Control of Laves phase in Inconel 718 GTA welds with current pulsing

Revisiting fundamental welding concepts to improve additive manufacturing: From theory to practice

Abstract: Additive manufacturing technologies based on melting and solidification have considerable similarities with fusion-based welding technologies, either by electric arc or high-power beams. However, several concepts are being introduced in additive manufacturing which have been extensively used in multipass arc welding with filler material. Therefore, clarification of fundamental definitions is important to establish a common background between welding and additive manufacturing research communities. This paper aims to review these concepts, highlighting the distinctive characteristics of fusion welding that can be embraced by additive manufacturing, namely the nature of rapid thermal cycles associated to small size and localized heat sources, the non-equilibrium nature of rapid solidification and its effects on: internal defects formation, phase transformations, residual stresses and distortions. Concerning process optimization, distinct criteria are proposed based on geometric, energetic and thermal considerations, allowing to determine an upper bound limit for the optimum hatch distance during additive manufacturing. Finally, a unified equation to compute the energy density is proposed. This equation enables to compare works performed with distinct equipment and experimental conditions, covering the major process parameters: power, travel speed, heat source dimension, hatch distance, deposited layer thickness and material grain size.

Numerical modeling of microstructure evolution during laser additive manufacturing of a nickel-based superalloy

Abstract: A multi-scale model that combines the finite element method and stochastic analysis is developed to simulate the evolution of the microstructure of an Nb-bearing nickel-based superalloy during laser additive manufacturing solidification. Through the use of this model, the nucleation and growth of dendrites, the segregation of niobium (Nb) and the formation of Laves phase particles during the solidification are investigated to provide the relationship between the solidification conditions and the resultant microstructure, especially in the morphology of Laves phase particles. The study shows that small equiaxed dendrite arm spacing under a high cooling rate and low temperature gradient to growth rate (G/R) ratio is beneficial for forming discrete Laves phase particles. In contrast, large columnar dendrite arm spacing under a low cooling rate and high G/R ratio tends to produce continuously distributed coarse Laves phase particles, which are known to be detrimental to mechanical properties. In addition, the improvement of hot cracking resistance by controlling the morphology of Laves phase particles is discussed by analyzing the cracking pattern and microstructure in the laser deposited material. This work provides valuable understanding of solidification microstructure development in Nb-bearing nickel-based superalloys, like IN 718, during laser additive manufacturing and constitutes a fundamental basis for controlling the microstructure to minimize the formation of deleterious Laves phase particles.

Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds

Abstract: Pulsed Nd-YAG laser welding characteristics of Inconel 718 sheet material (2 mm thick) were investigated. Welds were subjected to three different post-weld heat treatments: direct aging, 980 °C solution treatment + aging (980STA), and 1080 °C solution treatment + aging (1080STA). Weld microstructures and room temperature tensile properties were evaluated. Weld tensile properties in direct aged condition were found to be inferior in relation to the base metal due to the presence of Nb-rich brittle intermetallic Laves phase in fusion zone microstructure. Solution treatment at 980 °C was found to result in considerable dissolution of Laves phase, leading to some improvement in weld properties, though not to the level of base metal. The treatment was also found to result in precipitation of needle-like δ-phase around partially dissolved Laves particles. Solution treatment at 1080 °C was found to result in complete dissolution of Laves phase, but also in significant grain coarsening in the base metal, which led to considerable deterioration in base metal properties.

Characterization of heat affected zone liquation cracking in laser additive manufacturing of Inconel 718

Abstract: The heat affected zone liquation cracking behavior was studied in laser additive manufactured Inconel 718. Liquation cracking was found initiating from the weak site near the fusion line in the pre-deposited layer, propagating along the interdendritic region with the further deposition proceeding layer by layer. Total cracking length calculation results showed that when controlling the heat input and height increment constant, liquation cracking susceptibility increased with the increase of laser scanning speed; and when controlling the laser scanning speed and height increment constant, liquation cracking susceptibility increased with the increase of heat input. The effect of grain boundary misorientation on susceptibility to liquation cracking was also investigated through electron backscatter diffraction (EBSD) measurement, and the results showed that liquation cracking tendency increased with the increase of grain boundary angle, which was considered to be attributed to the higher stability of liquation film at larger grain boundary during the last stage of solidification.

