Warm Forging Research Articles

Thermo-micro-mechanical simulation of metal forming processes

The newly proposed microstructural constitutive model for polycrystal viscoplasticity in cold and warm regimes (Motaman and Prahl, 2019), is implemented as a microstructural solver via user-defined material subroutine in a finite element (FE) software. Addition of the microstructural solver to the built-in thermal and mechanical solvers of a standard FE package enabled coupled thermo-micro-mechanical or thermal-microstructural-mechanical (TMM) simulation of cold and warm metal forming processes. The microstructural solver, which incrementally calculates the evolution of microstructural state variables (MSVs) and their correlation to the thermal and mechanical variables, is implemented based on the constitutive theory of isotropic hypoelasto-viscoplastic (HEVP) finite (large) strain/deformation. The numerical integration and self-consistent algorithmic procedure of the FE implementation are explained in detail. Then, the viability of this approach is shown for (TMM-) FE simulation of an industrial multistep warm forging.

Simulation and Experiment of Manufacturing Process for Structural Aluminum Parts with Hard to Plastic Forming

A process comprising a hot extrusion process and a warm forging process was designed to form an umbrella-shaped aluminum structural component with a high degree of difficulty for the plastic forming method. A circular cylindrical part was extruded with a hot extrusion process, and then an embossing part was produced with a warm forging process. The formability and the maximum load required for forming were then determined using a forming analysis program. The hot extrusion process was executed at 450 °C under the extrusion speed at 6 mm/s, while the warm forging process was executed at 260 °C under the forging speed at 150 mm/s. The simulation results showed that the load required for hot extrusion was 1019 ton, while the load required for the warm forging was 534 ton. The umbrella-shaped part was manufactured by using a 1600 ton capacity press. The graphite lubricant was coated on the mold as well as the material. A forming experiment was performed under the same condition with the simulation condition. The portion where extrusion was done became elliptical with the α-Al phase elongated towards extrusion direction. Whereas, the α-Al phase became circular in the forged portion. The tensile strength value was found as 345 MPa, while elongation rate was 12%. Meanwhile, Vickers hardness value at the extruded portion was 105 HV, and it was 110 HV at the forged portion.

Microstructure and Mechanical Properties in Warm Forged 27MnSiVS6 Microalloyed Steel

The mechanical properties and microstructure of warm forged 27MnSiVS6 steel was evaluated at temperatures between 850 and 760 °C. The strength values showed enhancement by about 50 MPa, while the impact toughness was almost doubled. The microstructure showed increase in ferrite content with decreasing warm forging temperature, and the ferritic grains showed finer grain structure. While the increased ferrite content resulted in limiting the strength enhancement, the fineness of the grain structure led to a significant enhancement in impact toughness. The ductile-to-brittle transition temperature was pushed to a significantly low temperature in the warm forged steel compared to the hot-rolled steel. The microstructure evolution at various stages of processing was brought out.

Influence of Manganese and Boron Alloying and Processing Conditions on the Microstructure and the Mechanical Properties of 0.4% Carbon Steels

The performance of three steels with 0.4% carbon content was evaluated in the warm forging condition and benchmarked against the as-hot-deformed and hardening and tempering condition. The steels include a plain carbon steel S45C, a carbon-manganese steel 40Mn5 and a carbon-manganese-boron-alloyed steel 15B41. Such lean alloyed steels are preferred by automotive companies for component manufacture. In the as-hot-rolled condition, the steel with a ferrite pearlite microstructure shows, a good strength and lowest impact toughness. In the hardened and tempered condition at 650 °C, a tempered martensite microstructure with a moderate improvement in strength and significant enhancement in impact toughness, than the hot-rolled steel, is obtained. The steel subjected to warm forging at 760 °C, shows a ferrite pearlitic microstructure with a moderate improvement in strength but with the highest impact toughness level. The microstructure evolution at the various processing conditions has been brought out and correlated with the mechanical properties. The relative merits in processing the steels at the different processing conditions show that warm forging is an effective means to achieve the highest strength toughness combination in a ferrite pearlitic matrix.

Complex technology of stemping details “hub” from ball blank

In order to reduce the amount of metal wastes and improve the quality of press forgings, the new energy-and resource-saving technological complex of warm volumetric forging was developed, which has the following features: the metal is divided into billets on the rolling mill of cross-screw rolling; the induction heating of the billet to the temperature of warm forging (850-900°C); pressing of metal is performed, as a rule, in one pass in closed dies.

