Forming limits of sandwich sheet materials

Failure of sandwich sheet materials by tensile instability and localized necking was studied by performing punch-forming experiments on stainless steel clad aluminum. By using narrow blanks and no lubrication, lateral contraction was possible, and failures could be produced in the drawing area of the forming limit diagram. For this deformation regime, diffuse instability led to localized necking. As in monolithic materials, the development of the localized neck in stainless steel clad aluminum determined the forming limit, and predictions of the strain levels for the onset of local instability correlated well with the observed forming limit strains. By preventing lateral contraction, failures in stretching were produced. The forming limit strains in this case depended on the strains at the onset of diffuse instability in much the same manner as is observed for monolithic materials. The strains at the onset of diffuse instability were predicted using a generalized rule of mixtures, and agreement between measured values and values predicted from component properties was good when the strain-path dependence of the instability strain for the individual components was taken into account. The diffuse necking process in stretching of stainless steel clad aluminum led to local thinning when deformations involved small degrees of biaxiallity. On the other hand, nonuniform through thickness straining of the component layers in specimens strained close to balanced biaxial stretching appeared to control the localization process and gave rise to forming limit strains lower than expected from observations of punch formed monolithic sheet materials. For all deformation modes, localized flow culminated in delamination and fracture. S. L. SEMIATIN, formerly graduate student, Department of Metallurgy and Materials Science, Carnegie-Mellon University     

Deformation of sandwich sheet materials in uniaxial tension

The stable and unstable plastic flow of stainless steel-clad aluminum and aluminum-clad stainless steel sandwich sheet materials deformed in uniaxial tension have been investigated. For the clad sheet materials studied experimentally, stable deformations were uniform in the component layers, and the assumption of isostrain was used in modeling the deformation behavior. The rule of mixtures, an average of component properties weighted by cross-sectional area fractions, was applied to determine sandwich uniaxial true stress-true strain curves from those of the components. In addition, measurements of residual stress distributions in deformed tensile specimens gave insight into states of stress during loading. A model to determine the magnitude of stresses which are generated by component normal plastic anisotropy differences was developed as well. With this knowledge of the stress state, predictions of uniform elongation of the clad sheet materials were made which compared favorably to experimental measurements. As for ductile monolithic sheet materials, stable flow of sandwich sheet materials in tension was limited by diffuse necking, which leads to local instability at higher strains. This local instability gives rise to a through-thickness localized thinning which terminates macroscopic deformation. Conditions for local instability in uniaxial tension have been developed for sandwich as well as monolithic sheet materials. Predictions from these models are in agreement with measurements. S. L. SEMIATIN, forMetly Graduate Student, Department of Metal-lurgy and Materials Science, Carnegie-Mellon University.     

Shear band formation in plane strain compression

Shear band formation during plane strain compression of single crystals and polycrystals of an Al-3 wt% Cu alloy was studied. X-ray and electron diffraction, optical microscopy, and electron (TEM/STEM) microscopy were used to document the structure and micromechanisms of the localization process. Complementary finite element studies were performed using the measured single crystals' single slip system strain hardening data. The experimental observations and the computed deformation response are in very close agreement, and indicate that localization, through macroscopic shear band formation, occurs in continuously strain hardening, damage-free material. Shear band formation was preceeded by the development of very coarse slip which, like the shear bands, propagated across entire grains and, in single crystals, across the entire crystal. The computations and experiments showed that geometrical softening, caused by nonuniform lattice reorientation, is an important micromechanical influence on the localization process in both single crystals and polycrystals. Also, the propagation of shear bands across grain boundaries in polycrystals was looked at experimentally and computationally. Particular attention was paid to the crystallography of shear band transmission through grain boundaries.     

Lateral constraints in plane strain compression of single crystals

In plane strain compression tests where the walls of a channel suppress lateral flow, the lateral stress developed at the walls varies with the crystal orientation. We have applied a crystal plasticity analysis to calculate the ratio of lateral stress (σ_yy) to the compression stress (σ_xx) for fcc crystals compressed on the (110) plane and extended in various directions in (110). It was found that σ_yy/σ_xx is equal to zero from (110)[001] to \((110)[\bar 112]\) , at which point it rises discontinuously to a value of 1.5. It then decreases to a value of 1.0 for \((110)[\bar 111]\) and further to 0.5 for \((110)[\bar 110]\) . Tests on copper crystals generally confirmed the theoretical predictions. Deviations can be explained in a consistent way by recognizing the inability of the experimental apparatus to suppress small amounts of lateral flow.     

