Cold Forging Research Articles

Enhancing the sensitivity of a tribological testing method to enable development of lubricants for cold forging

Because of environmental issues, new water-based lubricants are being developed to replace the conventional lubrication approach in cold forging—phosphation plus soap coating. To support the development and application of water-based lubricants, a new testing method that is highly sensitive, accurate, and easy to use was established on the basis of the ring with boss compression test. The dimensions of ring specimens were optimized, and the method’s high sensitivity was demonstrated by obtaining scatter calibration curves and determining the distortion of the outer boss. The effectiveness and high sensitivity of the method were experimentally validated using two strategies. The method was then successfully employed to evaluate a commercial water-based lubricant; the effect of lubricant amount and lubrication mode was investigated.

Processing and Characterization of a New Quaternary Alloy Ti10Mo8Nb6Zr for Potential Biomedical Applications

The study of new metallic biomaterials for application in bone tissue repair has improved due to the increase in life expectancy and the aging of the world population. Titanium alloys are one of the main groups of biomaterials for these applications, and beta-type titanium alloys are more suitable for long-term bone implants. The objective of this work was to process and characterize a new Ti10Mo8Nb6Zr beta alloy. Alloy processing involves arc melting, heat treatment, and cold forging. The characterization techniques used in this study were X-ray fluorescence spectroscopy, X-ray diffraction, differential scanning calorimetry, optical microscopy, microhardness measurements, and pulse excitation technique. In vitro studies using adipose-derived stem cells (ADSC) were performed to evaluate the cytotoxicity and cell viability after 1, 4, and 7 days. The results showed that the main phase during the processing route was the beta phase. At the end of processing, the alloy showed beta phase, equiaxed grains with an average size of 228.7 µm, and low Young's modulus (83 GPa). In vitro studies revealed non-cytotoxicity and superior cell viability compared to CP Ti. The addition of zirconium led to a decrease in the beta-transus temperature and Young's modulus and improved the biocompatibility of the alloy. Therefore, the Ti10Mo8Nb6Zr alloy is a promising candidate for application in the biomedical field.

Controlling Damage Evolution in Geometrically Identical Cold Forged Parts by Counterpressure

Abstract It is investigated to what extent the evolution of ductile damage in cold forging can be controlled without changing the geometry of the produced part. Besides the effects of strain hardening and residual stresses, damage, which is the nucleation, growth and coalescence of voids on microscopic level, affects product properties of the manufactured components such as fatigue strength, impact strength, or elastic stiffness. Former investigations have shown that the load path-dependent damage evolution in forward rod extrusion, and thus, the performance of produced parts can be controlled by the process parameters extrusion strain and shoulder opening angle. As these parameters also affect the geometry of extruded parts, design requirements of components might be violated by varying these. Thus, counterpressure is used to superpose purely hydrostatic stresses to forward rod extrusion in order to decrease triaxiality in the forming zone without causing geometric variations in the produced parts. The counterpressure is either introduced by a counterpunch or by modified process routes. The achieved improvements in product performance are in agreement with results obtained by variation of extrusion strain and shoulder opening angle as described in the literature. In addition, it is observed in tensile tests that damage in cold extruded parts does not significantly affect flow stress. All advancements in product performance are realized without affecting the products’ geometries.

Ön Deformasyona Uğratılmış Hammadde Kullanılarak Paslanmaz Çelik Bağlantı Elemanının Simülasyon Destekli Soğuk Dövme Üretimi

The initial properties of the raw material used while producing by cold forging change the mechanical properties of the final product resulting from deformation. Depending on the amount and speed of deformation, there is an increase in strength and a decrease in formability after cold forging on the basis of the true stress - strain curve of the material. In this study, pre-deformation of 304Cu (1.4567) quality raw material used to produce fasteners in A2-70 class by wire drawing before forging and the change of the mechanical properties of the raw material, on the basis of this change, the simulation supported design of the fastener mold and process was carried out. Simufact.forming software was used in cold forging simulation studies. According to the simulation data, the yield strength value increased from 215 MPa to 620 MPa as a result of thinning the raw material from Ø7.80 mm to Ø6.95 mm with wire drawing operation. In this case, the properties of the raw materials entering the cold forging have changed according to the deformation percentage amount, and the material properties and mold design used in the finite element simulation supported production were made according to this data. As a result of the use of this raw material in the cold forging simulation, the HV10 hardness value in the fastener head region, where deformation is intense on the basis of the die geometry, has reached the levels of 320 to 340 HV. Material flow lines, product dimensional tolerances, rolling and thread cutting processes during deformation were verified in the computer environment and physical product production was made. Hardness, strength and structural checks were performed and compared with numerical model values. Especially, the changing material properties as a result of the wire drawing process applied to the starting raw material and its effect on cold forging were investigated mechanically and metallographically.

