Conventional Hot Forging Research Articles

Machine-Learning-Assisted Composition Design for High-Yield-Strength TWIP Steel

Twinning-induced plasticity (TWIP) steel is an ideal material for impact-resistant structures and energy absorption because of its high product of strength and elongation. However, compared with other advanced high-strength steels, the relatively low yield strength of TWIP steel is one of the important shortfalls that significantly limits its engineering applications. To enhance the comprehensive properties of TWIP steel, a machine learning design strategy that integrated comparative modelling, SHAP analysis, and multi-objective optimization were adopted in this study. Initially, various machine learning algorithms were compared for their predictive accuracy based on normalized data (273 entries) regarding the microstructure and properties of TWIP steel. Then, performance prediction models for yield strength, tensile strength, and elongation were established. SHAP analysis was subsequently employed to assess the significance and explicit laws of composition and microstructures in these three target properties, identifying key elements that enhance the overall performance. Furthermore, two new TWIP steels with high yield strengths and high products of strength and elongation were developed via multi-objective optimization. Under conventional hot forging + hot rolling + cold rolling + annealing processes, the two designed TWIP steels had yield strengths of 585 MPa and 560 MPa, tensile strengths of 1055 MPa and 1101 MPa, elongations of 55% and 58.5%, and products of strength and elongation of 58.0 GPa% and 66.4 GPa%, respectively. The yield strengths of the designed TWIP steels significantly improved while maintaining a reasonable product of strength and elongation. This work provides important references for the rational development of new TWIP steels.

Tensile and creep resistance improvement of GH4975 alloy for turbine discs via electron beam smelting layer solidification technology

A difficult-to-deform superalloy for turbine disks, which can be used at 850 °C for a long time, was prepared by electron beam melting layer solidification (EBSL) technology with high purity and fine structure. Subsequently, the ingot was processed by hot extrusion technology instead of conventional hot forging, and then the tensile and creep tests were performed after standard heat treatment. The results show that the EBSL alloy exhibits excellent high-temperature strength and creep rupture life compared to the conventional preparation method. Among them, the yield strengths at 20 °C and 850 °C are 8.1 % and 5.2 % higher than those in the literature, and the creep rupture life at 850 °C and 412 MPa is 1.53 times higher than that in the literature. The dislocation-precipitate interaction reveals that dislocation shearing γ′ phase is the major mechanism at 20 °C and 850 °C tensile, and Orowan looping and stacking fault shearing are the decisive mechanism during creep at 850 °C and 412 MPa. MC carbides remaining during solidification are the main source of cracks during deformation by fracture analysis. The reasons for refining the microstructure and improving the purity by electron beam melting are analyzed and explained. We believe this work can provide new ideas and methods for preparing superalloys.

Microstructure control of ferrous driven part fabricated by warm precision forging

A straight bevel gear for automobile differential unit is used to investigate the microstructures and dimensional change of warm forged part. Warm forging is the one of the good processes for near nest shaping since the warm forged parts have good microstructure, surface quality and others. Moreover, the process gives some advantages to reduce and eliminate following steps compared to the conventional hot forging. However, it is essential to have appropriate process parameters and then control the related metallurgical phenomena. Additionally, the control of dimensional accuracy of warm forged part is very important to reduce the following process, such as cold sizing or machining. SAE4130 is used as the material for bevel gear forging and high speed steel is used as die material. The hot compression tests are performed to select an appropriate warm forging temperature. A bevel gear is fabricated practically by warm forging using crank press. According to the microstructure evolution and FEM analysis of forging processes, the effective process data can be obtained.

