Key Engineering Materials Vol. 716

Abstract: In hot metal bulk forming and forging, the interface heat transfer and the friction between the tooling and the billet are of particular importance since they have a significant effect on material flow, deformation, forming forces, component surface finish and die wear. Several authors have used different characterization methods to measure the friction coefficient using cylindrical upsetting tests, ring compression tests, Spike tests and T-Shape tests among others.In the present paper, The T-Shape test has been used in order to measure the friction between aluminium billets and tool steel. In order to obtain the sensitivity of the test, a Finite Element (FE) parametric study has been performed which indicates that shape of specimen could be chosen to measure the friction. For this, compression tests for three specimens in dry conditions have been carried out and shape of specimen has been measured. These measurements and the use of adequate inverse modelling techniques enabled a precise characterization of the forging friction coefficient. Heat transfer coefficient (HTC) has been precisely characterised from the columnar upsetting thermal tests and later used in simulating the T-Shape tests to estimate the friction factor (m). Friction factor has been determined by comparing the experimental results with the numerical simulation results of T-Shape compression test. An encouragingly good agreement has been found between the experimental and numerical results.

Abstract: In this paper, the friction behaviour in a severe bending process of a thick plate was investigated, taking into account both dry and lubricated conditions. Early experimental tests were performed to obtain mechanical properties of the low carbon steel AISI 1006, to be used as input in FE solver. Besides a 3D thermo-mechanical model based on FEM was developed to predict stress and strain distributions and final component dimensions. The second experimental series was composed of a coining process and a forming operation to reach the size of the final part. The analysis and the control of the friction conditions has permitted to obtain a product of higher quality that permitted to avoid all the secondary machining operations previously required.

Abstract: Hot stamping process has been developed to produce the steel automobile parts with an ultra-high-strength of 1500 MPa. The effect of scale thickness on the formability in hot stamping was investigated by a hot deep drawing test in our previous research. The draw-in lengths of flange increased with decreasing the scale thickness. It is supposed that thin scale thickness resulted in low coefficient of friction at the flange area. The other reason is the temperature of wall zone would become low according to decreasing the scale thickness or increasing of the thermal transfer coefficient and it slightly inhibits local deformation at the wall area. It is difficult to separate these phenomena. To quantify the effect of scale thickness on the friction at the flange area during hot deep drawing, the coefficient of friction was directly measured. The coefficient of friction decreases with decreasing scale thickness.

Abstract: Oil with a chlorinated extremely-pressure additive (EP-additive) is effective to prevent galling in cold forming for stainless steel. However environmental issues have pressured the manufacture to replace this oil with high performance oil without chloride. Particularly, sulfur-based EP-additives are accepted as a practicable replacement for chlorinated EP-additives. Thus, the authors analyzed the structures of organic-sulfur compounds. Moreover, the effects of the molecular structures on the anti-galling performance are estimated by a cup internal ironing test. This test has been devised to classify the performance of sample oils by the ironing load and the damage on the workpiece after the test. Consequently, sulfurized olefin was superior to sulfurized ester and sulfurized fatty oil in cold ironing for stainless steel. It was also clarified that sulfur EP-additives generated iron sulfide and sulfate on the surface of stainless steel by using the surface analyses of X-ray photoelectron spectroscopy.

Abstract: The combination of different types of deformation can create a continuous method that ensures the formation of ultrafine-grain structure in medium carbon steel wire. The method is based on drawing operation combined with torsion and bending. Tools and equipment applied in the wire and cables manufacturing are used for the implementation of this method. As a result of the combined strain effect the ultrafine homogeneous structure is formed in the medium carbon steel wire. The wire has increased strength while maintaining the plastic properties when compared with the corresponding properties after drawing.