Optimization of a thermomechanical treatment of superelastic Ti-Zr-Nb alloys for the production of bar stock for orthopedic implants

Ti-19Zr-14 Nb (at%) shape memory alloy was subjected to low- and high-temperature thermomechanical treatments to fabricate long-length bar stock for the production of load-bearing orthopedic implants. The phase composition, structure, texture, uniaxial tensile and three-point bending fatigue behavior were studied. Low-temperature thermomechanical treatment (LTMT), combining cold rotary forging and post-deformation annealing at 550 °C led to the formation in the peripheral zone of the bar cross-section of a statically recrystallized fine-grained structure with a relatively strong [001] crystallographic texture and of a mixed statically recrystallized and polygonized substructure, in the central zone of the cross-section. In this state, the alloy exhibited an excellent combination of the static functional and mechanical properties: relatively high strength (UTS=680 MPa), low Young’s modulus (E < 40 GPa), and high superelastic recovery strain (εrsemax=3.4%). High-temperature thermomechanical treatment (HTMT), consisting in hot rotary forging at 700 °C, led to the formation of a dynamically polygonized substructure with a uniform crystallographic texture close to the [101] direction; this direction corresponding to the maximum theoretical recovery strain limit. As compared to the LTMT bar stock, its HTMT equivalent manifested slighly inferiour static properties but superior fatigue resistance in bending, thus being an optimal candidate for the production of orthopedic implants.

Deformation kinking and highly localized nanocrystallization in metastable β-Ti alloys using cold forging

• Microstructural refinement involving deformation kinking was investigated. • Formation of kink band involves dislocation gliding, pre-kinking and its ripening. • Nanograins ∼15 nm in diameter are produced but highly localized in kink bands. • Localized nanocrystallization results from the intrinsic soft nature of kink bands. • The softening of kink bands originates from the inner degraded dislocation density. Deformation kinking as an uncommon plastic deformation mechanism has been reported in several materials while the relevant microstructure evolution and grain refinement behavior at a large strain remain unclear so far. In this study, the issue was systematically investigated by utilizing cold forging to impose severe plastic deformation (SPD) on Ti-11 V metastable β -Ti alloys. It is found that the formation of kink bands experiences dislocation gliding, pre-kinking and the ripening of pre-kinks in sequences. The kink bands are subsequently thickened through the coalescence of multiple kink bands in a manner of high accommodation. Ordinary dislocation slip is developed as a dominant deformation mechanism when deformation kinking is exhausted. The resulting grain refinement involves transverse breakdown and longitudinal splitting of dislocation walls and cells, which fragment kink bands into small β -blocks. Further refinement of the β -blocks is still governed by dislocation activities, and finally nanograins with a diameter of ∼15 nm are produced at a large strain of 1.2. Alternatively, it is revealed that nanocrystallization is highly localized inside kink bands while the outer microstructure maintains original coarse structures. Such localized refinement characterization is ascribed to the intrinsic soft nature of kink bands, shown as low hardness in nanoindentation testing. The intrinsic softening of kink bands is uncovered to originate from the inner degraded dislocation density evidenced by both experimental measurement and theoretical calculation. These findings enrich fundamental understanding of deformation kinking, and shed some light on exploring the deformation accommodation mechanisms for metal materials at large strains.

Influence of Metal Flank Hardness of Machined and Cold Forged Gears on Wear within a Metal-Polyamide Gear Pair and Targeted Process Adaptation

Metal-polyamide gear pairs provide advantages but their application is limited due to wear. The properties of the metallic gearing significantly affect the wear behavior. However, the influence of varying metallic materials as well as flank hardness is not known. Within this contribution, the occurring wear mechanisms when applying steel, brass and aluminum with varying hardness resulting from manufacturing by machining and cold forging were identified. Depending on the hardness of the metallic tooth flank, the release of metallic particles (3-body abrasion) or surface roughening (2-body abrasion) results. The formation of a wear-reducing transfer film is only possible with sufficient strength of the metallic tooth flank and tribological compatibility. Maximum wear occurs at a metal hardness of about 120 HV due to 3-body abrasion with high abrasive effect of the metallic particles. The adaptation of the cold forging process enables a local increase in the plastic strain of the tooth flank by 84% resulting in an elevated tooth flank hardness (+ 53%) for aluminum and significantly reduced wear. Furthermore, the formation of a wear-reducing transfer film results. Aluminum pinions produced in the adapted cold forging process achieve performance level of steel within the investigated load case.

