Constrained groove pressing for sheet metal processing

Constrained groove pressing (CGP) is a modern technique for developing ultrafine grain structures in sheet metals for inducing superior material properties. In CGP, the sheet metal specimens are subjected to repetitive corrugating and straightening under the plane strain deformation condition by utilizing alternate pressing with the asymmetrically grooved dies and flat dies. This induces a great amount of plastic strain in the sheet metal specimen without changing its initial dimensions. Over the last few years, CGP has gained significant importance for it being one of the most suitable techniques to fabricate sheet metals with superior and attractive properties. CGP can effectively refine the grain structure to a sub-micrometre level and at times, even to a nanometer level. Materials processed by CGP exhibit very high strength, high hardness and many other desirable properties. Numerous investigations were made on different materials like aluminium, copper, low carbon steel and nickel, and their properties were well documented in the scientific literature. This review summarizes most of the scientific results that were obtained by applying CGP to various materials and gives an outline of the applications of CGP in various industries. This review also discusses about the future scope of research on CGP and its benefits.     

Wear properties of brass samples subjected to constrained groove pressing process

Ultrafine grained materials have experienced a rapid development during the last two decades. Constrained groove pressing (CGP) process is one of the severe plastic deformation methods to fabricate ultrafine grain sheet materials. In this research, wear behavior of brass sheet subjected to CGP process was investigated. Generally it is shown that CGP process enhances the wear resistance of the material and this behavior is improved by increasing pass number. Also, the effect of initial pass and lower applied normal load on the wear resistance is more profound than subsequent passes and higher applied normal load, respectively. In addition, the influence of normal load is more profound than pass number at the increment of friction force. Although CGP process results in reduction at the specific wear rate, the influence of the first pass is much higher than the subsequent ones. Furthermore, lower specific wear rate is occurred at the higher applied normal load. The scanning electron microscopy analyses indicated that the wear mechanism is transferred from adhesion, delamination, abrasion and oxidation for the annealed condition to abrasion and adhesion for the third pass CGP sample. Also, it is found that there is a reverse relationship between specific wear rate and hardness.     

Structural and mechanical behaviour of severe plastically deformed high purity aluminium sheets processed by constrained groove pressing technique

High purity aluminium sheets (∼99.9%) are subjected to intense plastic straining by constrained groove pressing method successfully up to 5 passes thereby imparting an effective plastic strain of 5.8. Transmission electron microscopy studies of constrained groove pressed sheets divulged significant grain refinement and the average grain sizes obtained after five pass is estimated to be ∼0.9 μm. In addition to that, microstructural evolution of constrained groove pressed sheets is characterized by X-ray diffraction peak profile analysis employing Williamson–Hall method and the results obtained fairly concur with electron microscopy findings. The tensile behaviour evolution with increased straining indicates substantial improvement of yield strength by ∼5.3 times from 17 MPa to 90 MPa during first pass corroborated to grain refinement observed. Marginal increase in strengths is noticed during second pass followed by minor drop in strengths attributed to predominance of dislocation recovery is noticed in subsequent passes. Quantitative assessment of degree of deformation homogeneity using microhardness profiles reveal relatively better strain homogeneity at higher number of passes.     

The flow stress of fractal dislocation arrangements

In metals deforming under multislip conditions, fractal dislocation arrangements may develop. The composite model of the flow stress of a dislocation cell structure is generalized for dislocation arrangements which exhibit a broad spectrum of local hardness. Both forest strengthening (cell walls) and the Orowan stress (cell interiors) are taken into account. Relations between the flow stress and the characteristic parameters of the dislocation arrangement such as fractal dimension, maximum and minimum cell size are established. The model is used to calculate stress dependences of fractal dimension and cell wall volume fraction of dislocation patterns in high-symmetry oriented f.c.c. single crystals. The results are in good agreement with experimental observations on Cu and Cu-rich Cu–Mn.     

Effect of pre‐processing annealing and ram speed on mechanical properties for low carbon steel processed by constrained groove pressing

Constrained groove pressing (CGP) has emerged for producing ultra‐fine‐grained materials with distinguished properties. Low carbon steel sheets were subjected to severe plastic deformation by constrained groove pressing process. The effect of pre‐processing annealing temperature, ram speed and number of passes on microstructure, mechanical properties and wear behaviour of the sheets were investigated. The 3 mm thick sheets were deformed by a constrained groove pressing die at ram speeds: 5 mm/min, 10 mm min^?1 and 20 mm min^?1. Furthermore, the as received sheets were annealed at 600 °C and 900 °C, then deformed at ram speed 20 mm min^?1. The annealing temperature 900 °C led to slightly coarser grains, lower strength and larger ductility compared to those obtained after annealing at 600 °C. With lowering the ram speed to 5 mm min^?1, the number of passes could be increased to 10 passes while increasing ram speed from 5 mm min^?1 to 20 mm min^?1 improved the mechanical properties; after 3 constrained groove pressing passes, the ultimate tensile strength increased from 420 MPa to 490 MPa, the hardness from 174 HV 1 to 190 HV 1 and the elongation from 7.6 % to 9.5 %. Finer grains were also obtained by increasing ram speed. Wear resistance was greatly enhanced by constrained groove pressing and by the increase in ram speed.     