Mechanical Behavior of Twinning Induced Plasticity Steel Processed by Warm Forging and Annealing

The present study shows that warmly forged and low-temperature annealed twinning induced plasticity (TWIP) steel exhibited very high dislocation density and apparent yield-point phenomenon in addition to very high yield strength. The initial density of dislocations significantly affected the evolution of dislocations during the subsequent tensile deformation. Original high dense dislocations prompted the rapid increase of dislocations, and intensified the complexity of dislocation configurations. All these effects made the twinning deformation weakened but the dislocation deformation enhanced, leading to increased strength but decreased plasticity.

Study on the mechanical behavior of twinning-induced plasticity steel processed by warm forging and annealing

The microstructures and mechanical properties of a 30Mn–3Al–3Si twinning-induced plasticity (TWIP) steel in the warmly forged and annealed states were investigated in the present study. It is shown that the as-forged and low-temperature annealed steels have similar grain morphologies and defect configurations, i.e., predominantly deformed grains together with recrystallized fine equiaxed grains as well as highly dense and complex dislocations. However, the high-temperature annealed steel showed completely different microstructures characterized by fully recrystallized grains and very few dislocations. The initially high density of dislocations in the forged and low-temperature annealed steels greatly promoted the multiplication of dislocations in the subsequently tensile deformation, leading to not only pronouncedly enhanced tensile strength but an apparent yield-point phenomenon. Compared with as-forged and low-temperature annealed steels, the high-temperature annealed steel exhibited relatively fewer increments of dislocations after tensile deformation but much more twins. These changes in the microstructures were responsible for relatively low strength and high plasticity of the steel.

Properties of direct-quenched aircraft forged component made of ultrahigh-strength steel 300M

PurposeThis paper aims to investigate the possibilities and determination of hot and warm forging of ultrahigh-strength steel 300M and subsequent quenching with accelerated air. Analysis of microstructure and mechanical properties of forged steel 300M focused on investigation of the effect of processing conditions on final properties, such as strength, impact strength and hardness, taking into consideration temperature gradients and within-part strain nonuniformity occurring in forging and direct cooling of aircraft landing gear.Design/methodology/approachThe research involved semi-industrial physical modeling of hot deformation and direct cooling, aided with numerical analysis of both deformation and kinetics of phase transformations on cooling, with process conditions determined on the basis of numerical simulation of industrial process. Examination of forged and quench-tempered samples involved testing mechanical properties (tensile properties, hardness and impact strength) and microstructure.FindingsThree major findings were arrived at: first, assessment of the effects of energy-saving method of cooling conducted directly after forging. Second, tensile properties, hardness and impact strength, were analyzed on the background of microstructure evolution during hot-forging and direct cooling; hence, applied temperature and cooling rates refer to actual condition of the material including varied deformation history. Third, the accelerated air cooling tests were carried out directly after forging with accurately measured and described cooling efficiency, which enabled the acquisition of data for heat treatment simulation with use of untypical cooling media.Research limitations/implicationsThe conclusions formulated on the strenght of studies carried out in semi-industrial conditions with the use of model samples, despite strain and strain rate similarity, wait for full-scale verification in industrial conditions. The direct cooling tests carried out in semi-industrial conveyor Quenchtube are of cognitive value. Industrial realization of the process for the analyzed part calls for special construction of the cooling line and provision of higher cooling rate for heavy sections.Practical implicationsThe results present microstructure properties’ relations for good-hardenability grade of steel, which is representative of several similar grades used in aircraft industry, which can support design of heat treatment (HT) cycles for similar parts, regardless of whether direct or conventional quenching is used. As they illustrate as-forged and direct-cooled microstructure and resultant mechanical properties, the studies concerning processing the steel of areas of lower temperature are transferable to warm forging processes of medium-carbon alloy steels. The geometry of the part analyzed in the case study is typical of landing gear of many aircrafts; hence, there is the high utility of the results and conclusions.Social implicationsThe direct heat treatment technologies based on utilization of the heat attained in the part after forging allow significant energy savings, which besides cost-effectiveness go along with ecological considerations, especially in the light of CO2 emission reduction, improving economical balance and competitiveness. The presented results may encourage forgers to use direct cooling, making use of the heat attained in metal after hot forging, for applications to promote environmentally friendly heat treatment-related technologies.Originality/valueDirect heat treatment typically seems to be reserved for micro alloyed steel grades and sections small enough for sufficient cooling rates. In this light, taking advantage of the heat attained in forged part for energy-saving method of cooling based on direct quenching as an alternative to traditional Q&T treatment used with application to relatively heavy sections is not common. Moreover, in case the warm-work range is reached in any portion of the forged part, effect of direct cooling on warm-forged material is addressed, which is a unique issue to be found in the related studies, whereas in addition to warm forging processes, the results can be transferable to coining, sizing or shot peening operations, where gradient of properties is expected.

