Bending Formability Research Articles

Microstructural characteristics and bending properties of extruded SEN9 and SEN11 magnesium sheets

Mg–Al-based alloys with small amounts of Ca and Y, referred to as stainless, environment-friendly, nonflammable (SEN) alloys, exhibit significantly improved resistance to corrosion and ignition compared to commercial Mg alloys. This study investigates the microstructural characteristics and bending properties of extruded sheets of Mg–(9, 11)Al–1Zn–0.3Mn–0.3Ca–0.2Y alloys (SEN9 and SEN11). These sheets contain particles containing Ca or Y, such as Al2Y, Al2Ca, Al8Mn4Y, and Al–Ca–Zn phases, along with dynamically precipitated Mg17Al12 particles. In the SEN9 sheet, Mg17Al12 particles are distributed in a band-like pattern along the extrusion direction, whereas in the SEN11 sheet, a more uniform distribution of abundant Mg17Al12 particles is observed. In both sheets, a macrocrack initiates on the bottom surface of a specimen and propagates into the specimen at an angle of approximately 35° during three-point bending tests. The SEN11 sheet exhibits a higher bending yield strength (by 27%) than the SEN9 sheet, primarily due to the stronger grain-boundary and particle hardening effects in the former, resulting from finer grains and a greater number of particles. In the SEN9 sheet, sharp deformation twins and subsequent cracks along these twins are formed during bending. Meanwhile, in the SEN11 sheet, the numerous Mg17Al12 particles induce stress concentration during bending, ultimately leading to premature bending failure before the formation of twins. As a result, the SEN11 sheet exhibits a 59% lower bending formability compared to the SEN9 sheet.

On the Competition between Intergranular and Transgranular Failure within 7xxx Al Alloys with Tailored Microstructures.

7xxx aluminium series reach exceptional strength compared to other industrial aluminium alloys. However, 7xxx aluminium series usually exhibit Precipitate-Free Zones (PFZs) along grain boundaries, which favour intergranular fracture and low ductility. In this study, the competition between intergranular and transgranular fracture is experimentally investigated in the 7075 Al alloy. This is of critical importance since it directly affects the formability and crashworthiness of thin Al sheets. Using Friction Stir Processing (FSP), microstructures with similar hardening precipitates and PFZs, but with very different grain structures and intermetallic (IM) particle size distribution, were generated and studied. Experimental results showed that the effect of microstructure on the failure mode was significantly different for tensile ductility compared to bending formability. While the tensile ductility was significantly improved for the microstructure with equiaxed grains and smaller IM particles (compared to elongated grains and larger particles), the opposite trend was observed in terms of formability.

Extraordinary Bending Formability of Mg–2.5Nd–0.5Zn–0.5Zr Alloy Sheet Through Pulsed Current

Extraordinary Bending Formability of Mg–2.5Nd–0.5Zn–0.5Zr Alloy Sheet Through Pulsed Current

The effects of laser-assisted heat treatment on warm formability of dual-phase materials

ABSTRACT In this study, the effects of local heat treatment using a low-power laser on the bending formability of DP800 sheet material were experimentally investigated. In the first step of the study, only the punch contact bending region of sheet metal was locally heated with a constant feed rate (1 mm s−1) and at different laser powers (50, 75, and 100% Watt). In the next step, the sheet material was transferred to the bending die for bending without any cooling process, and into the last stage, the sheet material was formed in different angled V-bending dies (30°, 60°, and 90°). It was observed that in heating up to 75-W power, the hardness decreased to 237 HV due to the dissolution of the martensite in the microstructure of DP material, and the springback value decreased by 39.285% due to the decrease in hardness.

Effects of pitch length of perforation on the crease bending characteristics of a polypropylene sheet subjected to indentation of a perforation blade

