Cold-worked Metal Research Articles

Strength enhancement of AlMg sheet metal parts by rapid heating and subsequent cold die stamping of severely cold-rolled blanks

A trade-off between high strength and formability is forcing the automotive industry today to use several different aluminum alloys in structural assemblies. This leads to a reduced recyclability, which ultimately affects the sustainability of the entire product. In this work, it is demonstrated that a severely cold-worked AA5182 sheet metal blank can be formed to a sound part through a heat treatment of 10 s at 300 °C and a subsequent cold die forming. The high dislocation density in the as-received feedstock is induced by means of cold-rolling and promotes a yield strength of 326 MPa. The rapid heating with a heating rate of 24 K/s allows to increase the local ductility of the investigated material in a warm condition by more than 200% while preserving the initial grain structure of the material. Combined with additional work-hardening during forming, the final yield strength of the formed part is 319 MPa. The recovery kinetics of the as-received AA5182-H19 sheet are examined between 150 °C and 300 °C with dwell times between 1 s and 30 s. It is found that above 250 °C the fraction of residual strain hardening is a function of temperature and time. The resulting dislocation structures are investigated by a series of SEM, EBSD, and TEM measurements. Furthermore, the ductility enhancement is studied by a series of tensile tests with in-situ contact plate heating and fractographic analyses. Because the process under discussion allows to combine high strength and formability within one alloy, it is a promising approach to substitute 6000-series aluminum alloys in mixed-material assemblies and therefore make them easy to recycle.

ПРО МОЖЛИВІСТЬ ПРОГНОЗУВАННЯ МЕХАНІЧНИХ ВЛАСТИВОСТЕЙ ХОЛОДНОДЕФОРМОВАНИХ МЕТАЛОВИРОБІВ ЗІ СТАЛЕЙ ПЕРЛІТНОГО КЛАСУ

In accordance with the latest global trends and modern needs of hardware factories, the demand for wire rod made of pearlitic steels has significantly increased, which can undergo cold plastic deformation with large degrees of reduction and is intended for the manufacture of various commercial products (cold-worked reinforcement, reinforcing ropes, spring wire, steel cord, wire for high pressure hoses, construction fiber, etc.). The most promising direction for increasing the strength class of hot-rolled steels is strain hardening during cold plastic deformation. The structure of cold-worked steels has a more uniform distribution over the cross-section, in contrast to thermally hardened ones, which in the latter case are characterized by annular structural zones formed due to different mechanisms of austenite decomposition. In a number of cases, structural heterogeneity causes instability of the mechanical properties of rolled steel, therefore, there is currently no alternative way to strain hardening. The manufacture of high-strength cold-worked metal products is a complex process and depends on the quality of the wire rod. If the cold-worked hardware does not meet the requirements of the normative documentation for the strength class, then it is no longer possible to ensure the correction of this defect using heat treatment. In this regard, scientific and practical interest has arisen to determine the possibility of creating a method for predictive determination of the mechanical properties of cold-deformed metal products made of pearlitic steels. The features of the influence of cold plastic deformation by on the formation of the strength class of pearlite grade steels with a carbon content of 0.7…0.9 % are considered It is established that the temporary resistance to fracture, at the known parameters of structure and carbon content, lends itself well to mathematical calculations and allows to create predictive models. According to the results of the research, a computer program was created that allows to automatically calculate the energy parameters of drawing and determine the tensile strength of the wire after depending on the total relative compression, parameters of the structure of wire rod and the carbon content in the steel.

Strengthening face centered cubic crystals by annealing induced nano-twins

Usually, cold working strengthen metals and alloys by introducing large population of dislocations, whereas annealing of cold worked metal recovers the structure, annihilates dislocations, forms new strain-free grains, and results loss of strength. Here, we report annealing-hardening at temperature well below stress relieving and recrystallization temperatures in contrast to the typical behavior. A large amount of structural defects, such as dislocations, grain boundaries, twins, and stacking faults, have been introduced in nanostructured α-brass by cryorolling. The interaction and rearrangement of these defects upon annealing at 165–200 °C have been monitored at an interval of 1 minute. Large increase of the yield strength up to 578 MPa has been achieved in annealed specimens, which is 23% higher than that of as-cryorolled, and 425% higher than that of as-cast brass due to the evolution of nano-twins. Our approach shows a new avenue on strengthening fcc crystals by incorporating annealing induced nano-twins.

