Thermodynamics and Phase Diagrams Equilibrium Single-Component Systems Binary Solutions Equilibrium in Heterogeneous Systems Binary Phase Diagrams Influence of Interfaces on Equilibrium Ternary Equilibrium Additional Thermodynamic Relationships for Binary Solutions Computation of Phase Diagrams Kinetics of Phase Transformations Exercises References Further Readings Diffusion Atomic Mechanisms of Diffusion Interstitial Diffusion Substitutional Diffusion Atomic Mobility Tracer Diffusion in Binary Alloys Diffusion in Ternary Alloys High-Diffusivity Paths Diffusion in Multiphase Binary Systems Exercises References Further Readings Crystal Interfaces and Microstructure Interfacial Free Energy Solid=Vapor Interfaces Boundaries in Single-Phase Solids Interphase Interfaces in Solids Interface Migration Exercises References Further Readings Solidification Nucleation in Pure Metals Growth of a Pure Solid Alloy Solidification Solidification of Ingots and Castings Solidification of Fusion Welds Solidification during Quenching from the Melt Metallic Glasses Case Studies of Some Practical Castings and Welds Exercises References Further Readings Diffusional Transformations in Solids Homogeneous Nucleation in Solids Heterogeneous Nucleation Precipitate Growth5 Overall Transformation Kinetics-TTT Diagrams Precipitation in Age-Hardening Alloys Precipitation of Ferrite from Austenite Cellular Precipitation Eutectoid Transformations Massive Transformations Ordering Transformations Case Studies Exercises References Further Readings Diffusionless Transformations Characteristics of Diffusionless Transformations Martensite Crystallography Theories of Martensite Nucleation Martensite Growth1 Premartensite Phenomena Tempering of Ferrous Martensites Case Studies Exercises References Further Readings Solutions to Exercises Compiled by John C. Ion

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Phase transformations in metals and alloys

Wire Arc Additive Manufacturing (WAAM) is attracting significant attention in industry and academia due to its ability to capture the benefits of additive manufacturing for production of large components of medium geometric complexity. Uniquely, WAAM combines the use of wire and electric arc as a fusion source to build components in a layer-by-layer approach, both of which can offer significant cost savings compared to powder and alternative fusion sources, such as laser and electron beam, respectively. Meanwhile, a high deposition rate, key for producing such components, is provided, whilst also allowing significant material savings compared to conventional manufacturing processes. However, high quality production in a wide range of materials is limited by the elevated levels of heat input which causes a number of materials processing challenges in WAAM. The materials processing challenges are fully identified in this paper to include the development of high residual stresses, undesirable microstructures, and solute segregation and phase transformations at solidification. The thermal profile during the build poses another challenge leading to heterogeneous and anisotropic material properties. This paper outlines how the materials processing challenges may be addressed in WAAM by implementation of quality improving ancillary processes. The primary WAAM process selections and ancillary processes are classified by the authors and a comprehensive review of their application conducted. Strategies by which the ancillary processes can enhance the quality of WAAM parts are presented. The efficacy and suitability of these strategies for versatile and cost effective WAAM production are discussed and a future vision of WAAM process developments provided.

https://purehost.bath.ac.uk/ws/files/183236429/StrategiesAndProcessesForHighQualityWireArcAdditiveManufacturing_Manuscript.pdf

Invited review article: Strategies and processes for high quality wire arc additive manufacturing

For joining type 316LN austenitic stainless steel to modified 9Cr–1Mo steel for power plant application, a trimetallic configuration using an insert piece (such as alloy 800) of intermediate thermal coefficient of expansion (CTE) has been sometimes suggested for bridging the wide gap in CTE between the two steels. Two joints are thus involved and this paper is concerned with the weld between 316LN and alloy 800. These welds were produced using three types of filler materials: austenitic stainless steels corresponding to 316, 16Cr–8Ni–2Mo, and the nickel-base Inconel 182 1 . The weld fusion zones and the interfaces with the base materials were characterised in detail using light and transmission electron microscopy. The 316 and Inconel 182 weld metals solidified dendritically, while the 16–8–2 (16%Cr–8%Ni–2%Mo) weld metal showed a predominantly cellular substructure. The Inconel weld metal contained a large number of inclusions when deposited from flux-coated electrodes, but was relatively inclusion-free under inert gas-shielded welding. Long-term elevated-temperature aging of the weld metals resulted in embrittling sigma phase precipitation in the austenitic stainless steel weld metals, but the nickel-base welds showed no visible precipitation, demonstrating their superior metallurgical stability for high-temperature service.

Microstructural features of dissimilar welds between 316LN austenitic stainless steel and alloy 800

Effect of heat treatment on the microstructure and anisotropy in mechanical properties of A357 alloy produced by selective laser melting

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary ? grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar ?-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior ? grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over?s, V-transitions, etc. By reconstruction of the high temperature ? microstructure, it has been shown that all the AM platforms showed primary columnar ? grains with a ? || Nz fibre texture with decreased texture strength from the WAAM to the EBSM and SLM processes. Due to a lack of variant selection, the room temperature ?-phase showed a weaker transformation ?-texture compared to the primary ?-texture with decreased texture strength in line with the reduction in ?-texture strength.The large ? grains observed in the WAAM process were not significantly affected by changes in the GTAW (Gas Tungsten Arc Welding) process parameters, such as travel speed, peak to base current ratio, pulse frequency, etc. However, an increased wire feed rate significantly improved the grain size. Another important finding from this work was that by combining deformation and AM the grain size was reduced to a greater extent than could be achieved by varying the arc or, heat source parameters. It has been shown that the large columnar ?-grain structure usually seen in the WAAM process, with a size of 20 mm in length and 2 mm in width, was refined down to ~ 150 �m by the application of a modest deformation, between each layer deposited. The EBSM process showed consistent average static tensile properties in all build directions and met the minimum specification required by ISO 5832-3 (for the wrought and annealed Ti6Al4V). The WAAM samples produced using more effective shielding and the standard pulsed GTAW system also showed average static properties that met the minimum specification required by AMS 4985C for investment casting and hipped Ti6Al4V alloy. Overall, the fatigue life of the samples that were produced by AM was very good and showed a better fatigue performance than the MMPDS design data for castings. However, there was a large scatter in the fatigue life due to the effect of pores.

Microstructure, texture and mechanical property evolution during additive manufacturing of Ti6Al4V alloy for aerospace applications

To an increasing extent the wide range of fundamental knowledge of solidification processes is being applied to the study of fusion-weld solidification. Initially this fundamental knowledge is surveyed concisely and those areas of particular importance to weld-pool solidification are indentified. This is followed by an examination of phenomenological studies of the solidification behaviour of fusion welds in which particular attention is given to factors influencing the development of the fusion-zone structure. Then, the ways in which the metallurgical structure of the fusion zone influences the mechanical properties of the weldment are reviewed. Attention is then given to methods of controlling the fusion-zone structure by using inoculants, stimulated surface nucleation, dynamic grain refinement, and arc modulation. The gains and advantages which accrue from the way in which structure control affects properties are then considered. The review concludes with a discussion of likely future developme...

https://canteach.candu.org/content library/20053409.pdf

J. G. Garland

Papers

Solidification Structures and Properties of Fusion Welds