Characterisation of mechanical behavior of 18CrNiMo7-6 steel with and without nb under warm forging conditions through processing maps analysis

Warm forging is a promising near net shape technique, giving microstructural control and good surface quality, besides reducing or eliminating finishing steps compared to the conventional hot forging. In order to optimize said process it is essential to have a good knowledge and control of a number of metallurgical phenomena. This paper intends to clarify how phenomena such as dynamic recrystallization and phase transformation affect the behavior of operating loads and the microstructure of a 18CrNiMo7-6 steel, a material typically used in the forging of large parts, such as helicopters axes, which are subsequently surface hardened by carbonization. When this steel is alloyed with Nb, complete or partial recrystallization can be avoided, depending of process parameters. Stress-strain curves for the temperature and strain rate ranges of 700–900°C and 0.001–10 s−1 were obtained by compression of cylindrical samples of the steel, with and without addition of Nb. The mechanical behavior of the material, here represented by the stress-strain curves, was related to the microstructural evolution whose dependence with the macroscopic processing parameters (T,ε•) was worked out. Further, the influence of said parameters on the efficiency of Nb as an austenite conditioner was determined. Finally, in order to enrich the analysis, process maps were constructed based on the Dynamic Material Model of the material (DMM) and the kinetics of recrystallization was described by modified Avrami's equation.

외경이 큰 환형 부품의 다중형상 열간 링 롤링 공정의 개발

열간 링 롤링 공법은 열간단조 후의 잠열을 이용하기 때문에 가공 부하가 적고 최종 환형제품에 가까운 제품을 제조하는 장점이 있다. 본 연구에서는 열간 링 롤링 공법에 의한 대형 직경의 환형 부품의 다중형상 링 제조를 목표로 AFDEX2D 및 AFDEX3D/HEXA/RING 시뮬레이터(simulator)를 이용하여 열간 단조공정부터 링 롤링 공정까지 연계된 공정을 유한요소 해석하였다. 또한 링 롤링공정의 유한요소해석 시간을 줄이기 위해 소성변형이 일어나고 있는 영역에서만 요소망을 조밀하게 재구성하는 이중요소 시스템을 적용하였다. 이 유한요소해석 결과를 바탕으로 실제 공정을 설계하고 실험하여 개발된 제품이 품질 규격을 만족하는지를 확인하였다. 또한 링 롤링 실험 완성품을 통하여 부품의 품질수준 및 공정능력을 평가하였다. 그 결과, 개발된 환형 부품은 40%의 투입재료 감소과 최종 제품의 원소재 회수율이 24% 증가하였다. 또한 링 롤링 제품은 내, 외경의 진원도가 0.5mm로 우수하였으며 공정능력 분석결과 외경 Cpk 1.49, 내경 Cpk 0.85를 확보하여 환형 부품의 다중 형상 열간 링 롤링 공정의 최적화를 실현하였다. 【In this study, multilayered rings with a large outer diameter have been developed using a hot ring rolling process. The ring rolling process has been analyzed by rigid plastic finite element analyses (FEA) using the AFDEX2D and AFDEX3D/HEXA/RING simulators, where the finite element meshes received severe plastic deformation are remeshed into a fine mesh-size using a dual-mesh system. According to the simulated results, the design variables of the multilayered rings were determined and real tests were conducted to check the validity of the simulation results. By adopting the hot ring rolling process, the input weight of raw materials was reduced by 40% against the conventional hot forging process and that the recovery rate was increased by 24%. The measurement of the averaged roundness was satisfied within 0.5 mm for both the inner and outer diameters. Moreover, the hot ring rolling processes yielded 1.49 Cpk for the outer-diameter and 0.84 Cpk 0.84 for the inner-diameter.】

Impact of Process Parameters on the Auto Claw-Pole Thixoforming Process

Thixoforming of steel is a potential forming technology, which can realize near-net-shape forming process with good quality in one forming step. In this paper, thixoforming process was used to replace the conventional hot forging process to form the auto claw-pole. The finite element code Forge2008Ó was used to simulate the auto claw-pole thixoforming process. The impact of three main process parameters such as initial billet temperature, punch speed and die temperature on the forming process were investigated. The reasonable process parameters for the auto claw-pole thixoforming were obtained: initial billet temperature 1430~1440°C, punch speed 100~200mm/s and die temperature 300~400°C.

Mechanical properties of as-forged Ni-free Co–29Cr–6Mo alloys with ultrafine-grained microstructure

We have succeeded the fabrication of Ni-free Co–29Cr–6Mo alloys with ultrafine-grained (UFG) microstructures via conventional hot forging process with the reduction up to 83.4% (true strain of 1.8). Grain refinement to submicron order is an effective method for improvement of strength and athermal martensite ɛ phase formed during quenching seems to determine the ductility of this alloy system.