Conditions for Complex Precipitate Formation and Their Effect on Low-Carbon Steel Cold-Rolled Product Properties

The effect of chemical composition and recrystallization annealing regimes in continuously operating units on the mechanical properties of cold-rolled product of type 08Yu steels is studied. It is established that with a sulfur content of 0.012–0.018% during hot rolling an optimum amount and size distribution of manganese sulfide particles is achieved providing the formation on them of cementite and aluminum nitride precipitates. Complex MnS-cementite particles formed in an equalization chamber of a continuously operating hot galvanizing unit facilitate an increase in formability as a result of a reduction in the degree of the difference in grain size of the microstructure and an increase in cementite distribution uniformity. Precipitation of AlN on MnS particles in the course of metal hot rolling and cooling provides more complete removal of nitrogen from solid solution, and consequently a reduction in steel tendency towards strain ageing. A significant reduction in the proportion of fine AlN particles increases the rate of cold worked metal recrystallization, and this leads to a reduction in fineness of the microstructure and a corresponding reduction in yield strength and an increase in relative elongation.

Estimation of plastic deformation in the technologies of bulk cold deformation

A technique for determining the degree of reduction upon forming by cold bulk shaping has been proposed. The technique is based on a comparison of the measurement data on the hardness of a cold-worked metal with that of the same metal subjected to a compression test at a certain strain intensity. The technique has been tested in technological operations of reduction and thread rolling.

Formability of Friction Stir-Welded Blanks with Different Thickness Ratios

Welded sheets with different thicknesses are one of the interesting types of tailor-welded blanks (TWBs) that are widely used in metal-forming industries. In the present work, the formability behavior of different 1100-aluminum TWBs was studied. In this regard, the TWBs were made with different thickness ratios by using friction stir welding (FSW) at different welding rotational speeds (ω). The thickness ratios of 1.0, 1.3, and 1.7 were investigated where the thinner sheets had 1.5 mm thick for all conditions; i.e., the volume of welded material increased when the thickness ratio increased. Macrostructural observations, mechanical investigations, and sheet-forming limit tests were conducted. The results indicate that increasing ω leads to increasing the weld nugget size up to a maximum level and welding became impossible at higher ω. Furthermore, increasing heat input during FSW, the ultimate tensile strength of welds reduced in comparison with the initial cold-worked base metal. However, the ductility improved by increasing the heat input, which produced the sound welds. Formability studies of the friction stir-welded blanks with equal thicknesses have shown that the forming ratio improves up to 2.8 times the base metal. Forming limit curves also illustrate that increasing the thickness ratio of TWB causes the formability ratio to decrease steadily. Thus, when the thickness ratio becomes 1.7, the formability of TWB decreases approximately to the thinnest base metal.

Impact of Forming, Welding, and Electropolishing on Pitting and the Surface Finish of SRF Cavity Niobium

A broad range of coupon electropolishing experiments are described to ascertain the mechanism(s) by which large defects are formed near superconducting radio-frequency (SRF) cavity welds. Cold-worked vs. annealed metal, the presence of a weld, and several variations of electropolishing (EP) parameters were considered. Pitting is strongly promoted by cold work and agitation of the EP solution. Welding also promotes pitting, but less so compared with the other factors above. Temperature increase during EP did not strongly affect glossiness or pitting, but the reduced viscosity made the electrolyte more susceptible to agitation. The experiments suggest that several factors that are rather benign alone are combined by the cavity forming, welding, and processing sequence to promote the formation of defects such as pits. Process changes to mitigate these risks are discussed.

Nanomechanical Testing of Gum Metal

“Gum Metal” is a newly developed β-Ti alloy that, in the cold-worked condition, has exceptional elastic elongation and high strength. The available evidence suggests that Gum Metal does not yield until the applied stress approaches the ideal strength, and then deforms by mechanisms that do not involve conventional crystal dislocations. To study its behavior, submicron-sized pillars of solution-treated and cold-worked Gum Metal were compressed in situ in a quantitative compression stage in a transmission electron microscope. Solution-treated specimens and half of the cold-worked specimens exhibited essentially monotonic hardening during compression, but with serrated load-deflection curves that included periodic partial relaxations of the stress. The other cold-worked specimens exhibited pronounced shear instability. These samples deformed by a stick-slip motion along a well-defined shear plane, with a serrated load-deflection curve demonstrating partial stress relaxation at each sliding event. The pattern of deformation is consistent with prior work showing deformation by the formation and growth of shear bands and faults in a matrix that is densely decorated with defects.