Frictional and material deformation of aluminium alloy in cold forging test under different derivatives of palm oil lubrication condition

Frictional and material deformation of aluminium alloy in cold forging test under different derivatives of palm oil lubrication condition

Recrystallisation, concurrent precipitation, and texture development in cold forged Mg-6Al-3Sn magnesium alloy

ABSTRACT The present investigation has highlighted the effect of annealing temperatures from 200°C to 300°C on the recrystallisation behaviour and texture evolution of cold-forged Mg-6Al-3Sn (AT63) magnesium alloy. A new microstructure with a modified grain size was formed under different annealing conditions. The dominant nucleation sites of recrystallised grains were found at twin/grain-boundary and twin/twin intersections and twin interiors. Recrystallisation was incomplete at an annealing temperature of 200°C, even for a prolonged time. It may be ascribed to concurrent precipitation (Mg17Al12 and Mg2Sn), which interferes with grain boundary motion during recrystallisation. However, at high annealing temperatures, the volume of these precipitates was very low. They did not effectively hinder the recrystallisation process, resulting in faster recrystallisation kinetics. The average grain size was extensively refined from 85 to ∼7 µm with cold forging and annealing approach. The cold-forged deformation texture of AT63 alloy shows the typical basal structure (0002) in nature. The recrystallised material retained the basal texture after isothermal annealing of cold-forged AT63 alloy.

Fracture mechanism of AISI 1025 rod shearing in automatic multi-stage cold forging and critical shearing speed

Experimental and numerical studies on shearing processes that focused on automatic multi-stage cold forging (AMSCF) are conducted to reveal the mechanisms of the fracture phenomena that occur in the cold rod shearing process in AMSCF. First, AISI 1025 (treated for AMSCF, specifically called SWCH25F) coil-rod of radius 17.5 mm was straightened and cut by an AMSCF machine at operating speeds of 20–90 rpm to elucidate the effects of speed on the surface quality of the sheared billet. Second, a universal testing machine was used in combination with a special shearing apparatus for experimental cold rod shearing to measure the shearing load-time (or stroke) curve and examine fracture mechanisms at sufficiently low speeds. Fractographical observations showed that speed clearly affects the cold shearing of a straightened coil-rod. The surfaces of billets sheared at low speeds showed clear evidence of ductile fracture, whereas those sheared at normal speeds did not. The experimental results were used in thermoviscoplastic finite element (FE) analysis together with the flow behaviors of the material in the cold-forging temperature range to explain the brittle fracture of the ductile material during cold coil-rod shearing in AMSCF. The concept of a critical coil-rod shearing speed for acceptable surface quality is presented.

Impact of a Splined Mandrel Geometry on Die Filling in a Cold Forging Process with Adjustable Deformation Zone

Manufacturing of hollow components with local functional internal surfaces often requires complex process routes with several individual operations. By using a special hollow cold forging process with an adjustable deformation zone it is possible to manufacture hollow shafts with varying wall thickness over its axial length parts just within a single stroke of the press. Combining this principle with a splined mandrel allows manufacturing of tailored hollow shafts with local internal splines. However, the impact of geometry of mandrel and other process parameters on the shape of the cold forged internal splines have not been investigated yet. Furthermore, an underfilling phenomenon can occur on the outer surface of shaft during specific process states. In this contribution, several mandrel geometries and their impact on the part shape and filling / underfilling phenomenon in the mentioned process are investigated. To determine those effects, a numerical investigation has been conducted. Besides the geometry of the splined mandrel, also other parameters such as die geometry and tool kinematics were considered. The numerically calculated workpiece geometries are compared to the ideal geometries using a deviation analysis.