<abstract> <p>This study reveals the bending formability of a polypropylene (PP) sheet indented by a perforation blade when changing the pitch of the dashed-ruled line and the indentation depth. Creasing is a folding method of a carton sheet in which a score (called as a ruled line) is made at the bent portion. When making a creased line on a resin sheet, the scored sheet thickness decreases by applying half cutting or creasing (pressing) at the bent portion to make it easier to fold. To smoothly process a folding line on the resin sheet, a dashed line using a perforation blade is sometimes considered. The pitch length of the dashed line, and its nicked (uncut) length, affect the crease bending characteristics of the resin sheet scored by the perforation blade. However, only a limited number of studies have analyzed the dashed line bending moment response. In this study, to clarify the bending formability of a 0.5-mm-thick PP sheet indented by a developed perforation blade, first, the influence of the perforation pitch length on the crease bending characteristics of a scored PP sheet was investigated from a 0.5-mm fine pitch up to an 8-mm commercially sold pitch with a cutting-to-pitch length ratio of 50%. Second, the nicked zone depth against the cutting tip was set as 50% of the 0.5-mm thickness of the PP sheet. Furthermore, it was revealed that burrs (wedged bottom) in the cut part of the perforated (dashed) line affected the bending moment resistance in the folding process of the scored PP sheet, when changing the indentation depth of the perforation blade.</p> </abstract>

Variations in microstructure and bending formability of extruded Mg–Al–Zn–Ca–Y–MM alloy with precompression and subsequent annealing treatment conditions

In this study, the bending formability of an extruded Mg–9Al–1Zn–0.3Mn–0.9Ca–0.6Y–0.5 MM (AZXWMM91100, wt%) alloy at room temperature is significantly improved through application of a combined precompression and subsequent annealing (PCA) treatment. As the amount of precompression applied along the extrusion direction (ED) (i.e., the total strain) increases from 4% to 6%, the area fraction of ED-oriented grains of the PCA-treated alloy increases, which consequently causes an improvement in its bending formability because these grains accommodate larger tensile strain along the ED during bending. As the temperature of the subsequent annealing treatment increases from 350 °C to 450 °C, both the area fraction of the ED-oriented grains and the average grain size increase, and the residual dislocation density decreases owing to the promotion of boundary migration and occurrence of the recovery process at higher temperatures. Consequently, the bending formability of both the 4%-precompressed and the 6%-precompressed samples increases with an increase in the annealing temperature. However, when the precompressed samples are annealed at 300 °C, their bending formability is lower than that of the extruded alloy because the dislocations formed by precompression remain even after the subsequent annealing at this temperature.

Breaking bending limit of difficult-to-form titanium tubes by differential heating-based reconstruction of neutral layer shifting

Achieving small-radius bending of difficult-to-form tubular materials with large-diameter, thin-walled (LDTW, D/t > 20) geometry — as well as the properties of high-strength poor-hardening (HSPH) — is a continual target and challenging issue in the aerospace, marine, and healthcare industries. Based on the theoretical analysis of neutral layer shifting reconstruction (NLSR) by positively utilizing the temperature-dependent tension-compression (T-C) asymmetry of tubes, an innovative differential heating-based rotary draw bending (DH-RDB) method is designed, i.e., applying differential working temperatures at the intrados (Tin) and extrados (Tout) of tubes during forming to proactively coordinate the non-uniform deformation and thus prevent multiple forming defects to break the bending limit. To verify the above concept, the temperature-dependent evolution of the T-C asymmetry of titanium tubes was characterised — using commercial pure titanium (CP–Ti) tubes as case materials — and numerically implemented into the thermo-mechanical coupled three-dimensional finite element (3D-FE) model of the DH-RDB process. The influence of two heating schemes–i.e., overall heating and differential heating at the extrados and intrados of the tube — on bending formability was quantitatively assessed regarding multiple bending defects, such as wrinkling, over-thinning, and cross-section flattening. It was found that by introducing differential heating, the design and optimisation space for NLSR in tube bending is enlarged. The overall heating increases the work hardening of titanium tubes and thus restrains wrinkling during bending. Furthermore, the coupling effects of the increased compression-tension flow stress ratio (CTFSR) and decreased friction force under differential heating of Tin < Tout slightly aggravated the inward neutral layer shifting (NLS) but reduced wall-thinning and cross-section flattening. Following the above critical understanding, a series of carefully designed bending experiments were conducted to validate the capability of the DH-RDB to break the bending limit. This proves that the differential heating-based NLSR can be used to positively coordinate T-C non-uniform deformation to break the bending limit of difficult-to-form tubular materials.

