Detailed diffraction and electron microscopy study of inertia-friction-welded dissimilar high-strength steels

A detailed characterization of two dissimilar high-strength steels, SCMV and Aermet 100, joined by inertia friction welding (IFW)—a solid-state welding technique—was undertaken using high energy synchrotron X-ray diffraction and advanced electron microscopy in order to understand the dramatic hardness variation across such a weld. It was found that the severe high-temperature deformation in the thermomechanically affected zones (TMAZs) of the weld, stabilized ordered, and nanosized FeCo zones in Aermet 100 and about 12 to 14 vol pct austenite in SCMV (Ni equivalent 9 wt pct). The ordered FeCo zones in Aermet 100 resulted in exceptionally high hardness values of 700 to 725 HV. Very close to the weld line, the TMAZ of Aermet 100 also displayed a region with about 15 vol pct austenite, while in the parent material, 8 to 9 vol pct was typically observed. No indication of martensite was found in the weld region of Aermet 100. Ferrite texture analysis at different locations within the TMAZs on either side of the weld showed that SCMV develops a very strong a-fiber texture near the weld line and, in addition, a c-fiber texture toward the heat-affected zone (HAZ), suggesting the presence of ferrite during welding near the weld line and recrystallization further away. The ferrite texture development in the TMAZ of Aermet 100 was relatively weak, suggesting that austenite is a dominant phase in the TMAZ during IFW

3‐D observations of short fatigue crack interaction with la2mellar and duplex microstructures in a two‐phase titanium alloy

International audienceno abstrac

3-D observations of short fatigue crack interaction with lamellar and duplex microstructures in a two-phase titanium alloy

cited By 25International audienceIn situ observations of short crack growth in powder-processed Ti-6246 have been undertaken using synchrotron X-ray microtomography to investigate crack tip interaction with microstructure. Together with post-mortem analysis using electron backscatter diffraction (EBSD), it was possible to identify a number of microstructural features that affect crack propagation rates by causing crack bifurcation, crack bridging and crack deflection. Three samples with different microstructures were tested in this way: lamellar, duplex and a heterogeneous microstructure that showed regions of lamellar and duplex microstructure. The in situ fatigue experiments were carried out with a maximum stress of 410 MPa and R = 0.1. The three microstructures showed significantly different short crack propagation rates, with the lamellar microstructure displaying the fastest and the duplex microstructure the slowest rate. It was found that the lamellar microstructure develops a deeper crack than the duplex microstructure that is related to significant crack bifurcation taking place near the surface region in the lamellar but not duplex microstructure. It was also found that a columnar lamellar microstructure creates a relatively smooth crack front while a basket-weave-type microstructure forces the crack tip to undulate on the lath width scale. Crack bridging of the fine lamellar region of the duplex microstructure was observed, which seems to hinder significant crack bifurcation to occur, but still provides improved crack growth resistance that explains the low crack propagation rate. In the third microstructure the crack tended to grow slightly asymmetrically due to the heterogeneous nature of the microstructure, resulting in the intermediate growth rate. EBSD grain orientation and Schmid factor analysis of regions including the crack revealed that the crack path is strongly influenced by the crystallographic orientation of the α lamellae and grains. While in the lamellar microstructure the crack tends to grow across lamellae favourably orientated for basal slip, in the duplex microstructure the crack path follows primary α grains favourably orientated for prismatic slip. It is suggested that the change to basal orientation in the lamellar microstructure is related to the requirement of α/β slip transfer in the lamellar microstructure and the Burgers relationship favouring basal slip in such a case. In addition, the crack growing in primary α tends to get diverted within the α grain and is not seen in the lamellar microstructure. This crack deflection in primary α grains might further slow down crack propagation and could be related to the availability of different (11-20) planes while basal planes only display one orientation in a hexagonal close-packed crystal. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Explaining microstructural and physical variations in rapid additive manufactured waspaloy parts through the laser-deposition thermal cycle

Laser direct metal deposition has the potential to increase the material efficiency during the manufacture of superalloy aerospace components and also to reduce the need for welded joints to produce the necessary geometries. However, a possible problem is inconsistent microstructure, and therefore physical properties, in the deposited part. In this work, the factors that affect microstructure formation and evolution were examined in simple Waspaloy parts deposited using a high power diode laser and coaxial powder nozzle across a range of process parameters. Thermal imaging and embedded thermocouples were used to record the thermal cycle during deposition at different positions within the part and this is correlated with the wall characteristics and the final microstructures formed. Results allow the extent of the variations in sample properties with process parameters and position to be determined and indicate that intra melt pool factors such as local fluctuations in temperature gradients due to Marangoni flow and changes in nucleation density due to injected powder are significant