Numerical simulation of a thixocasting process for AISI420 stainless steel air-turbine blade

A new technology, semi-solid casting (thixocasting) method, was used to replace the conventional hot forging process to form AISI420 stainless steel air-turbine blade. The power law cut-off (PLCO) material model in Procast software was used to simulate the thixocasting process. The thixocasting process was simulated. The results show that the reasonable technology parameters for air-turbine blade thixocasting process are obtained: billet temperature 1 483-1 485 °C, piston velocity 12-15 m/s and die temperature is about 400 °C.

Warm forging: new forming sequence for the manufacturing of long flat pieces

Warm forming of steel is an economic and ecologic alternative to the conventional hot forging technology. It offers several advantages like decreased energy input, reduced scale formation, better surface quality and closer tolerances. Warm forming is common for rotation symmetric parts but has not been applied for long flat pieces yet. The main obstacles prohibiting the transfer are the missing of a suitable preforming technology and the problems to keep the narrow temperature tolerances despite the increased number of forming operations compared to rotation-symmetric process routes. In a cooperative effort within a European consortium of forges and scientific institutes, closed-die warm forging processes for long flat pieces have been developed. To cover a wide range of longitudinal geometries, two model products, a connecting rod and a steering link, with unequal requirements towards the production process have been selected. The process developed for the steering link is discussed in this article.

AZ80 and ZC71/SiC/12p closed die forgings for automotive applications: technical and economic assessment of possible mass production

In this study, redesigning for equal functional properties and rapid prototyping of hot forged automotive parts based on commercially available wrought magnesium alloys (AZ80) and composites (AZ80/SiC/12p) were performed. To achieve the same functional properties in a forged automotive component irrespective of the structural material selected, an automotive connecting rod, mass produced in aluminium alloy 6061, was redesigned to use wrought magnesium alloy (AZ80A) and particle reinforced magnesium alloy matrix composite (ZC71/SiC/12p). By applying conventional hot forging technology, prototype connecting rods were forged, trimmed, and heat-treated on a semiindustrial scale. The microstructure of both as extruded and hot forged specimens was examined and the tensile properties of AZ80, ZC71/SiC/12p, 6061 and 6061/SiC/15p test bars machined from hot forged connecting rods were measured. Weight reduction in the redesigned connecting rods was determined and the substitution of magnesium and aluminium based materials for steel was compared in terms of cost.

열간 단조 공정에서 금형 수명 향상을 위한 공정 설계

This paper explains the process design for improving tool life in the conventional hot forging process. The thermal load and the thermal softening are happened by contact between the hotter billet and the cooler tools in hot forging process. Tool life decreases considerably due to the softening of the surface layer of a tool was caused by a high thermal load and long contact time between the tools and the billet. Also, tool life is to a large extent limited by wear, heat crack and plastic deformation in hot forging process. Above all, the main factors which affect die accuracy and tool life we wear and the plastic deformation of a tool. The newly developed techniques for predicting tool life are applied to estimate the production quantity for a spindle component and these techniques can be applied to improve the tool life in hot forging process.

Effect of processing parameters on shear bands in non-isothermal hot forging of Ti -6Al -4V

