Plastic flow behavior of twinning induced plasticity steel from low to warm temperatures

This work investigates the tensile behavior of twinning induced plasticity (TWIP) steel under room to warm temperatures. Stress-strain flow curves have been obtained from steady-state uniaxial tensile tests performed at different temperatures, that is, from 20 to 800 °C, and different engineering strain rates, 0.003 and 0.5 s−1. The yield and ultimate tensile strength, elongation at fracture, reduction of area, strain hardening exponent and strain hardening rate have been defined to describe the plastic behavior of TWIP steel. It has been found that the plastic flow behavior of TWIP steel is characterized by negative strain rate sensitivity and strain hardening, at temperatures from 20 to 300 °C, until the twinning mechanism occurs. At these temperatures, dynamic strain aging starts at the beginning of plastic deformation, with the appearance of fluctuations in the work hardening rate and local strain rate. Pronounced local serrations also appear at 300 °C and 0.003 s−1. Plastic deformation is mainly driven by dislocation gliding at 550 and 800 °C, and mechanical twins are absent. As a result, negative strain rate sensitivity and local serrations disappear in the stress-strain curves. Creep also contributes notably to plastic deformation at 800 °C, as was also observed on the fracture surfaces of tensile samples. Mechanical twins were only visible for the lower temperatures. The grains in the sample tested at 800 °C and 0.003 s-1 were fully recrystallized. Keywords: Twinning induced plasticity steel, Plastic flow behavior, Warm temperature, Mechanical strength, Strain hardening, Dynamic strain agin

On plastic notch effects in quenched and tempered steels

Presentazione orale al convegno TMS 2010 - 139th Annual Meeting & Exhibition, svoltosi a Seattle (USA) dal 14/02/2010 al 18/02/201

Use of electrode displacement signals for electrode degradation assessment in resistance spot welding

In resistance spot welding, the quality of welds is not only affected by the correct design of the welding cycle, but also by the electrode degradation that occurs over time. This work proposes a novel approach to indirectly monitor the electrode degradation during welding by analyzing the electrode displacement signal from a non-contact sensor embedded in the welding machine and the electrode tip shape obtained from carbon imprint tests. As a result of an experimental campaign involving more than 1200 weld spots, the electrode speed during the final hold stage has been determined as the most explanatory feature describing the electrode displacement. Based on the mechanical strength of spot welds, the electrode contact face area has been defined as the most representative feature characterizing electrode degradation. A regression analysis has been carried out to infer a relationship between the electrode speed and the contact area representative of tool wear. A Neural Network has been built to use some features extracted from the electrode displacement signals to predict the contact area and thus indirectly the electrode degradation. The model has shown a good accuracy, with a mean error of the contact area about 1.61 mm2, and with a standard deviation of about 3.73 mm2. The data-driven approach proposed allows through the evaluation of the electrode contact area to have better real-time knowledge and control of the welding process

Weldability and monitoring of resistance spot welding of Q&P and TRIP steels

This work aims at investigating the spot weldability of a new advanced Quenching and Partitioning (Q&P) steel and a Transformation Induced Plasticity (TRIP) steel for automotive applications by evaluating the effects of the main welding parameters on the mechanical performance of their dissimilar spot welds. The welding current, the electrode tip voltage and the electrical resistance of sheet stack were monitored in order to detect any metal expulsion and to evaluate its severity, as well as to clarify its effect on spot strength. The joint strength was assessed by means of shear and cross tension tests. The corresponding fracture modes were determined through optical microscopy. The welding current is the main process parameter that affects the weld strength, followed by the clamping force and welding time. Metal expulsion can occur through a single large expulsion or multiple expulsions, whose effects on the shear and cross tension strength have been assessed. Longer welding times can limit the negative effect of an expulsion if it occurs in the first part of the joining process. The spot welds exhibit different fracture modes according to their strengths. Overall, a proper weldability window for the selected process parameters has been determined to obtain sound joints

Fatigue crack growth in inhomogeneous steel components

Massive low-alloy high-strength steel components often exhibit microstructure variations from surface to core due to decreasing cooling rates when moving towards the interior. Since different steel constituents exhibit different fatigue crack growth (FCG) behaviors, both the overall and local FCG rates are expected to be influenced by the microstructural change that in turn affects the crack shape. The case of slack-quenched components with simple geometries, having a surface flaw, and subjected to constant-force-amplitude tensile fatigue, is first examined theoretically. The microstructural variations are hypothesized by considering low-alloy steel hardenabilities and medium quench severities; thereafter, the FCG is computed by considering (during each integration step) the stress intensity factor amplitude and the FCG behavior of different points of the crack front, the pointwise FCG properties being determined by the local steel constituents fractions. Simulation results are compared with experimental evidences from a recent failure in a 90 mm diameter low-alloy steel connection rod of a 2460 kW naval diesel generato