Deformation mechanism of forging tool for multi-stage forming of deep groove ball bearing

In this study, two design configurations routinely adopted in actual mass production for deep groove ball bearing rings of multi-stage warm forging process are investigated numerically and experimentally. The deformation mechanism, forging defects, microstructures, and compositions analysis are the main focus of this study. For type A design with a stepped configuration, inner and outer rings are automatically pierced and separated in the multi-step forging machine. On the other hand, for the type B geometry, additional step is required by the customized milling machine. For type A design, the main issue encountered during actual forging process is the inadequate material filling at the upper corner radius and folding defects at the transition area of inner wall of forgings. For type B design, the material flow is unsatisfactorily directed and lower outer radius is insufficiently filled. Therefore, variations of forging parameters include billet weight, punch/knock out pin geometry and the effect of lubricating fluid is systematically investigated. In addition, the finite element method has been performed and compared with the actual forging experiments. In summary, the modification of tooling design, dimension variation of billet weight, and the forging temperature difference as impacted by the lubricating fluid, which are identified as the three major factors of the forging integrity and stability of the mass production process. The results are particularly useful for the advanced tooling design and contribute largely to minimize the tool failure and the integrity of the bearing forged.

Micro-voids quantification for damage prediction in warm forging of biocompatible alloys using 3D X-ray CT and RVE approach

This study aims to quantify the three-dimensional (3D) micro-voids for damage prediction in warm forging through non-destructive X-ray computed tomography (CT) and RVE approach. Typical biocompatible alloys, i.e., stainless steel 316 L (SS316L) and titanium alloy Ti-6Al-4V, were used as specimen materials in warm-forging a medical implant, i.e., a basal thumb implant. X-ray CT scanning was performed for both the preforms and forged components. Volumetric CT images were then reconstructed and the 3D micro-void distribution and evolution inside the materials were detected and analysed quantitatively. Furthermore, three typical local strain regions, i.e., the small tensile strain region (STSR), small compressive strain region (SCSR) and large compressive strain region (LCSR), were established as the 3D representative volume element (RVE) models for both SS316L and Ti-6Al-4V preforms. The spatial location, size and volume of each micro-void were obtained from defect analysis of the 3D CT images and considered explicitly for subsequent damage prediction. An improved thermo-mechanical coupled micromechanics-based damage (micro-damage) model, which considered the variation of volume fraction of micro-voids (VFMVs), was implemented into finite element (FE) package ABAQUS for localized damage prediction of the RVE models. The damage distributions of the RVE models at different strain levels were visualized and identified. Finally, the localized damage evolutions at both compressive and tensile deformations were predicted and found to match quite well with the findings acquired from CT scanning. Thus, the application of non-destructive X-ray CT measurement of micro-voids, incorporating the RVE approach, was able to play a significant role leading to a more reliable damage prediction in the warm forging process.

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.

Forgeability Study of Al 6061 Alloy During Warm Forging of Cylindrical Billets

Warm forging is a metal forming operation which is used to obtain near net shaped components and is considered as a trade-off between cold and hot forging as optimum deformation load and temperature can be used for forging. In this present work, Al 6061-T6 alloy is selected and cylindrical billets were prepared for warm forging. Effect of homogenization on forgeability is also studied and thus half of the samples are homogenized at 550°C for 8 hours. The initial raw material and the homogenized samples are warm forged at two different temperatures i.e. 100°C and 150°C respectively. It is observed that all the samples are forgeable and no cracks are observed and thus all the samples are further analysed for its various parameters. Longitudinal contraction (ln (ho/hf)) and diametrical expansion (ln (do/df)) are calculated and their relationship is found to be rectilinear. Radius of curvature and maximum barrelling diameter is measured and their relationship proved that curvature of barrel follows the geometry of circular arc. New geometrical shape factor (NGSF) is also calculated for the samples.