Monte Carlo simulation of recrystallization with hardness input of cold worked metal

A Monte Carlo model on the basis of hardness input is developed to predict the annealing microstructure of deformed specimens in tensile, compression, and tensile + compression tests. From experimental value of hardness, the stored energy of the deformed specimens is calculated and entered into the Monte Carlo model. The consistency between the simulation results and experimental data shows that the developed model based on hardness input can be more practical since the effect of different deformation states is considered for estimating of stored energy.

Characterization of twinning in electrodeposited Ni–Mn alloys

Twinning is ubiquitous in electroplated metals. Here, we identify and discuss unique aspects of twinning found in electrodeposited Ni–Mn alloys. Previous reports concluded that the twin boundaries effectively refine the grain size, which enhances mechanical strength. Quantitative measurements from transmission electron microscopy (TEM) images show that the relative boundary length in the as-plated microstructure primarily comprises twin interfaces. Detailed TEM characterization reveals a range of length scales associated with twinning beginning with colonies (∼1000 nm) down to the width of individual twins, which is typically <50 nm. We also consider the connection between the crystallographic texture of the electrodeposit and the orientation of the twin planes with respect to the plating direction. The Ni–Mn alloy deposits in this work possess a {110}-fiber texture. While twinning can occur on {111} planes either perpendicular or oblique to the plating direction in {110}-oriented grains, plan-view TEM images show that twins form primarily on those planes parallel to the plating direction. Therefore, grains enclosed by twins and multiply twinned particles are produced. Another important consequence of a high twin density is the formation of large numbers of twin-related junctions. We measure an area density of twin junctions that is comparable to the density of dislocations in a heavily cold-worked metal.

Rotation‐rate continuity in bi‐axial plastic deformation

AbstractThe hypothesis of rotation‐rate continuity, which is associated with the deformation of homogeneous, isotropic bodies by slip, is applied to the plastic flow of a cold‐worked metal (or strain‐aged mild‐steel) under the assumption of bi‐axial strain. In a special case, two classes of rotationally continuous slip‐line fields are shown to exist on curved principal surfaces. The first of these reduces to Hill's equiangular net under plane strain, further special cases of this field being the circular‐centred‐fan and the rectangual‐net. A second class of special solutions consists, again under plane‐strain, of two orthogonally intersecting families of logarithmic spirals. A slip‐line field that occurs in the drawing of a circular cup obeys this condition, as it applies to a curved principal surface. The general condition for continuity of rotation‐rate within a slip‐line net, located on a curved principal surface, is then derived and it is shown that Prandtl's solution to the plane‐strain compression problem obeys this general condition; except in a pair of thin layers adjacent to the platens.

The effect of defect annihilation on the segregation kinetics of sulphur in both quenched and cold worked nickel

Impurity segregation to grain boundaries and free surfaces of pure metals is thought to take place according to two mechanisms. One involves the thermodynamic and kinetic laws of equilibrium, as is the case, for example, in annealing for sufficiently long periods at sufficiently high temperatures (≈0.5T M, T M being the melting temperature). The other takes place while a metal, initially in a non equilibrium state is returning to equilibrium, as is the case, for example, during the annealing of a quenched or cold worked metal. In this work, a kinetic study of sulphur segregation taking place during the annealing of both quenched and cold-rolled nickel is carried out. It is found that the sulphur segregation kinetics measured during the elimination of vacancies (quenched nickel) are two–three orders of magnitude higher than the ones predicted by McLean for equilibrium segregation. In the case of the cold worked material, it is found that the kinetics, compared to equilibrium ones, are accelerated by both the vacancy annihilation and the recrystallization stages of the heating process. A considerable reduction in strength and ductility, coupled with severe intergranular corrosion as shown by scanning electron micrographs, may be taken as clear evidence of intergranular segregation taking place during the recovery by the material of its equilibrium state.

The Effect of Multiaxiality on the Evaluation of Weldment Strength Reduction Factors in High-Temperature Creep

The conventional way to define the weldment creep strength reduction factor is usually based on uniaxial creep data of weld metals and parent metals. In order to take the multiaxial effect into consideration, this paper has defined a structural transfer function which can be evaluated from general creep stress analysis. An analytical model is then proposed in the light of the function. Two numerical examples of typical weld properties show that the transfer function has a load-independent feature, which allows one to obtain multiaxial stress components in a weldment through minimal computation effort. Fairly good estimation of the stress level in the weld metal is achieved. On the basis of the present semi-analytical procedure, the weldment creep strength reduction factors are evaluated. For a 0.5Cr0.5Mo0.25V butt-welded tube under internal pressure, which has a higher weld metal creep-rupture strength, and lower weld metal creep strain rate, the reduction factors range from 0.9 to 0.95. For the AISI 316 butt-welded tube of cold-worked parent metal and creep soft weld metal, lower strength reduction factors are found, but they may still be nonconservative due to stress enhancement in the heat-affected zone.

