Advanced High-strength Materials Research Articles

Effect of laser welding on microstructure and mechanical behaviour of dual phase 600 steel sheets

Dual Phase steels are one of the most used advanced high-strength materials in the industry, due to its combination of a ductile ferritic matrix and disperse hard martensite islands, which provide outstanding mechanical properties for components to be cold stamped. This work investigated fiber laser welding applicability in Dual Phase 600 1.6 mm thick steel sheets, evaluating potential welding impacts on properties of the material for industrial applications. A first set of bead-on-plate welds was generated to define best parameters for subsequent tests. A second set was prepared, consisting of butt joints welded in the optimized condition. Weld microstructure was characterized as 100% martensitic at fusion zone (FZ), with growing fractions of ferrite at Heat Affected Zone (HAZ) as one moves away from fusion line. Hardness is around 60% higher at FZ than at BM, being maximum at supercritical HAZ due to its highly refined microstructure and HAZ softening was not observed. Tensile and Erichsen cupping tests presented similar strength results between welded and non-welded specimens, with slight ductility reduction. Finally, numeric simulations based on Finite Element Analysis were carried out to estimate temperature evolution, phase proportions, residual stresses and distortion levels, achieving excellent agreement with experimental results.

Effect of Laser Welding on Microstructure and Mechanical Behaviour of Dual Phase 600 Steel Sheets

Dual Phase steels are one of the most used advanced high-strength materials in the industry, due to its combination of a ductile ferritic matrix and disperse hard martensite islands, which provide outstanding mechanical properties for components to be cold stamped. This work investigated fiber laser welding applicability in Dual Phase 600 1.6 mm thick steel sheets, evaluating potential welding impacts on properties of the material for industrial applications. A first set of bead-on-plate welds was generated to define best parameters for subsequent tests. A second set was prepared, consisting of butt joints welded in the optimized condition. Weld microstructure was characterized as 100% martensitic at fusion zone (FZ), with growing fractions of ferrite at Heat Affected Zone (HAZ) as one moves away from fusion line. Hardness is around 60% higher at FZ than at BM, being maximum at supercritical HAZ due to its highly refined microstructure and HAZ softening was not observed. Tensile and Erichsen cupping tests presented similar strength results between welded and non-welded specimens, with slight ductility reduction. Finally, numeric simulations based on Finite Element Analysis were carried out to estimate temperature evolution, phase proportions, residual stresses and distortion levels, achieving excellent agreement with experimental results.

Failure surfaces of high-strength materials predicted by a universal failure criterion

Advanced high-strength materials including metallic glasses (MGs) and nanocrystalline (NC) metals have attracted great attentions due to their promising mechanical properties. However, since they usually exhibit relatively brittle fracture behavior, a proper failure criterion is urgently required for the safety design of structural components and devices using these high-strength materials. It has been found that the conventional failure criteria may not soundly explain the fracture behaviors of MGs, while the ellipse criterion shows promising universality for well describing the fracture behaviors of several kinds of high-strength materials under various loading conditions. In this study, the principal stress form of the ellipse criterion was derived for the first time, and the two-dimensional failure surfaces according to this criterion were plotted and compared with the other predictions, as well as the experimental and simulation results of MGs and NC metals. The results confirm the applicability of the ellipse criterion for predicting the failure of high-strength materials, and may also improve the understanding of strength and fracture of various materials.

ACEROS AVANZADOS: COMPARACIÓN DEL COMPORTAMIENTO MECÁNICO EN EL ESTAMPADO DE UN TRAVESAÑO DE AUTOMOVIL

One of the fundamental objectives of the automotive industry today is the fuel economy; therefore, reducing vehicle weight has been pushed to the limit especially in the new body designs, without neglecting the passengers passive safety that these structures should provide. Thus, the achievement of these goals is only possible through the implementation of advanced high strength materials along with new design techniques and their associated manufacturing processes. In this investigation, a study of the implementation of three steels, categorized by the AISI, as advanced high strength steels (AHSS) in the manufacture of a cross bar is done. The results obtained are compared each other and with similarly obtained results considering a conventional high-strength steel (CHSS) possessing superior forming properties, but lower resistance properties regarding AHSS considered in this study . To this goald the foundation of the simulation process all necessary operations are presented to perform the sheet forming of a cross car for commercial vehicle floor and a comparative performance takes place between the four selected steels. A safety factor regarding the FLD curve is proposed. The use of appropriate material models of deformation processes involved in forming sheet and the correct definition of the mechanical properties associated with them are essential.