Differential scanning calorimetry study of constrained groove pressed low carbon steel: recovery,recrystallisation and ferrite to austenite phase transformation

Abstract

Low carbon steel sheets are subjected to severe plastic deformation (SPD) via constrained groove pressing (CGP) up to five passes. As a result of this process, strain magnitude up to 5·8 is imposed to the sheets, which leads to grain size of 225 nm. These nanostructured steel sheets, due to their high dislocation density and ultrafine microstructure, are very sensitive to heating. In the present study, recovery, recrystallisation and ferrite to austenite phase transformation phenomena for the SPD steel are investigated using differential scanning calorimetry method. The results show that with increasing the strain in steel sheets, the deformed stored energy (released through recovery and recrystallisation) and enthalpy of ferrite to austenite phase transformation are significantly increased and varied in 38·5–85·8 and 109–156·1 MJ m^?3 ranges respectively. In addition, transformation temperature is decreased from 761 to 750°C after five CGP passes. However, recovery stored energy, recovery and recrystallisation peak temperatures are not changed, considerably. Experimental data show that with increasing the hardness, the stored energy is increased. One empirical equation is developed for relationship between hardness and stored energy of severely deformed low carbon steel. In addition, using the dislocation model, this mentioned relationship is justified.     

On dislocation-based artificial neural network modeling of flow stress

Computational design of materials processes has received great interests during the past few decades. Successful designs require accurate assessment of material properties, which can be influenced by the internal microstructure of materials. This work aim to develop a novel computational model based on dislocation structures to predict the flow stress properties of metallic materials. To create sufficient training data for the model, the flow stress of a precipitation–hardening aluminum alloy was measured by characterizing the dislocation structure of specimens from interrupted mechanical tests using a high resolution electron backscatter diffraction technique. The density of geometrically necessary dislocations was calculated based on analysis of the local lattice curvature evolution in the crystalline lattice. For three essential features of dislocation microstructures – substructure cell size, cell wall thickness, and density of geometrically necessary dislocations – statistical parameters of their distributions were used as the input variables of the predictive model. An artificial neural network (ANN) model was used to back-calculate the in situ non-linear material parameters for different dislocation microstructures. The model was able to accurately predict the flow stress of aluminum alloy 6022 as a function of its dislocation structure content. In addition, a sensitivity analysis was performed to establish the relative contribution of individual dislocation parameters in predicting the flow stress. The success of this approach motivates further use of ANNs and related methods to calibrate and predict inelastic material properties that are often too cumbersome to model with rigorous dislocation-based plasticity models.     

Full density consolidation of pure aluminium powders by cold hydro-mechanical pressing

Gas atomized pure Al powders were successfully consolidated into full density at room temperature by newly developed cold hydro-mechanical pressing (CHMP). The effects of the configuration of pressures on the relative density and microstructure of consolidated specimens were investigated. The full density consolidation of metal powders at room temperature resulted from severe shear deformation of the particles which ruptured the surface oxide layers. Our works demonstrate that the novel CHMP provides an economical approach to consolidate particulate material into high quality bulk preforms for engineering applications.     

Effect of plasticity on the residual stress measurement using the groove method

The mechanical methods based on milling rectilinear or annular grooves on a component's surface and measurement of relaxed strains are some of the most used semi-destructive methods for the determination of residual stresses. These are evaluated from the relaxed strains by means of equations based upon the linear elastic theory. In this paper the errors due to yielding localised at the bottom of the groove have been investigated. The analyses were carried out by means of the finite element technique varying the most important parameters involved. The experimental results show a good agreement with the numerical ones.     