X-ray CT assisted damage identification in warm forging

This study aims to identify the 3-dimensional interactions of micro-voids evolution before and after warm-forging a typical titanium alloy component by employing a micro-focus X-ray CT system so as to achieve a more accurate micro-void location and distribution as critical data input for damage prediction in warm forging. A specific forging tool was designed for warm-forging the component. Serval local regions that represented different strain levels were established as representative volume element models for the Ti-6Al-4V preform, in which, the micro-void distributions were obtained directly from the X-ray CT images. Furthermore, the micro-void evolutions of the corresponding representative volume element models were analyzed from the forged component. Thus, the damage evolution, i.e., the variation of void volume fraction, in the warm forging process was identified.

Preform Optimization for Bevel Gear of Warm Forging Process

Preform design is an important aspect in consideration of the improvement of quality and reduction of cost in warm forging process. Bevel gear being a key component of drive mechanisms in automotive industry requires lot of attention to its complex shape design optimization. A multi-objective optimization problem in designing preforms of warm forging for bevel gear by using genetic algorithm is solved and presented in this paper. The main objective of this study is to design an optimal preform part, which can get a better quality and the reliable preform shape systematically and theoretically.

Strain Evolution in Cold-Warm Forged Steel Components Studied by Means of EBSD Technique.

Electron BackScatter Diffraction (EBSD) in conjunction with Field-Emission Environmental Scanning Electron Microscopy (FEG-ESEM) has been used to evaluate the microstructural and local plastic strain evolution in different alloys (AISI 1005, AISI 304L and Duplex 2205) deformed by a single-stage cold and warm forging process. The present work is aimed to describe the different behavior of the austenite and ferrite during plastic deformation as a function of different forging temperatures. Several topological EBSD maps have been measured on the deformed and undeformed states. Then, image quality factor, distributions of the grain size and misorientation have been analyzed in detail. In the austenitic stainless steel, the γ-phase has been found to harden more easily, then α-phase and γ-phase in AISI 1005 and in duplex stainless steel, sequentially. Compared to the high fraction of continuous dynamic recrystallized austenitic zones observed in stainless steels samples forged at low temperatures, the austenitic microstructure of samples forged at higher temperatures, 600–700 °C, has been found to be mainly characterized by large and elongated grains with some colonies of fine nearly-equiaxed grains attributed to discontinuous dynamic recrystallization.

Effect of Processing Conditions on Forgeability and Properties of Hot and Warm-Forged Steel 300M

AbstractResults of investigation of the effect of processing conditions of medium-carbon alloy steel AISI 300M on forgeability and microstructure-properties are presented here, including as-forged and heat treated condition of the material. The presented results concern two vital aspects of plastic forming of high-duty impression-die forgings, which underlay a selection of technological conditions which enable the accomplishment of the required quality of the forged part. These are: firstly, the microstructure and mechanical properties and their uniformity within a part and secondly, technological realization of forging the required geometry in the given processing conditions.In order to define a favourable processing window, dynamic behaviour modeling in variable forging conditions was carried out, establishing a coefficient of energy dissipationη% and the metal flow instability areas, which indicated the temperature regime and strain rate range for the forging process. The constructed processing maps were subject to experimental verification in the die-forging tests, carried out on a screw press. Hot and warm forging conditions were applied accordingly to selected areas of the processing maps, representative for unique forging conditions occurring in the industrial practice.

Pinning effect of strain induced Nb(C,N) on case hardening steel under warm forging conditions