A Model to Construct and Predict Flow Curve of Materials from Compression Test Results with Machine Learning Models Using Python

In order to obtain flow curves from compression test results of a cold forging material and predict flow curves of the material at intermediate temperature and strain rate values, a model was developed using Python programming language in this study. The model consists of two parts: Flow curve determination and flow curve prediction. The compression test data including Force-Stroke values was processed to determine the flow curves in the first part, and the flow curve data constructed for certain temperature and strain rate values of the material was used as input for the machine learning algorithms to predict flow curve at desired intermediate temperature and strain rate values in the second part. Moreover, Ludwik material model parameters were estimated by using curve fitting methods in order to define the material model into the simulation software. Machine learning algorithms and various regression models in Python libraries were tested to predict the flow curves. The performances of different machine learning and regression models were compared with respect to the mean squared error and coefficient of determination performance measures. Support vector regression, k-Nearest Neighbour (kNN) and artificial neural network models were used to predict flow curves of cold forging materials and kNN regression model was able to found predictions with the lowest error rate. As a result, a model that can process the compression test data to predict flow curves at intermediate temperature or strain rate values was developed.

Experimental Estimation of Wear Behavior of Al6082 and Al7075 Alloys During Cold Forging

One of the most important metal forming process includes forging. Estimating wear, coefficient of friction and frictional force on Al6082 and Al7075 alloys has become widely considerable and research factor in industries. In this paper, a mathematical model is proposed to estimate the required forging force to be applied on a pin-on-disc machine during the actual forging process. Cold forging process experiments are conducted on a pin-on-disc machine with different loads from 40N to 70N. Experiments are carried out for a time period of 5 mins at 150 rpm at temperatures such as 50℃, 100℃ and 150℃. Outcomes exhibit that the friction force fluctuates constantly with time. The experimental investigation revealed that the wear changed with temperature, and it was distinctive for various alloys. The chemical composition of aluminum can also affect the performance of Al alloys. Furthermore, it also proven that there is a decrease in the wear of Al alloys with a decrease in the real area of contact due to the formation of wear debris as well as an increase in the content of silicon and zinc that is added to form the alloy.

Investigation on Billets and Tools Geometry in Cold Forging of the Straight Bevel Gear

Straight bevel gears are an essential component of mechanical transmissions; they are widely used in the automotive, aerospace, shipbuilding industries and are made by shaping methods such as metal forming, casting, or machining. To market products, with proper function and properties, at a low cost, this gear component is usually fabricated by precise cold forging (the billet is forged in a closed-die at room temperature). In cold forging, the geometry of the billet and the forming tool plays an important role. It determines the ability to fill the die cavity, creating a finished product profile that meets the required geometric parameters. In this study, by 3D numerical simulation, some geometric shapes of workpieces and tools were investigated to find the optimal parameters. The results obtained from the simulation method were determined that a cylindrical workpiece with a tapered end and a die bottom with a convex profile will increase material flow velocity, improving cavity filling, uniform distribution stress in the forming specimen, and forging products without defects. Experimentally also verified the simulation results, which were cold forging with optimized workpiece and tool geometry, the straight bevel gear part was fully shaped, ensuring geometrical accuracy. The result of this study is a suggestion to apply to the design of a cold forging die for similar features.

Interfacial heat transfer coefficients in cold forging of stainless steel

In numerical simulations of cold forging processes, the heat transfer coefficient has a significant influence on the resulting temperature fields. However, the values determined so far for steel materials are not in the load range of cold forging processes. In this paper, a test stand for the determination of the interfacial heat transfer coefficient under typical loads of metal forming processes is presented. The results are discussed at an example of a cold extrusion forming process. It can be shown that the value increases significantly with increasing contact pressures and are higher compared to values known from literature. The investigations also disclose the influences of lubricants and the mechanical surface treatment on the heat transfer coefficient.

Investigation of Through-Thickness Residual Stress, Microstructure and Texture in Radial Forged High-Strength Alloy Steel Tubes

Gradient variations of through-thickness residual stress, microstructure and texture greatly affect the performance of cold radial forged tubes. In this work, the through-thickness distribution of residual stress was measured based on the Debye ring. The microstructure was characterized with the electron backscattering diffraction technique. The texture was measured by the X-ray diffractometer. The influence of microstructure and texture on the strength and anisotropy of forged tubes with different thickness reductions was analyzed. The results show that the residual stress varies gradually from compressive to tensile from the outer to inner surface. The microhardness of the outer surface is lower than the inner. The dislocation density and low-angle grain boundary fraction are the smallest in the one-third thickness. The dislocation density and low-angle grain boundary fraction increase gradually from the one-third thickness to the inner surface. The main texture components of the forged tube include {111}&lt;110&gt;, {001}&lt;110&gt; and {114}&lt;110&gt;. Texture {111}&lt;110&gt; deflects gradually toward {114}&lt;110&gt;, {112}&lt;110&gt; and {110}&lt;110&gt; from the external tube to the internal tube. The gradient variation of strength mainly resulted from the difference of the dislocation density. The difference of strength along the radial direction is reduced with a larger thickness reduction. This work has important significance for improving the performance of high-strength alloy steel tubes processed by cold radial forging.