Investigation of Strength and Formability of 6016 Aluminum Tailor Welded Blanks

The automotive industry is constantly looking for innovative techniques to produce lighter, more efficient, and less polluting vehicles to comply with the increasingly restrictive environmental regulations. One of the latest technologies, which is still developing, is based on the fabrication of the body-in-white and car parts through the stamping of aluminum tailor welded blanks. Tailor welded blanks (TWBs) are generally a combination of two/three metal sheets with different thicknesses and/or mechanical strengths, which are commonly laser butt-welded. Even though the aluminum TWBs have the main advantage of producing lightweight parts, their use is still limited by the lower formability than their parent materials and by the fact that laser welding of aluminum sheets still remains a process easily subjected to weld defects (i.e., internal porosity) and, hence, requires strict control of process parameters. This study has investigated the effects of the main laser welding process parameters (laser power, welding speed, and focus position) on the mechanical properties and formability of aluminum TWBs made of the 6xxx series. The research results show that the welding conditions highly influence the weldability of such alloys. Heat input over 70 J/mm is responsible for excessive porosity and molten pool (and consequent root concavity), which are responsible for the lowest mechanical strength and formability of joints. Differently, low amounts of imperfections have a limited influence on the mechanical behaviors of the TWB joints. Overall, a narrow weldability window is required to ensure welded joints with proper strength and limited or no porosity

Investigation of Mechanical Properties of Aluminum Tailor Welded Blanks

Nowadays, the reduction of CO2 emissions and the decrease in energy consumption are the main aims of several industries, especially in the automotive sector. To comply with the increasingly restrictive regulations, the automotive industry is constantly looking for innovative techniques to produce lighter, more efficient, and less polluting vehicles. One of the latest technologies, and still developing, is based on the fabrication of the body-in-white and car parts through the stamping of aluminum Tailor Welded Blanks. Tailor Welded Blanks (TWBs) are generally the combination of two/three metal sheets with different thicknesses and/or mechanical strengths, which are commonly butt-welded together by laser sources. The use of aluminum TWBs has several advantages such as low density and corrosion resistance adequate. However, their use is still limited by the lower formability with respect to the parent materials and the more intrinsic difficulty of laser welding of aluminum sheets (i.e., internal porosity) that, although its use in automated industries is constantly growing, remains a process to be further developed and improved. This study has investigated the effect of the main laser welding process parameters (laser power, welding speed, and focal distance) on the mechanical properties of aluminum TWBs made of 6xxx series. The research results show that a narrow weldability window can be found to ensure welded joints with high strength and limited or no porosity

TIG welding of advanced high strength steel sheets

To reduce self-weight, car manufacturers and machine part producers tend to parts made of high strength steels and advanced high strength steels. The load bearing capacity of these components depends largely on the quality of the welds, therefore the joining of such high strength steels sheets by automatized tungsten inert gas (TIG) welding without filler metal was investigated. The selected steel grades were transformation induced plasticity (TRIP) steel (with 1000 MPa ultimate tensile strength) and twinning induced plasticity (TWIP) steel (with 800 MPa ultimate tensile strength). The weldability of both steel grades without filler metal and without pre heating and post weld heat treatment was investigated also in dissimilar joints. The visual and metallographic examinations, hardness measurements and tensile testing showed that the usability of this welding process to weld good TWIP-TWIP joints is good, to TRIP-TRIP joints is limited and to TRIP-TWIP joints is poor

Fracture behavior in Cu46.5Zr46.5Al7 and Cu46.5Zr41.5Al7Y5 bulk metallic glasses

The interplay between chemical composition, plastic behavior, and fracture modes of Cu46.5Zr46.5Al7 and Cu46.5Zr41.5Al7Y5 bulk metallic glasses (BMGs) was investigated by compression tests and fracture surfaces analyses. The aim was to explore the possibility of coupling physical, chemical, and hardness properties, with adequate macroscopic compressive plasticity. Cylindrical test samples, having a height-to-diameter ratio equal to 2, were machined and ground from as-cast bars and were tested in compression between lubricated plates, the displacement being measured by a clip-gage inserted between the plates. Y free BMG engineering stress-strain curves show a plastic behavior consisting of successive sudden stress drops and linear reloading segments. A detailed analysis of these features was performed to yield a correlation between the plastic deformation steps and the released elastic energy associated with each serratio

Effects of microalloying on an innovative quenched and tempered plastic mold steel

presentazione orale al TMS 2008 Annual Meeting & Exhibition (TMS2008), New Orleans, LA, USA, 9/3/2008 - 13/3/200