C. Maldonado

Modeling of the hot deformation behavior of boron microalloyed steels under uniaxial hot-compression conditions

Abstract The present study shows that the hot deformation behavior of boron microalloyed steels can be quantitatively described by constitutive equations. These equations take into account both dynamic recovery and recrystallization phenomena. They have been fitted using experimental data taken from hot compression tests of four boron microalloyed steels in order to determine their characteristic parameters. The tests were carried out over a wide range of temperatures (950, 1000, 1050 and 1100°C) and strain rates (10−3, 10−2 and 10−1 s−1). The analysis of the characteristic parameters of the constitutive equations describing the hot flow behavior of these steels shows that boron additions play a major role in softening mechanisms rather than on hardening. A quantification of the boron effect is also presented. The experimental data were compared with the predictions of the proposed model and an excellent agreement between measured and predicted values for all boron micro-alloyed steels over a wide range of temperatures and strain rates was obtained.

Softened zone formation and joint strength properties in dissimilar friction welds

The mechanical properties of dissimilar MMC/AISI 304 stainless steel friction welds with and without silver interlayers were examined. The notch tensile strengths of MMC/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds increased when high friction pressures were applied during the joining operation. The higher notch tensile strengths of dissimilar MMC/AISI and MMC/Ag/AISI 304 stainless steel friction welds resulted from the formation of narrow softened zones in MMC material immediately adjacent to the bondline. The influence of softened zone width and hardness (yield strength) on the notch tensile strengths of dissimilar welds was analysed using finite element modelling (FEM). FEM in combination with the assumption of a ductile failure criterion was used to calculate the notch tensile strengths of dissimilar joints. The key assumption in this work is that dissimilar weld failure wholly depended on the characteristics (mechanical properties and dimensions) of the softened zone formed in MMC material immediately adjacent to the bondline. The modelling results produced based on this assumption closely correspond with the actual notch tensile strengths of dissimilar MMC/Ag/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds.

Nanoparticle and Intermetallic Formation in Dissimilar Friction Welds Produced with Silver Interlayers

The introduction of a silver interlayer during MMC/AISI 304 stainless steel friction welding promoted the formation of intermetallic compounds and silver nanoparticles. It is suggested that nanocrystal formation and the formation of Ag3Al intermetallics regions resulted from mechanical mixing during the friction welding operation. The nanoparticles were analyzed by a Philips FEG Tecnai F20. Experimental Procedure The chemical compositions of the MMC and AISI 304 stainless steel base materials are indicated in Table 1. All dissimilar friction welds were made using 19-mm bars 0f diameter of AISI 304 stainless steel and Al 6061/Al2O3 (W6.A.l0A-T6) base material containing 10 vol.% Al2O3 particles. The contacting surfaces of the steel and MMC substrates were machined perpendicular to the axes of the as-received bars. The stainless steel test bars were polished using 1µm diamond particles and coated with 5 µm-thick nickel strike layers.