A universal fracture criterion for high-strength materials

Recently developed advanced high-strength materials like metallic glasses, nanocrystalline metallic materials, and advanced ceramics usually fracture in a catastrophic brittle manner, which makes it quite essential to find a reasonable fracture criterion to predict their brittle failure behaviors. Based on the analysis of substantial experimental observations of fracture behaviors of metallic glasses and other high-strength materials, here we developed a new fracture criterion and proved it effective in predicting the critical fracture conditions under complex stress states. The new criterion is not only a unified one which unifies the three classical failure criteria, i.e., the maximum normal stress criterion, the Tresca criterion and the Mohr-Coulomb criterion, but also a universal criterion which has the ability to describe the fracture mechanisms of a variety of different high-strength materials under various external loading conditions.

Polyol-assisted functionalization of carbon nanotubes—a perspective

Since the 1991 rediscovery of carbon nanotubes (CNTs), methods to decorate these materials with well-dispersed, functional nanoparticles have been greatly pursued. Nanoparticle-functionalized carbon nanotubes (CNTs) have potential applications within advanced high-strength materials, sensors, and catalysis. However, CNTs generally have to be surface-activated, using complex and time-consuming treatments, before they can be decorated with nanoparticles via traditional methods like wet impregnation, which in itself has additional drawbacks (mainly the energy penalty from the drying step). Polyol-mediated functionalization is a viable, economical alternative, as it is a rapid and simple technique that uses biocompatible solvents that are available in large industrial quantities. Additionally, although they are sometimes implemented, strong surface activation treatments are not necessarily required to get even distributions of functional nanoparticle on CNTs. Further, polyols are good dispersion media and have the ability to dissolve a variety of metal salt precursors, a definite advantage that allows the method to be used in the synthesis of various target functional metal nanoparticles. Current research in this field is mainly focused on functionalizing CNTs with metallic and metal-oxide nanoparticles. A perspective of the current literature shows varying deposition results and a lack of continuity across reports, though process metrics like temperature and pH clearly impact supported-nanoparticle morphology. This review describes some of the advances in this field, as well as the significant drawbacks that currently restrict the widespread implementation of polyol-mediated CNT functionalization.

The Use of Explosive Energy for Joining Advanced High Strength Low Alloy Steels

This article deals with an alternative method of joining advanced steels for frame structures. These steels cannot be joined by a conventional process due to the impact of temperature on the base material. Therefore, a simple and cost-effective method of forming a high-strength joint, intended for advanced high-strength materials, was designed using explosive forming. One of its key advantages is that it preserves the microstructure of the high-strength material being joined. At the same time, the design of the joint allows it to undergo further plastic deformation if the yield stress is exceeded, thus preventing the step change in load-carrying capacity and the instability of the structure. The alternative joint was intended for materials with yield stress above 1000 MPa and elongation of 10%, under quasi-static conditions. However, the design is also suitable for materials with ultimate tensile strength higher than 2000 MPa. Testing of the load-carrying capacity of the joint in a mechanical testing shop showed that the larger the flow stress of the material, the higher the load-carrying capacity of the joint. The selected joint designs with good load-bearing capacity values were manufactured by forming using products of detonation of the SEMTEX industrial blasting explosive. In a compression test, the demonstration joints showed the axial load-bearing capacity of 200 kN with up to 20-mm displacement to failure.

Laser-assisted Milling of Advanced Materials

Advanced high-strength materials offer a huge application potential within highly stressed components in various industrial areas. But their machinability is still limited when applying established and conventionally available technologies. Aiming at the reduction of process forces, increased material removal rates and longer tool service life without application of cooling lubricants the Fraunhofer IPT has developed a novel process concept for laser-assisted milling with local laser-induced material plastification before cutting. The following paper comprises the novel approach as well as first machining results on nickel-base alloy Inconel 718.

Laser-assisted milling of advanced materials

Advanced high-strength materials offer a huge application potential within highly stressed components in various industrial areas. But their machinability is still limited when applying established and conventionally available technologies. Aiming at the reduction of process forces, increased material removal rates and longer tool service life without application of cooling lubricants the Fraunhofer IPT has developed a novel process concept for laserassisted milling with local laser-induced material plastification before cutting. The following paper comprises the novel process approach, fundamental process investigations, the design of a spindle-tool system with integrated beam guidance, laser control and first investigations with the new system.

