applied in combination with a hardening process performed directly in the closed forging dies after the forging step similar to press hardening of sheet metal. This complex forging process is completely analysed by means of a finite element simulation including the local inductive heating phase of the workpiece as well as the cooling process of the final stub shaft inside the forging dies. All relevant process parameters and the whole simulation model are documented in detail and the simulation results are discussed and validated by means of experimentally measured data, showing good agreement. Parameter studies for various properties of the model are carried out in order to investigate their influence on the geometry and the temperature field development, whereby a deeper understanding of the entire process is gained. Thus, a finite element benchmark analysis is provided for such a complex thermo-mechanically coupled structuring process.     

高强度低屈强比铆螺钢轧制工艺的研究

为消除螺栓在热处理中产生的各种缺陷,缩短生产周期,使铆螺钢免热处理.采用热模拟试验机、实验室轧机和多工位冷镦机对铆螺钢轧制实验,铆螺钢原料和成品分别在拉力试验机和万能试验机上进行拉力试验,并对其组织进行了分析.结果表明,铆螺钢经过控轧控冷,获得具有多边形铁素体、细片状珠光体、粒状贝氏体、残余奥氏体和少量MA岛的多相组织.由于控轧控冷后的多相组织及TRIP效应,改善了螺栓的强韧性.铆螺钢因低的屈强比可以直接由热轧棒材冷镦成螺栓,螺栓无需最终热处理,产品的力学性能满足8.8级螺栓国家标准的相应要求.     

Numerical shape optimization of cold forging tools by means of FEM/BEM simulation

The procedure of a numerical shape optimization of the cold forging tool geometry allows for a reduction and a homogenization of tool load stresses. This procedure considers the workpiece-tool-machine interaction by means of the FEM/BEM coupling. The coupling requires modifications to insure accurate integration in TOSCA software for further shape optimization. The resulting distribution of the nodal displacements and changes of tool geometry are discussed and analyzed. Additionally, the numerical validation of the results by means of mechanical simulation of the optimized geometry with the extended FEM/BEM model is given. The equivalent stresses distribution at the end of lateral extrusion in the tool and in workpiece is presented. The numerical shape optimization leads to the maximum equivalent stress value reduction on the press shoulder on 856 MPa, corresponding to a percentage decrease of 24.3 % in comparison with the initial geometry. The approach for the compensation of load induced workpiece deviations, involving the described optimization procedure, is presented as well.     

Barrelling of aluminium solid cylinders during cold upset forging with constraint at one end

Abstract

Experiments were carried out to generate data on the cold upset forging of solid cylinders of annealed aluminium using different lubricants, with a die at one end providing a constraint. The die had created back extrusion of metal through its hole and developed two barrels in the unrestrained portion of the cylindrical specimens. The curvatures of the barrels, top and bottom, developed in the unrestrained portion were found to conform one to one to calculated values using experimental data. The calculations were made on the assumption that the curvatures of the barrels followed circular arc geometry. It was further found that the measured radius of curvature of both barrels exhibited a straightline relationship with the new geometrical shape factor, irrespective of the aspect ratios of the cylinders. Further empirical relations were found to exist between the measured radius of curvature of both barrels and other variables, namely hydrostatic stress and stress ratio parameter.     

Failure analysis of mould dies of an industrial punching machine

In this research, the high failure rate of indigenously produced mould die of an industrial punching machine utilized for the production of cable trays was investigated. This was aimed at discovering the reason(s) for the disparity in the service life of the indigenously produced die as against the imported ones that had longer service life. The failure investigation was carried out utilizing visual examination, microstructural examination, hardness test and chemical composition determination. The analysis show that the short service life of the indigenous die component is due to incorrect heat-treatment which did not remove the cold-worked structure in built in the material during production, thus resulting in inferior toughness and/or fatigue resistance. It was equally identified that occasional misalignment of the mould upper die teeth and lower die plate due to over exertion of the machine contributes to failure of the die material.     

盘类锻件成形过程变形模式的模块化分析

针对塑性体积成形过程复杂且金属流动行为难于分析的现状,从塑性加工力学角度,首次将复杂盘类锻件模锻成形过程的不同阶段和区域抽象分解为圆柱压缩、圆环压缩和类挤压3个基本变形模式,使盘类锻件模锻问题的分析模块化.抽象构造出三维复杂构件模型,针对模锻成形过程的不同阶段和区域,利用基本变形模式的成形规律对其进行深入分析,给出了耦合成形过程中金属变形及流动的规律,及不同模腔内金属流动分界面的形状和塑性区的分布特征.通过深入分析不同变形模式及互相耦合流动行为,揭示了盘类构件模锻成形过程中金属的变形流动规律,为盘类锻件精确塑性成形的变形流动控制提供理论据.     

Failure analysis of an open die forging drop hammer

This paper analyses the failure of a large open die forging drop hammer. The analysis had to be semi-destructive as it was wanted to repair the equipment and to return it into service. A visual examination was performed at the forging shop and fatigue initiated at two internal defects blamed for the failure. A small sample was extracted from the fracture surface. Unfortunately, due to the desire of recovering the drop hammer this sample did not belong to those zones which were identified to be the origins of the failure but to the zone of progression of the crack by fatigue close to the overload final failure. Fractographic analysis by scanning electron microscopy revealed the presence of some ductile dimples pointing to an acceleration of the cracking process at this stage.     

Numerical and experimental investigation of cold rotary forging of a 20CrMnTi alloy spur bevel gear

Due to the complex three-dimensional (3D) geometry and tooling design, mass production of precision spur bevel gears by machining or conventional forging technology is impeded by numerous stubborn barriers to date. Cold rotary forging, an innovative incremental metal forming process, has great potential to improve the current situation owing to its flexibility and low tool load requirement. In the present study, a sound 3D rigid-plastic finite element (FE) model of cold rotary forging of a 20CrMnTi alloy spur bevel gear is developed under the DEFORM-3D platform. To ensure the precision of the proposed FE model, a series of experiments are well performed for the identification of the mechanical properties of 20CrMnTi alloy and the realistic friction conditions prevailing at the die-workpiece interface. By utilising this FE model, the workpiece geometry is optimised with the intention of achieving a better filling of gear shape and a lower forming load requirement, and then the distribution of different field-variables such as flow velocity and effective strain are thoroughly investigated. For verification purposes, the cold rotary forging experiments of 20CrMnTi alloy spur bevel gears are subsequently conducted. Good agreement between the experimental results and the simulation ones is highlighted by comparing the gear shape after cold rotary forging and the axial forging force, which validates employed model.