A New General Fatigue Limit Diagram and Its Application of Predicting Die Fatigue Life during Cold Forging.

Traditional fatigue fracture theory and practice focus principally on structural design. It is thus too conservative and inappropriate when used to predict the high-cycle fatigue life of dies used for metal forming, especially cold forging. We propose a novel mean stress correction model and diagram to predict the high-cycle fatigue lives of cold forging dies, which focuses on the upper part of the equivalent fatigue strength curve. Considering the features of die materials characterized by high yield strength and low ductility, a straight line is assumed for the tensile yield line. To the contrary, a general curve is used to represent the fatigue strength. They are interpolated, based on the distance ratio, when finding an appropriate equivalent fatigue strength curve at the mean stress and stress amplitude between the line and curve. The approach is applied to a well-defined literature example to verify its validity and shed light on the characteristics of die fatigue life. The approach is also applied to practical forging and useful qualitative results are obtained.

Tensile and Fatigue Characteristics of WC-20wt%Co Cemented Carbide with Sintering Methods

WC-Co cemented carbide has excellent mechanical properties and is widely used in many industrial applications including cold forging die and cutting tools. WC-Co cemented carbide is manufactured by liquid phase sintering (LPS), and a new sintering process for densification has been developed. In this study, cylindrical rod shaped tensile and fatigue specimens of WC-20wt%Co cemented carbide were fabricated using commercial sintering methods such as vacuum sintering (VS), low pressure sintering (LP), sinter HIP (S-HIP), and HIP after vacuum sintering(VS+HIP). The effect of the sintering methods on microstructure, tensile properties and fatigue life was investigated. The relative density and average size of the WC particles increased after additional HIP treatment. Also large abnormal WC particles were observed. In the tensile test, the deviation of tensile properties (maximum tensile strength, elongation) was minimized in the VS+HIP specimen due to densification. Based on the uniaxial tensile-compression fatigue test, the SN curve of WC-20wt%Co cemented carbide was concluded to be logσa = -0.11244logNf +3.66817. Furthermore, the increase in WC particle size in the HIP process caused the VS+HIP specimen to have a lower fatigue life than the VS specimen. However, when high pressure was applied to the WC-Co cemented carbide during the sintering process, internal defects and the standard deviation of fatigue life were reduced.

Near Net Forming Research for Fin-Typed LED Radiator.

Pure aluminum radiator is the best choice for heat dissipation of various LED products at present. Its forming methods include common extrusion, die casting, forging, etc. Compared with other forming technologies, the LED radiator formed by cold forging has good heat dissipation performance, but there are some disadvantages in the forming process, such as uneven deformation, large material consumption and low die life. The cold forging process of pure aluminum fin-typed LED radiator is analyzed by the finite element method. The calculation results show that equal fillet structure of concave die is improper, leading to serious uneven flow velocity distribution during aluminum forging, inconsistent fin length, and warpage tendency. The gradient fillet structure of concave die is adopted. Numerical simulation and production test show that the gradient fillet structure design can significantly reduce the uneven metal flow. The extruded fins have a uniform length, which is conducive to reducing subsequent machining and production cost.

Heat Transfer Coefficient of Zinc Phosphate Coating with Metal Soap during Cold Backward Extrusion

In cold forging, the temperature difference between the workpiece and the tool is small and thus deformation analysis is rarely coupled with thermal analysis. However, the friction coefficient of zinc phosphate coating with metal soap has a large temperature dependence. Consideration of the effects of workpiece and tool temperature change on the friction coefficient is thus expected to improve the analytical accuracy of cold forging. Thermally coupled cold forging analysis requires thermal conductivity, specific heat, heat transfer coefficient between the workpiece and the tool, in addition to the temperature dependence of flow stress and friction coefficient. The heat transfer coefficient between workpieces coated with zinc phosphate with metal soap and tools is investigated in this paper. Cold backward extrusion was performed with a 50% reduction of area, and the temperature history in the punch was measured with a thermocouple. The forging speed was 1, 3, and 10 spm. FEM analysis was performed to simulate the experiment by considering the temperature dependence of flow stress and friction coefficient. The heat transfer coefficient was estimated at 20 kW/(m2•°C) by comparing the experimental result and calibration curves.