An X-ray analysis of stacking disorder in kaolinite by fourth moment

Mathematically, it is well established that besides the Fourier coefficients, the natural characteristics of any distribution are its moments. The use of the second moment, called variance, as a measure of line broadening has already gained wide popularity because of its several merits relative to other empirically devised parameters. In recent decades, the fourth moment has been recognized as an important parameter; its theoretical significance has been discussed by Mitra (1964), who developed the moment range function for the estimation of crystallite size and distortion in cold-worked metal. However, owing to the requirements of high degree of accuracy of intensity measurement coupled with cumbersome calculations, this parameter did not find much general application. Modern diffractometers and sophisticated computers have helped ease the problem and in recent years several workers (Kagan &amp; Snovidov, 1965; Kulshreshtha et al., 1971; Mukherjee, 1981) have successfully used this parameter for defect analysis in various non-ideal forms of materials.

Fabricability of and microstructural development in cold worked metal matrix composites

In some cases, in situ composites may be fabricated by mechanical working even when they contain phases which are inherently “mechanically incompatible” in bulk form. An extreme case is provided by composites containing low volume fractions of “brittle” chromium. The initial microstructure plays an important role in the fabricability of these composites and several examples of this are provided by our work. During wire drawing, interphase spacing is reduced in an axially symmetric manner irrespective of whether or not a lamellar or ribbon-like morphology is developed. The reduction in interphase spacing with rolling strain is similar to that observed during drawing in spite of the externally imposed plane strain conditions accompanying deformation by rolling.

Spectral absorptivity of rough copper and brass surfaces

The spectral absorptivity of brass and copper in the optical range (400–650nm) has been measured using a calorimetric method, for a series of rough cold-worked metal surfaces. Results depend strongly on the surface roughness and weakly on the light wavelength. The total absorptivity for white light (2870 K) reaches 34% on copper samples.

Surface roughness effect on optical absorptivity of metals

The white light (3700 K) optical absorptivity has been measured, using a calorimetric method, for a series of rough cold-worked metal surfaces. Results show a strong maximum (∼30%) in absorptivity vs roughness. The depth variance of surface imperfections (from 20 to 700 nm) has been determined by measuring infrared reflectance of the samples.

The structure of MgO abrasive wear debris

An X-ray line-broadening study of MgO abrasive wear debris suggests that the finest particles have a structure comparable with that of a heavily cold-worked metal and that they are produced by deformational processes whereas the larger particles are not homogeneous in structure and are probably produced by fracture.

Effect of carbon, manganese, nickel, and nitrogen concentrations on the phase composition and properties of wrought high-manganese steel

1. Strain hardening of high-manganese steel is characterized not only by the formation of dislocations but also by precipitation of second phase (primarily α martensite with a carbon concentration 1.2%), lowering the ductility of the cold worked metal, which induces the formation on the working surfaces of defects due to contact fatigue. 2. Reducing the quantity of second phase precipitated during deformation of high-manganese steel and the addition of 0.040% N with 0.85–1.12% C and a simultaneous increase of the Mn↮C ratio to 14–16 provides high values of the ductility and fracture toughness of the hardened metal, due to which the service life increases for cast machine parts operating under high dynamic loads under wear conditions, especially at negative temperatures.

The Effect of Prior Cold Work on the Degree of Sensitization in Type 304 Stainless Steel

Abstract The effect of prior cold work of 0 to 60% on subsequent sensitization of Type 304 stainless steel at 660 C was studied from anodic polarization scans in 1N HClO4 + 0.2N NaCl and 0.5M H2SO4 + 0.01 M KSCN solutions and from SCC behavior in room temperature 1N H2SO4 at 150 mVSHE at a strain rate of 10−6 sec−1. Small amounts (5 to 20%) of prior cold work sharply enhanced the susceptibility to subsequent sensitization. With increasing amounts of prior cold work, heating at 660° for 1 or 16 hours resulted in progressively more widespread precipitation of carbides within and across the grains. This presumably shortened diffusion paths for chromium and, along with the expectedly faster chromium diffusion rate in cold worked metal, resulted in partial to total absence of sensitization in specimens that were highly cold worked (>40%) prior to the sensitization treatment.