Hot plane strain compression testing of aluminum alloys by channel-die compression

A thoroughly tested, high-temperature channel-die compression (CDC) rig is described for simulating hot plane strain compression of metallic alloys up to 500 °C. The equipment is currently used to characterize the flow stress and microstructure evolution in hot-rolled Al alloys. It has been validated by several tests involving (1) metallographic analysis of deformed samples; (2) flow stress comparisons with the same, or similar alloys deformed in conventional uniaxial or plane strain compression; and (3) microstructure and texture measurements. The use of modern lubricants enables one to obtain accurate flow stresses and true plane strain deformations that are homogeneous over 80 pct of the sample. The equipment also features rapid heating and cooling systems to minimize thermally-induced microstructure changes. Some results on high-temperature slip systems, hot deformation textures, and microstructures, and the behavior of constituent particles are outlined to illustrate the advantages of the technique.     

平面应变过程中摩擦对金属流变规律的影响

研究了平面应变压缩过程中摩擦对金属流变规律以及力能参数的影响.通过有限元软件MSC/Superform,采用二维以及三维热力耦合有限元理论对不同摩擦条件下的力能参数、宽展情况以及变形金属的流动情况进行了分析;在自主研制的大试样平面应变热模拟试验机上,利用室温下工业纯铝探讨了上下接触面摩擦不一致时金属流动的规律.结果显示:随着摩擦的增大,变形负载将增大,宽展减小;当上下接触面间摩擦条件不同时,变形后的试样将出现\     

Temper rolling of sheet steel

Temper rolling with relatively small deformation (usually around 1%) forms the final mechanical properties, planarity, and surface microrelief of sheet steel. The aspects of temper rolling that affect the final strip quality may be identified on the basis of theoretical and experimental data and production experience with hot- and cold-rolled thin-sheet steel at various metallurgical enterprises. Practical recommendations are made regarding temper rolling.     

Monitoring the surface quality in sheet rolling

A system for monitoring the surface quality of hot-rolled strip produced on a 2000 continuous broad-strip mill is introduced on an experimental basis. Data on the surface defects permit improvement in sheet-production technology.     

Force required in significantly asymmetric sheet rolling

A method of determining the force in asymmetric sheet rolling is considered. Theoretical concepts regarding rolling with asymmetry are considered. A mathematical model is formulated. Software that permits the calculation of the rolling force from relevant factors is developed. Optimization of the significantly asymmetric rolling of thick disks on the basis of computer experiments is considered.     

Formability behavior of brass alloy sheet: the role of twins in microstructure

Quantitative understanding of the process and formability parameters involved in grain size and the formation of annealing twins after plastic straining is important in the control of the manufacturing process. There is a synergistic effect of strain and temperature on the density of annealing twins. Formability of brass alloy sheets was studied after annealing of 65% cold worked (CW) samples at different temperatures (300–600°C). Tensile, deep drawing and Erichsen tests were carried out at room temperature to evaluate formability of alloy. Effect of annealing temperature on density, distribution and size of twins is investigated. It was shown that annealing of brass alloy resulting in formation of annealing twins which at higher annealing temperature were reduced by increasing grain size. Best deep drawability would be achieved by annealing at moderate temperature 400–450°C which microstructure consists of fine grain and twin bands. Work hardening exponent of samples was calculated based on the tensile test data and correlated with stretch ability of annealed brass sheets. It was found that the sheets annealed at 600°C possess best ductility and high average n-value.     

Shear-band orientations in plane strain

Plane-strain experiments on a wide variety of materials show that localized shear bands are generally not inclined at 45° to the in-plane axes of principal stresses. In metals the shear bands are usually inclined at angles less than 45° to the direction of the maximum (most tensile) principal stress, whereas in polymers and geological materials they are usually inclined at angles greater than 45 . It is shown here that the orientations of shear bands in a metal, a polymer, and a sand are very well predicted by a bifurcation analysis that uses an incrementally linear rate constitutive law exhibiting pressure-sensitivity, dilatancy, and noncoaxiality.     

One-zone rolling of composite materials

The energy–force parameters of free rolling of a strip without its tension and rolling with one backward or forward creep zone in the deformation zone are compared. The limiting backward or forward tensions are determined, and the change in the linear sizes of a composite billet during deformation in a rolling mill is considered.