The alloy system Ti- 6Al- 4V is the prominent Ti alloy system for aerospace and biomedical applications, as a result of its mechanical property balance and biocompatibility. Since the mechanical characterisation of Ti- 6Al- 4V is strongly sensitive to processing parameters there is relationship between processing variables, i.e. strain rate and temperature, microstructure, and properties under different loading conditions. Two phase (α + β) titanium alloys undergo flow instabilities and are susceptible to shear bands or regions of localised deformation crossing many grains during hot forging under non-isothermal conditions (dies and workpiece at different temperatures). Under such conditions shear bands can be generated even in materials without flow softening attributes. This occurs if the forging parameters lead to large amounts of heat transfer between the dies and the workpiece. This study investigates the occurrence of shear bands during non-isothermal, hot forging of Ti -6Al- 4V in order to evaluate the process parameters that generally lead to shear bands in conventional hot forging of metals. Upset compression tests on cylindrical specimens were conducted in a mechanical press and lateral side pressing tests on long, round bars were performed in either a mechanical press or a hydraulic press. The tests ranged from axisymmetric to plane strain compression. In upset specimens shear bands occurred at an angle of 45° to the compression axis and bands of intense deformation separated chill zones from the deforming bulk. Observation also demonstrated that the fracture might be owing to microvoids nucleated at weak points in sections of the shear surfaces. For plane strain deformation, shear bands were found to initiate along zero extension directions in a manner analogous to the formation and propagation of shear bands in isothermal hot forging. Although the shear band features at hot forging temperatures were similar to each other, there was a difference in the hardness and thickness of the shear bands depending on deformation mode, amount, and temperature.

Die design for flashless forging of complex parts

In conventional hot forging of connecting rods, the material wasted to the flash accounts approximately 20–40% of the original workpiece. In order to reduce the cost of forged products, the forging must be performed in a closed cavity to obtain near-net or net shape parts. In flashless forging, the volume distribution of the preform must be accurately controlled to avoid overloading the dies and to fill the cavity. Additionally, the preform must be simple enough to be mass-produced. This study deals with the preform design for flashless forging of a connecting rod and introduces a new tooling concept for forging of complex parts with a controlled amount of flash. In both studies the use of process simulations has been helpful in the performance of several iterations without requiring the construction of expensive tooling.

Thixoforming of a novel layered metal matrix composite

Thixoforming is a semisolid processing route capable of producing near net shape components with low levels of porosity in a single, rapid operation. It has potential for the processing of aluminium alloy based metal matrix composites (MMCs). In contrast with casting of fully liquid metal, the flow of material into the die in thixoforming is primarily laminar. Thus, a novel layered MMC structure, generated by spray forming, consisting of alternate lamellae of reinforced and unreinforced alloy, might be preserved in thixoforming. The laminated structure is intended to give an increase in toughness, compared to a homogeneously reinforced material. The material was produced by spray forming alternate layers of aluminium matrix alloy 2014 (Al–4Cu–0·8Si–0·8Mn–0·5Mg) and matrix alloy reinforced with ∼17 vol.%SiC particles. Following thixoforming, the layered structure was well preserved with good retention of layer integrity and arrangement. Impact measurements for estimating toughness (Charpy and Izod) showed limited improvement with the layered materials, relative to homogeneously reinforced ones. However, in slow crack growth (Tattersall–Tappin tests), toughness was nearly doubled when estimated in terms of an average energy of fracture from the area below the load–crosshead displacement curve. This was attributed to the reduction in the energy of the propagating crack at low strain rates, which led to a greater potential for the structure, and in particular for the unreinforced layer, to stop or slow the crack. Additionally, these layered MMC materials have provided a striking record of the die filling behaviour for metallic materials during thixoforming. The flow pattern is akin to that in conventional hot forging. This may assist in the establishment of design rules for thixoforming dies.

Investigation of metal flow and preform optimization in flashless forging of a connecting rod

In conventional hot forging of connecting rods, the material wasted to the flash accounts approximately 20 to 40% of the original workpiece. In order to reduce the cost of forged products, the forging must be performed in a closed cavity to obtain near-net or net shape parts. In flashless forging, the volume distribution of the preform must be accurately controlled to avoid overloading the dies and to fill the cavity. Additionally, the preform must be simple enough to be mass produced. This study deals with the design of the optimum preform to forge a connecting rod without flash. The initial preform design was obtained from physical modeling experiments. The optimization of this preform was found through 3D FEM process simulations. The advantage of performing simulations is that no tooling has to be built and the number of experimental tryouts can be significantly reduced. A preform optimization methodology was derived for this investigation.