The formation and distribution of haloacetic acids in copper pipe during chlorination

The formation and distribution of HAAs in copper pipe during chlorination was investigated. To determine the material influence of copper pipe, parallel experiments were performed in glass pipe. Results showed that there was no obvious difference between the sum of haloacetic acids (HAAs) and trihalomethanes (THMs) produced in copper pipe compared to that produced in glass pipe over a 12h period. However, significant differences were observed about the distribution of five haloacetic acids in copper pipe and in glass pipe. Relatively less trichloroacetic acid (TCAA) and more monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), dibromoacetic acid (DBAA) and trihalomethanes (THMs) were produced in copper pipe than those in glass pipe. Corrosion scale on the wall of copper pipe was analyzed using X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results showed the scales on the pipe surface mainly consisted of Cu2O, CuO and Cu (OH)2 or CuCO3. During 24h stagnation, copper released gradually from copper pipe. The influences of copper (II) and copper oxides on the distribution of HAAs were investigated in designed experiments. Results showed that less amount of TCAA, more amounts of DCAA and MCAA were formed with increasing concentration of copper (II). It was because the accelerative effect of copper (II) on the depletion of chlorination restricted the formation of TCAA precursor and the further formation of TCAA. Owing to the transformation of DCAA precursor to TCAA precursor was limited, more DCAA precursor could yield DCAA. The influences of Cu2O and CuO on the distribution of TCAA and DCAA were the result of copper released at higher content.     

Texture and mechanical properties of single crystal copper during equal channel angular pressing by route C

Single crystal copper has excellent electrical and thermal conductivity, but the lower strength seriously limited its application. Traditional strengthening methods, such as alloying, will severely damage its conductivity. Severe plastic deformation is the most effective methods for increasing the metals strength and not reducing the conductivity. The microstructure and texture evolution of single crystal copper (99.999 %) during equal channel angular pressing by route C was investigated by scanning electron microscopy, X‐ray diffraction, electron backscatter diffraction and transmission electron microscopy, the mechanical properties and conductivity were tested, and the influence mechanism of texture and microstructure on mechanical properties and conductivity were analyzed. The results show that during equal channel angular pressing, the original <111> orientation gradually changed to <001>, accompany lots of low‐angle grain boundaries were formed. With strain increasing, the high‐angle grain boundaries increased gradually, and the deformation bands with <110> orientation was formed in the single crystal structure, which plays a positive role on the conductivity. After 5 passes, the tensile strength of single crystal copper increased from 168 MPa to 415 MPa by route A and 385 MPa by route C, and the elongation declined sharply from 63 % to 30 % and 27.9 %, respectively. After 16 passes, the hardness increased from 60.4 HV to 130.8 HV and the conductivity only slightly down.

     

Effect of hydrogen on stress corrosion cracking of copper

The effects of hydrogen on electrochemical behavior and susceptibility of stress corrosion cracking (SCC) of pure copper were studied. SCC susceptibility of pure copper in a 1 M NaNO₂ solution was increased by pre-charged hydrogen. The effect of hydrogen on the susceptibility is more obvious in the low stress region due to the longer fracture time, which resulted in a longer time for more hydrogen to diffuse toward the crack tip. Synergistic effects of hydrogen and stress on corrosion and SCC processes were discussed. The results showed that an interaction between stress and hydrogen at the crack tip could increase the anodic dissolution rate remarkably.     

Prediction of the flow stress of Al6061 at hot deformation conditions

Isothermal compression tests were carried out on Al6061 using a Gleeble-1500 thermal simulator at temperatures ranging from 573 to 723 K and strain rates from 0.5 to 30 s⁻¹. The flow stress of Al6061 was characterized based on an analysis of the true stress-true strain curves. A mathematical mode coupling flow stress with strain, strain rate and temperature for Al6061 has been proposed by using a hyperbolic sinusoidal type equation. The material constant α is 0.01 MPa⁻¹ in the model, whereas other material constants n, lnA and Q are found to be functions of strain. The predicted results from this proposed model are found to be in good agreement with the experimental flow stress curves which can be used to predict the required deformation forces in hot deformation processes.     

电解铜粉和雾化铜粉对SPS过程中显微组织演变的影响

以电解铜粉和雾化铜粉为SPS烧结原材料,对它们在SPS烧结过程中的烧结曲线、烧结体显微组织的演变过程,以及烧结温度对烧结体相对密度的影响进行了分析、比较,并分析了其中的原因。得到电解铜粉和雾化铜粉在SPS烧结过程中显微组织演变特征和规律不同的结论。     

Microstructure of the pure copper produced by upsetting with ultrasonic vibration

The propagation of the ultrasonic wave in the solids can greatly affect the microstructure of the material. In this paper, upsetting with ultrasonic vibration as a novel method to produce the ultrafine grained material was presented to refine the pure copper grains. For the ultrasonic vibration upsetting, during the upsetting process, the ultrasonic wave propagates in the specimen at the same time. Under the coupling action of the ultrasonic and plastic deformation, the pure copper grains with the initial grain size of ~ 50 μm_? were refined to about 100-300 nm after one time forming process. By comparing the ultrasonic vibration upsetting with the conventional upsetting process, the role that the ultrasonic wave played during the upsetting process was revealed.     

Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP

The effect of deformation temperature on microstructure evolution during equal channel angular pressing (ECAP) was studied in a coarse-grained aluminum alloy 2219 in a wide temperature interval from 250 to 475 °C. The structural changes taking place during ECAP up to strains of 12 are classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by grain fragmentation at moderate strains; (3) rapid development of new grain at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected at higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. This can be attributed to relaxation of strain compatibility between grains due to frequent operation of dynamic recovery and grain boundary sliding at higher temperature. The mechanism of grain refinement is discussed in detail.     

Microstructure development and hardening during high pressure torsion of commercially pure aluminium: Strain reversal experiments and a dislocation based model

The effect of strain reversal on hardening due to high pressure torsion (HPT) was investigated using commercially pure aluminium. Hardening is lower for cyclic HPT (c-HPT) as compared to monotonic HPT (m-HPT). When using a cycle consisting of a rotation of 90° per half cycle, there is only a small increase in hardness if the total amount of turns is increased from 1 to 16. Single reversal HPT (sr-HPT) processing involves torsion in one direction followed by a (smaller) torsion in the opposite direction. It is shown that a small reversal of 0.25 turn (90°) reduces hardness drastically, and that decrease is most marked for the centre region. These behaviours and other effects are interpreted in terms of the average density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs). A model is presented that describes the experimental results well. A key element of the model is the assumption that at the very high strains developed in severe plastic deformation processes such as HPT, the dislocation density reaches a saturation value. The model indicates that the strength/hardness is predominantly due to GNDs and SSDs.     

Modeling of flow and debris ejection in blasting erosion arc machining in end milling mode

Blasting erosion arc machining (BEAM) is a typical arc discharge machining technology that was developed around 2012 to improve the machinability of difficult-to-cut materials. End milling BEAM has been successfully developed and preliminarily applied in industry. However, owing to the high complexity of the flow field and the difficulty of observing debris in the discharge gap, studies of the flow and debris in end milling BEAM are limited. In this study, fluid dynamics simulations and particle tracking are used to investigate the flow characteristics and debris ejection processes in end milling BEAM. Firstly, the end milling BEAM mode is introduced. Then the numerical modeling parameters, geometric models, and simulation methods are presented in detail. Next, the flow distribution and debris ejection are described, analyzed, and discussed. The velocity and pressure distributions of the axial feed and radial feed are observed; the rotation speed and milling depth are found to have almost no effect on the flow velocity magnitude. Further, debris is ejected more rapidly in the radial feed than in the axial feed. The particle kinetic energy tends to increase with increasing milling depth, and smaller particles are more easily expelled from the flushing gap. This study attempts to reveal the flow field properties and debris ejection mechanism of end milling BEAM, which will be helpful in gaining a better understanding of BEAM.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00328-9     

SA508Gr.4N钢热变形过程微观组织演变及流变应力模型

利用Gleeble-1500D热模拟试验机研究Ni-Cr-Mo系低合金SA508Gr.4N钢在变形温度为850~1200℃,应变速率为0.001~1 s^-1,真应变为0.9条件下的等温热变形行为,建立包含动态回复和动态再结晶的基于物象的流变应力模型与动态再结晶晶粒尺寸模型,并提出避免粗大晶粒组织遗传性的适宜锻造工艺。结果表明:随着变形温度的升高,应变速率的降低,动态再结晶体积分数和晶粒尺寸逐渐增加;SA508Gr.4N钢的真应力-真应变曲线具有明显的不连续动态再结晶现象;通过实验值和模型预测值对比可得流变应力模型的相关系数(R)及平均相对误差(MRE)分别为0.998和4.76%,动态再结晶晶粒尺寸模型的相关系数(R)及平均相对误差(MRE)分别为0.991和8.69%,两个模型均具有较高的准确性。     

Superconformal copper filling of a nano-scale trench by nucleation suppression at the trench entrance during metal organic chemical vapor deposition

Superconformal filling of copper (Cu) into the nano-scale SiO₂ trench was investigated by controlling the nucleation and growth conditions during the metal organic chemical vapor deposition of Cu. Inductively coupled H₂/Ar plasma pretreatment of the Ru-deposited trench pattern with a substrate biasing prior to deposition led to suppression of Cu nucleation on the top and entrance areas of the trench. In turn, Cu grows preferentially inside the trench. Controlled nucleation by plasma pretreatment enabled the achievement of superconformal Cu gap filling of sub-60 nm trenches without voids. Suppression of nucleation was attributed to deposition of sputtered silica (Si)-containing species on the top and entrance areas of the trench from the quartz window of the plasma reactor.