Mechanical Properties of Ultrafine Grained Steels Processed by Large Strain-High Z Deformation - A Review

Ultrafine-grained steels with a grain size of about one micron offer the prospect of high strength coupled with high toughness among conventional steel compositions and are attracting the attention of researchers worldwide. Application of these ultrafine-grained steels to potential engineering structures demands extensive study of their mechanical properties. While there are many studies on the development of ultrafine-grained microstructures through various deformation processing techniques on a spectrum of compositions, fewer studies were reported on the more important aspect of evaluating their mechanical properties. This is to verify the basic assumption that the microstructural refinement at bulk level indeed improves the mechanical properties offering the prospect of a realistic replacement of the existing conventional steels in the near future. As we move towards the ultimate goal of applying these advanced high strength materials, this review article attempts to present a comprehensive picture on the mechanical properties of ultrafine-grained steels with varying carbon contents fabricated by large strain warm deformation. Finally, it is believed that time is ripe for exploring the possible applications of these materials for structural applications.

Deformation and fracture of Ti-base nanostructured composite

Abstract The in-situ formation of nano-eutectic/primary dendrite bulk composites in Ti-base multicomponent alloy systems allows the design of advanced high strength materials, where a nanostructured matrix is combined with ductile β-Ti solid solution dendrites as a toughening phase. This microstructure can be achieved over a wide range of cooling rates. The multicomponent recipe stabilizes the β-Ti phase and helps to reduce the eutectic spacing to the nanometer scale. The superiority in the mechanical properties, i. e. high room temperature ductility (up to 30 %) as well as high strength (up to 2600 MPa), stems from the possibility to tailor the alloy composition leading to the formation of different volume fractions of dendrites in a nanostructured eutectic matrix. All composites with high volume fraction of dendrites offer a high ductility (∊p = 8 – 30 %) and a final failure angle in the range of 46 – 51°. The role of the volume fraction of the dendrites in the composite in enhancing the ductility as well as the fracture angle is critically assessed. The deformation and fracture mechanisms are linked to the macroscopic fracture features as well as to impingement of the shear bands leading to rotation of the shear plane and lattice distortion at the atomic level.

Strengthening of multicomponent glass-forming alloys by microstructure design

The combination of high strength with high elastic strain renders bulk metallic glasses (BMGs) or nanostructured Zr-base multicomponent alloys attractive candidates as advanced high-strength materials for structural applications. To circumvent the limited ductility of these glassy alloys, we prepared a heterogeneous microstructure combining glassy or nanostructured matrix along with a dendritic second-phase particles. The alloys were designed by adding different amounts of Nb to the Zr-based multicomponent glass-forming alloy system. The refractory metal Nb promotes the formation of a primary bcc β-Zr phase having dendritic morphology while the residual melt solidifies to a nanostructured/amorphous matrix. A correlation between the alloy composition and adopted casting method with evolved microstructures and mechanical properties is revealed. These composites exhibit a unique combination of high fracture strength up to 1922MPa as well as plastic strain over 15.8% under uni-axial compression testing at room temperature. We emphasize the possibilities to tailor such composite microstructures in favor of either strength or ductility, or a combination of both, and also discuss the acquired ability to synthesize such in situ composite microstructures in bulk form through inexpensive processing routes.

Ti-base bulk nanostructure-dendrite composites: Microstructure and deformation

Bulk nanostructured/glassy Ti-base multicomponent alloys are attractive advanced high-strength materials for structural applications or as functional materials, e.g. for medical applications. Typically, the strength and elastic strain limits of such alloys are significantly higher than for conventional microcrystalline alloys. It is of strong interest to develop a one-step process that can directly produce such alloys in bulk form, e.g. through casting, with an in situ formed composite microstructure that combines phases with different desirable properties. Along this line, a series of (Ti–Zr–Ta)–Cu–Ni alloys designed from glass-forming alloy compositions with different volume fractions of β-Ti phase have been investigated. The introduction of the β-Ti dendritic solid solution phase enhances the ductility of the material. A further improvement can be achieved by adding isomorphous β-phase stabilizers in (Ti–Nb–Ta)–Si/In alloys. These alloys exhibit low elastic modulus (50 GPa) and very large deformability (>100% plastic strain) at room temperature without appearance of any microcracks. The microstructure of these alloys consists of a high volume fraction of bcc β-Ti phase coexisting with a nanostructured interdendritic matrix. The superplastic-like deformability of the near beta alloys is correlated with copious twinning in the micrometer-size β-Ti-dendrites leading to a nano/ultrafine filamentary structure.