Abstract: The demand of microforming is increasing as one of the economical production methods for small metallic parts. However, the formability of metallic foils decreases with decreasing ratio of thickness to grain size. In the present study, a process combining step motion and ultrasonic vibration is proposed to enhance the formability by stress relaxation. To investigate the effect of stress relaxation on forming limit of metallic foils in different stress states, micro bulge tests were carried out. The material used was brass foils with a thickness of 0.03, 0.05 and 0.08 mm. For calculating the strains of the deformed specimens, a pattern of dots with a diameter and a pitch of 50 and 60 μm was fabricated on the surface of the specimens by photolithography. The results of micro bulge tests showed that the forming limit increases by the stress relaxation regardless of stress states, except for the foil with a thickness of 0.03 mm. The possibility of enhancing the formability of metallic foils by stress relaxation was experimentally demonstrated.

Abstract: Strain gradient is known as an important factor that influences springback of bent components in microscale. Compared with thicker foils, thinner foils usually indicate more strain gradient due to non-uniform material deformation. A resistance heating (RH) method is an effective approach to obtain homogenous material flow by heating foils within only several minutes. To predict springback of foils bent at elevated temperatures, an investigation of the influence of strain gradient on springback is indispensable. To achieve this, microbending tests assisted by RH were conducted at different temperatures ranging from 298 to 723 K in the present study. 0.05 mm-thick pure Ti foils with varying grain sizes of 2.7, 14.7, and 24.5 μm were used. As results, normalized bending moment decreased with increasing temperature and with increasing grain size. The less strain gradient of the foils with larger grain size and at elevated temperatures was confirmed to be the reason according to a theoretical analysis of springback using the constitutive model considering statically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs). The predicted normalized bending moment by theoretical calculation showed good agreement with experimental results at the temperature of 573 K or higher but not at the temperature lower than 573 K. It was found that the springback of the foils was influenced by the strain gradient at low temperatures. Furthermore, the size effects caused by strain gradient reduced as the bending temperature increases.

Abstract: Investigation of the mechanical behaviour of multilayered metallic materials obtained during novel joining technique called Constrained Compression (CC) is presented. 316L stainless steel material was used in CC to achieve multi-layered structure. Microstructural study based on light microscopy was performed focused presumably on the joining areas of the deformed metallic laminate. The qualitative and quantitative assessment of the processing conditions, microstructure development and microhardness distributions showed the possibility of achievement good bonding quality. Experimental study was supported by numerical stress and strain analysis. It has been shown that determination of the optimum processing parameters allowed for improvement of the joining process, which in turn will enable to produce multilayered metallic materials on a larger scale.

Abstract: A new multi-resolution slip system-based material hardening law has been developed for micro-forming simulation using Crystal Plasticity Finite Element (CPFE) Approach. Material hardenings are formulated based on global and local hardening of dislocations for each slip system and defined with distinct physical meaning. Plasticity is assumed to arise solely from crystalline slip and the overall mechanical response with any crystallographic system, such as FCC, BCC, etc, can be addressed by a local hardening parameter, c, from 0 (pure anisotropic) to 1 (fully isotropic). No interaction matrix is necessary, since the latent hardening can be realized by the hardening factor , c , and the new dislocation density based hardening law can be implemented into existing FE software efficiently. The proposed equations are an extension of the existing hardening law from macro mechanics descriptions down to micro mechanics level, therefore unified constitutive equations had been established at multiscale resolution. Some features of the proposed hardening law will be demonstrated with a single cubic crystal under tension load.

Abstract: The microstructure and texture evolution in commercially pure aluminium (AA1050 alloy) and copper have been characterized after change in strain path to elucidate the mechanisms of shear bands (SBs) formation and propagation across grain boundaries. Samples were pre-deformed in equal channel angular pressing (ECAP) and further compressed in a channel-die to form two sets of macro-SBs. The deformation-induced sub-structures and local changes in crystallographic orientations were characterized by scanning electron microscopy equipped with a high-resolution electron backscattered diffraction facility. It was found that the mechanism of micro-/macro-SBs formation is strictly crystallographic. In all the grains of the sheared zone a strong tendency to strain-induced re-orientation could be observed. Their crystal lattice rotated in such a way that one of the {111} slip planes became nearly parallel to the shear plane and the <011> (or <112>) direction became parallel to the direction of maximum shear. This crystal lattice rotation led to the formation of specific SBs components which facilitates slip propagation across grain boundaries without any visible variation in the slip direction.