Bending properties of extruded AZ91–0.9Ca–0.6Y alloy and their improvement through precompression and annealing

In this study, an extruded Mg–9Al–1Zn–0.3Mn–0.9Ca–0.6Y (AZXW9110) alloy is subjected to precompression and subsequent annealing (PCA) treatment for improving its bending formability, and the three-point bending properties of the as-extruded material and the precompressed and subsequently annealed (PCAed) material at room temperature are compared. During bending, microcracks formed in undissolved Al 2 Ca, Al 2 Y, and Al 8 Mn 4 Y particles propagate along the grain boundaries or twin boundaries and consequently form a macrocrack in the tension zones of the bending samples of both the materials. However, the bending formability of the PCAed material is 75% higher than that of the as-extruded material owing to the promoted twinning and slip behaviors in the former. The as-extruded material has a strong normal direction (ND) texture and a weak transverse direction (TD) texture, and the ND- and TD-oriented grains are unfavorable for both {10–12} twinning and basal slip in the tension zone of the bending sample of this material, which eventually results in its low bending formability. The PCAed material has a strong extrusion direction (ED) texture, weak TD texture, and spread ND texture. During bending, {10–12} twins are fully formed in the ED-oriented grains, and this {10–12} twinning effectively accommodates the plastic strain in the tension zone. In addition, the activation of basal slip in the ND-oriented grains of the PCAed material is promoted owing to the spread ND texture of this material. Consequently, this material exhibits substantially superior bending formability because of the vigorous {10–12} twinning in its ED-oriented grains and promoted basal slip in its ND-oriented grains.

Effects of post-heat treatment on microstructure, tensile properties, and bending properties of extruded AZ80 alloy

This study investigates the effects of post-heat treatments on microstructural characteristics, uniaxial tensile properties, and three-point bending properties of an extruded AZ80 alloy. The as-extruded material (no heat treatment) has a fine recrystallized grain structure containing Mg17Al12 precipitates formed along grain boundaries. During the 10-h 400 °C treatment in Ar–SF6 (treatment T4), precipitates fully dissolved into the matrix and grain coarsening occurred with a decrease in the dislocation density. Since the as-extruded material has a higher dislocation density, it exhibited accelerated precipitation behavior during aging; consequently, its peak aging time was shorter than that of the T4 material. The peak-aged, as-extruded material (15-h at 200 °C, treatment T5) and peak-aged T4 material (30-h at 200 °C, treatment T6), i.e., T5 and T6 materials, respectively, have higher tensile strengths and lower elongations than the as-extruded and T4 materials owing to the presence of more abundant Mg17Al12 precipitates in the former two materials. The tensile yield strengths of the materials are on the order of T5 > T6 > as-extruded > T4. This order is related to the differences in grain size and precipitate amount. The bending yield strengths of the as-extruded, T4, T5, and T6 materials are linearly proportional to their tensile yield strengths and their bending formability linearly increased with the tensile elongation. The relations between the tensile and bending properties of the materials are attributed to the occurrence of macrocracking in the outer region of the bending specimen, where the tensile stress along the longitudinal direction is dominant.

Improvement in the anisotropic mechanical properties and formability of Al–Si–Mg–Cu-based alloy sheets

The anisotropic mechanical properties and formability of Al–Si–Mg–Cu-based alloy sheets were investigated by varying their recrystallized grain sizes. By adding Cr and Mn and controlling the homogenization heat treatment, the recrystallized sheets with average grain sizes of 85, 61, 54, and 26 μm were prepared. As the average grain sizes of the recrystallized sheets decreased from 85 to 26 μm, the yield strength (YS) increased from 69.5 to 86.4 MPa due to grain boundary and particle strengthening. At the same time, the elongation was improved because the reduction in the stress concentration delayed the formation of the large-scale necking. The anisotropy in YS and elongation significantly decreased because a large number of fine grains with various orientations operated similar slip systems in all directions. Furthermore, grain refinement can increase and decrease the plastic strain ratio (r‾) and planar anisotropy (Δr), respectively, due to the formation of random textures. A bending formability map was proposed, which showed the acceptable and non-acceptable bending regions based on the grain size and flanging radius. This study provided valuable insight regarding the effects of refining recrystallized grains on anisotropic mechanical properties and formability.