The effectiveness of Nb in retarding dynamic recrystallization (DRX) during warm forging process (700∘C–900 ∘C) and its dependence on the process parameters(ε,ε˙,T), which are the definers of formation conditions and morphology of the strain induced Nb(C,N), were studied. Due to its capacity of interaction with the austenite phase through solute drag and precipitate pinning effects, Nb is an important variable in the production of high-strength low-alloy steel (HSLA). However, the effectiveness of Nb as austenite conditioner is strictly dependent on its correct manipulation within each manufacturing step of finished or semi-finished products. Computer simulation of the precipitation kinetics of a forging steel 0.03wt. pct. Nb was performed in order to design a thermomechanical processing route based on the time-temperature-precipitation diagram (TTP), thereby ensuring high precipitate phase fraction in short processing time. Characterization of the steel mechanical behavior and its dependence with the formation of Nb(C,N) was done through flow stress curves analysis, the relaxation rate for the microalloyed steel is remarkably lower than for the Nb free steel being a typical stress plateau (carbonitride formation) and a peak stress (recrystallization) respectively observed. Transmission electron microscopy (TEM) was performed in order to characterize the distribution, geometry and dimension of Nb(C,N) generated by different processing routes at different process stages. The solubilization capacity of the adopted and computational designed austenitizing process was also experimentally analyzed. The energy-dispersive X-ray spectroscopy (EDX) method was used in the characterization of the precipitates chemical composition, allowing the identification of Ti-rich precipitation remnants of the austenitizing process and the absence of the element in the Nb(C,N) formed during deformation (strain induced precipitates). The strain, strain rate, temperature and isothermal holding time contribution upon the formation of precipitates were evaluated separately which reveals that it is so important as the processing temperature, the strain rate regime play decisive role in the formation of the particles. In addition, the strain rate dependency of strain induced Nb-precipitates formation at optimal processing temperature was indirectly confirmed by the capacity of the formed precipitates in hinder the static recrystallization (SRX) progress under relaxation tests.The strain induced precipitation formed at optimal processing conditions have circular geometry and size below 10nm, causing high precipitate pinning effect observed in the conditioning of austenite phase by hindering the DRX under warm forging condition.

High Strength of Mg-9%Al-1%Zn Alloys Achieved by Severe Working

To obtain the excellent mechanical properties of AZ91 magnesium alloy (Mg-8.9%Al-0.6%Zn-0.2%Mn), the microstructural characteristics of AZ91 alloys are modified by various forging and heating processes. High tensile properties (ultimate tensile strength σ UTS = 420 MPa and fracture strain e f = 3%) are obtained for the alloy made by the following process: solution treatment (ST) at 410 °C for 24 h plus water quenching, multidirectional forging (MDF) with 5% strain applied in 15 forgings at room temperature, and warm unidirectional forging (WUF) at a forging rate of 75% at 225 °C. The high tensile strength is a reflection of improved microstructural characteristics, namely a fine α-Mg phase and a high dislocation density. Moreover, brittle β-phase is significantly attributed to the mechanical properties of AZ91 alloy. Because of the severe deformation undergone by the alloy during the MDF process, the solution treatment is important to achieve high ductility with low internal strain, i.e., normalization. In fact, the e f value for the ST sample is as high as 10%, leading to severe work hardening during the tensile test, with deformation twins and slips. The WUF process is conducted immediately after the sample has been heated to 225 °C, for less than 5 min, to avoid material softening. A relatively high tensile strength (σ UTS = 305 MPa) is also achieved using the WUF process (with a forging rate of 75% at 200 °C) after the ST and aging process (200 °C for 12 h) although low ductility is found (e f = 0.7%), with hard and brittle β-phases being precipitated around the grain boundaries.

A Novel Technology on Unfilled Corner Occurred in Gear Two-Step Warm Forging Process

Based on the geometric feature of back gear with double bosses, a new two-step warm forging process before cold precision forging was developed to reduce the number of process and the overall cost of manufacture. However, after the new process being conducted, unfilled corners were found at the bottom profile of all teeth. Two different methods were adopted to solve the issue but the results were unsatisfied. In order to solve the issue, a rigid plastic FE model was established to simulate the process. The velocity, stress and strain fields were investigated. In the mean time the metal flow rules were discussed along with experimental results. Through expanding the radial wave shape by means of a groove established in die and a qualified forge piece was achieved finally.

Process-integrated gas- and spray-quenching for high-strength ductile forged components

The metal working industry is continuously seeking for process chain improvements, involving, for instance, forging techniques, principally aiming at reducing the process costs, and improving the mechanical properties of the automotive parts. Promising new forging materials (e.g., high-strength ductile bainite ‘HDB’ steel alloy) offer new fields for process combinations, such as cold and warm forging, machining, and controlled in-process quenching. In the present paper, the design, implementation, and validation of a controlled gas and spray-quenching unit within an automated forging unit is described. A two-fluid, flat-spray nozzle is characterized for spray-quenching purposes (droplet size, mass flux, and heat-transfer distributions). Experiments are utilized to validate transient heat transfer simulations in order to extend the numerical model for quenching scenarios of complex specimen geometries (a stepped shaft and a common rail), involving various steel grades from the forging heat. The scenarios are validated using the automated forging/quenching unit, through hardness and microscopy tests. The high potential of the combination of forging and heat treatment (gas and spray-quenching) into a single manufacturing unit to ensure the homogeneous bainitizing of forged components is demonstrated in the present paper.