Development and Applications of Titanium Alloy SP-700 with High Formability

Abstract A new β-rich α + β titanium alloy, SP-700, was designed to improve hot workability and mechanical properties over Ti-6A1-4V alloy. The chemical composition of the alloy is Ti-4.5A1-3V-2Mo-2Fe, and particularly enhanced properties include superplasticity, hardenability, and fatigue strength. Owing to its extremely fine microstructure and low β-transus temperature, SP-700 is superplastically formable at temperatures below 1073 K without significant increase in flow stress. More advantageously, diffusion bonding is also accomplished around this temperature. The low temperature SPF/DB process not only saves die material life and process costs, but also reduces alloy degradation from exposure at elevated temperatures, e.g., grain growth and oxidation. Isothermal and conventional hot forging are also performed better with SP-700 than ordinary titanium alloys like Ti6A1-4V. The alloy is heat treatable and one of its many mechanical properties is that high strength over 1200 MPa is attained in solution treatment and aging condition. The advantage to this is that the moderately stable β phase tolerates air cooling for solution treatment. Thermal distortion in work pieces, which would be caused by rapid cooling, is consequently avoided. In addition, the alloy is cold workable in such cases as rolling and blanking, when annealed around 1073 K. Furthermore, fatigue life and resistance to hydrogen-assisted stress corrosion cracking are improved due to reduced aluminum content. The superior properties of SP-700 over conventional titanium alloys make new applications possible in non-aerospace fields including sporting goods, hand tools, and wrist watches. Golf drivers, with club heads made of SP-700, are sold commercially. Crampons, a new product of SP-700, withstand cold blanking and bending which reduces production costs and extends life. Due to its high forgeability, SP-700 is expected to extend its application to the production of steam turbine blades. Details of practical applications have been demonstrated together with advantages of properties and fabrication for the blades.

Microstructure development during conventional and isothermal hot forging of a near-gamma titanium aluminide

The breakdown of the lamellar preform microstructure in the ingot metallurgy near-gamma titanium aluminide, Ti-45.5Al-2Cr-2Nb (atomic percent), was investigated. Microstructures developed during canned, conventional hot forging were compared to those from isothermal hot forging. The higher rate of deformation in conventional forging led to considerably finer and almost completely broken-down structures in the as-forged condition. Several nontraditional approaches, including the isothermal forging of a metastable microstructure (so-called “alpha forging”) and the inclusion of a short static recrystallization anneal during forging, were found to produce a more fully broken-down structure in as-isothermally forged conditions. Despite the noticeable microstructure differences after forging, conventionally and isothermally forged material responded similarly during heat treatment. In both cases, almost totally recrystallized structures of either equiaxed gamma or transformed alpha grains surrounded by fine gamma grains were produced depending on the heat-treatment temperature. Metallography on forged and heat-treated pancake macroslices was useful in delineating small differences in composition not easily detected by analytical methods.

The occurrence of shear bands in nonisothermal, hot forging of Ti-6AI-2Sn-4Zr-2Mo-0.1Si

The occurrence of shear bands in nonisothermal, hot forging of Ti-6Al-2Sn-4Zr-2Mo-0. ISi (Ti-6242) was investigated in order to establish the material properties and process parameters which generally lead to shear bands and shear cracks in conventional hot forging of metals. Upset compression tests on cylindrical samples and lateral sidepressing tests on long, round bars were performed to determine the modes by which flow localizes in deformation states ranging from axisymmetric to plane strain. In axisymmetric deformation, it was found that nonisothermal, hot compression leads to chill zones and bands of intense deformation separating the chill zones from the deforming bulk. For plane strain deformation, shear bands were found to initiate along zero extension directions and subsequently localize flow in a manner analogous to the formation and propagation of shear bands in isothermal, hot forging. For both deformation states, it was found that material properties, such as the flow stress dependence on temperature, and process parameters, such as forging speed and die temperature which strongly influence the amount of heat transfer, play critical roles in the flow localization process. A simple model quantifying these effects was developed to predict the occurrence and severity of the shear bands observed in the Ti-6242 alloy hot forged at various temperatures and rates. In addition, the occurrence of shear cracking under certain forging conditions was rationalized in terms of the chilling brought about by nonisothermal, hot forging conditions and the inferior workability of Ti-6242 at temperatures far below the transus temperature.