Effects of Basal-texture Inclination on Bending Formability in Mg-Al-Zn-Ca Alloys

Flame-resistant magnesium alloy containing Ca is attracting attention in the transportation field because it is safe and lightweight. In this study, the effects of texture on bending formability were investigated for samples having different forms of texture that were cut from an extruded Mg-Al-Zn-Ca alloy plate at different angles ranging from 0° to 90° from the extrusion direction. Tensile and compressive deformation behaviors were also examined to discuss the shift of a neutral layer in bending. For bending, it was shown to be preferable that the basal texture not be inclined in low-strain region because of tension/compression asymmetry. However, in high-strain region, the influence of the shift of the neutral layer was weakened. In the experiment, the sample with a cutting angle of 45° exhibited the best bending formability. Furthermore, the result of a verification test using a sheet fabricated by a lateral extrusion, named friction-assisted extrusion (FAE), in which the basal texture was controlled to be inclined, showed that FAE improves the bending formability. This was confirmed by microstructural observation of the cross section near the tension surface at the same bending radius. The number of deformation twins that cause voids and cracks was reduced by controlling the basal texture to be inclined.

Texture tailoring and bendability improvement of rolled AZ31 alloy using {10–12} twinning: The effect of precompression levels

Abstract In this study, the texture of a rolled Mg alloy is effectively modified through the application of precompression and subsequent annealing treatment, leading to a remarkable improvement in the bending formability of the alloy at room temperature. Precompression induces lattice reorientation through {10–12} twinning, and annealing treatment reduces the stored strain energy of the precompressed material, which results in the formation of a stable grain structure with two dominant texture components. With an increase in precompression, the tensile strain in the outer region of the bending samples is accommodated to a greater extent due to more pronounced {10–12} twinning and basal slip. As a result, the bending formability of the material at room temperature improves with greater precompression. The variation in microstructure, texture, and bending behavior in relation to the degree of precompression is discussed in detail.

Effect of additional axial tension on formability of equal curvature-diameter bending without mandrel for 0Cr18Ni9 stainless steel tube

The equal curvature-diameter (ECD; R0/D0 = 1.0; where R0 is the bending radius, and D0 is the outer diameter of tube) bending process without mandrel was regarded as the tube’s minimal bending condition. Compared with pure bending, this represented a very complex physical process with multifactor effects. The quality and forming limit of tube bending are affected by additional axial tension (AAT); therefore, in this study, the ECD bending process without mandrel for tubes was performed by combining the finite element (FE) method and experimental research. The effects of AAT on the bending formability, such as wall thickness variation, cross-section distortion, and springback, were investigated. The results showed that: (1) under a certain friction coefficient between the tube and pressure die, AAT was constant, the maximum wall thickness variation rate, the maximum cross-section minor axis variation rate, and the springback angle all increased with the increase of the bending angle; (2) under a certain bending angle, maximum wall thinning rate, maximum cross-section short axis variation rate, and springback angle were positively associated with the friction coefficient, while the wall thickening rate was negatively associated with it. These data implied that the maximum wall thinning rate, the maximum cross-section short axis variation rate, and the springback angle all increased, while the maximum wall thickening rate decreased with the increase of AAT; (3) when the friction coefficient was too small, AAT was too small. Although the degree of the wall thinning, cross-section distortion, and springback were relatively small, the degree of wall thickening was large and even wrinkled. On the contrary, when the friction coefficient was too large, AAT was too large. Although the degree of maximum wall thickening was relatively small, the degree of maximum wall thinning, maximum cross-section short axis variation rate, and the springback angle were large, and the tube’s surface was extensively scratched. When the friction coefficient was too large, it would have a negative impact on the comprehensive bending formability of tube. In conclusion, our study indicates that in order to ensure a tube’s comprehensive bending formability, AAT should be properly controlled by reducing the friction coefficient between the tube and pressure die.

Improvement in bending formability of rolled magnesium alloy through precompression and subsequent annealing

A remarkable improvement in the bending formability of a rolled magnesium alloy at room temperature is achieved through application of precompression and subsequent annealing. This combined treatment results in the formation of a new grain structure with two texture components whose basal poles are oriented nearly along the rolling direction and normal direction. The tensile deformation occurring in the outer region of the samples during bending is effectively accommodated by the activated {10–12} twinning and promoted basal slip, which consequently results in a significant improvement in the bending formability of the alloy.

Effect of Forming Conditions on Draw Bending Formability in Hot Stamping Process

Effect of Forming Conditions on Draw Bending Formability in Hot Stamping Process

Formability of Thin-Walled Commercial Pure Titanium Tube upon Rotary Draw Bending

Under variations of bending radii and material properties, the multi-defect constrained formability of thin-walled commercial pure titanium (CP-Ti) tube was identified to explore the forming potential of this advanced lightweight material via explicit/implicit 3D-FE models and the experiments. The major results show that the variations of hardening exponents, thickness anisotropy exponent and Young's modulus, have great effects on bending formability; and compared with the wall thinning and cross-section deformation, the wrinkling instability is the dominant defect to restrain the bending formability of the large diameter thin-walled (LDTW) CP-Ti tube. The bending formability of CP-Ti tube, especially the anti-wrinkling capability, can be improved at elevated temperatures.

An insight into neutral layer shifting in tube bending

Abstract Neutral layer shifting (NLS) is the most fundamental phenomenon during tube bending, which characterizes tension-compression (T-C) non-uniform bending deformation and directly affects multi-defect constrained formability and bending limits. However, the essentials of NLS in tube bending have not yet been clarified, viz., why does the neutral layer (NL) generally shift inward, can the NL shift outward possibly and even can be positively reconstructed? To address these issues, via analyzing the equilibrium conditions of moment and force during tube bending, an axial force equilibrium (AFE)-based hybrid analytical-numerical framework for NLS calculation is established and numerically implemented, in which the tubular materials properties such as the anisotropy/asymmetry behaviors, and the geometry parameters such as bending radius, tube diameters and wall thickness can be comprehensively considered. Taking rotary draw bending of Ti-3Al-2.5 V tube and press bending of AZ31 and A6063 tubes as the cases, the above NLS model is fully evaluated and validated. Using the above platform, the NLS rules related to the geometrical parameters and intrinsic material parameters are quantitatively identified, and within the above framework combined with the experiments, a thorough insight into the NLS upon tube bending is thus presented, viz., given geometrical parameters of bending, under combined action of the evolution of material properties and multi-tool induced additional forces, the inevitable transient T-C non-uniform deformation can be positively coordinated to satisfy the equilibrium of force, then the NLS can be reconstructed to propose innovative processes for improving bending formability.

Mg AZ31B 판재의 기계적 특성과 성형성 분석

Abstract Magnesium alloys are currently expected to be widely used for weight reduction of cars and as high efficient materials in the automotive and electronics industries. Although the specific strength of magnesium is excellent, it cannot be easily formed at room temperature due to its HCP structure. However in order to improve the formability of magnesium, it is necessary to investigate its formability in the warm temperature range. In the current study, the aim was to add to the magnesium property database so that the mass production of a magnesium car body can be accomplished. Warm tensile tests were conducted and the forming limit diagram was determined to confirm formability characteristics of magnesium AZ31B alloy sheet. In addition the bending formability and the magnesium damping capacity were evaluated for AZ31B and compared to SPRC440E which is a sheet steel used for car bodies. Key Words : AZ31B Magnesium Alloy, Mg Mechanical Properties, Mg Sheet, Mg Forming, Mg Car Body, Warm Forming

Experimental Study on Three-Dimensional Flexible Stretch Bending Formability for Complex Cross-Sectional Profiles of TWB

Taking the upper strip of automobile doorframe which is complex profiles roll bending part of high-strength steel welded blanks as the research object, and then to Investigate its three-dimensional flexible stretch-bending formability under force control mode; the law of tensile force and clamp rotation angles’ influences on forming precision of the product is obtained. The results show that the springback of the line contour precision is largely affected by the comprehensive impact of tensile force and the clamp rotation angles, the values mentioned above reached minimum when the tensile force is 1.1 times the material yield strength, the springback will increase when greater or less than this value. Because of the surface torsion springback, internal and external surface contour accuracy mainly affected by the stage rotation angle when the profile bending curvature is large enough, and the effect is not obvious when the bending curvature is small.

Influence of Rare Earth on Hot-Dipped 55%Al-Zn Alloy Coating

Simulation experiment was done to investigate the effects of rare earth on hot-dipped Zn-55%Al alloy coating. The results show that the rare earth has little effect on the zinc dross and its burning loss is about 10%. The microstructure of coating is similar to that of solidification bath, and which is made up of phases of rich aluminum, rich zinc, rich silicon and rare earth, and intermetallic layer of Al-Zn-Fe-Si. The rare earth phase is needle or rod, and mainly distributed inside rich zinc phase and on the interface between the coating and steel substrate. The rare earth has no obvious influence on coating grain and spangle size. The appropriate addition of rare earth would be helpful to improve the coating bending formability and corrosion resistance.