Microstructural Evolution and Mechanical Properties of Inconel 625 Alloy during Pulsed Plasma Arc Deposition Process

Abstract: Pulsed plasma arc deposition (PPAD), which combines pulsed plasma cladding with rapid prototyping, is a promising technology for manufacturing near net shape components due to its superiority in cost and convenience of processing. In the present research, PPAD was successfully used to fabricate the Ni-based superalloy Inconel 625 components. The microstructures and mechanical properties of deposits were investigated by scanning electron microscopy (SEM), optical microscopy (OM), transmission electron microscopy (TEM) with energy dispersive spectrometer (EDS), microhardness and tensile testers. It was found that the as-deposited structure exhibited homogenous columnar dendrite structure, which grew epitaxially along the deposition direction. Moreover, some intermetallic phases such as Laves phase, minor MC (NbC, TiC) carbides and needle-like δ-Ni3Nb were observed in γ-Ni matrix. Precipitation mechanism and distribution characteristics of these intermetallic phases in the as-deposited 625 alloy sample were analyzed. In order to evaluate the mechanical properties of the deposits, microhardness was measured at various location (including transverse plane and longitudinal plane). The results revealed hardness was in the range of 260–285 HV0.2. In particular, microhardness at the interface region between two adjacent deposited layers was slightly higher than that at other regions due to highly refined structure and the disperse distribution of Laves particles. Finally, the influence of precipitation phases and fabrication strategies on the tensile properties of the as-deposited samples was investigated. The failure modes of the tensile specimens were analyzed with fractography.

Progress in joining of advanced materials

Abstract: Advanced materials generally require novel joining techniques. Developments in new materials research should be conducted hand in hand with work on weldability and joining capacity aspects. Sound joint quality for any new material has always been considered a milestone in a research and development scheme for a new material, particularly in terms of widespread applications. Better understanding of the microstructure-mechanical properties relationships of the bonded or welded joints will feed back to the materials development activities both in conventional and new materials areas. The two joining processes diffusion bonding and laser welding are considered in this literature review, since these processes are capable of joining a wide range of materials of interest in the aerospace industry, as well as in many other industrial applications, and offer remarkable advantages over conventional fusion welding processes. Of particular interest is the ability to join the more difficult aerospace alloys wi...

The formation and control of Laves phase in superalloy 718 welds

Abstract: Weld heat input/cooling rate (affected by welding process, parameters, technique, tooling, etc.) was found to influence the microstructural characteristics and segregational features in alloy 718 welds, with low heat inputs proving beneficial. Laves phase formed in the interdendritic regions of the weld metals as a result of segregation. The morphology and composition of Laves phase depended strongly on heat input/cooling rate and influenced its response to subsequent homogenization post-weld heat treatment. The various factors affecting the formation and control of Laves phase in alloy 718 welds are highlighted.

A review on particulate-reinforced titanium matrix composites

Abstract: The current status of particulate-reinforced titanium matrix composites is reviewed. The different types of reinforcements being used, together with the alternative processing routes, are described. The mechanical properties of these composites are influenced by a wide range of factors. Particulate reinforcements can affect properties by enhancing modulus and strength or by refining the grain size. Mechanical properties such as elastic modulus, low and high temperature strength, fracture toughness and compressive creep are discussed as functions of reinforcement size/shape and volume fraction. The review concludes by underlining the importance of further research in some critical areas to fully realise the industrial potential of these composites.

Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds

Abstract: Pulsed Nd-YAG laser welding characteristics of Inconel 718 sheet material (2 mm thick) were investigated. Welds were subjected to three different post-weld heat treatments: direct aging, 980 °C solution treatment + aging (980STA), and 1080 °C solution treatment + aging (1080STA). Weld microstructures and room temperature tensile properties were evaluated. Weld tensile properties in direct aged condition were found to be inferior in relation to the base metal due to the presence of Nb-rich brittle intermetallic Laves phase in fusion zone microstructure. Solution treatment at 980 °C was found to result in considerable dissolution of Laves phase, leading to some improvement in weld properties, though not to the level of base metal. The treatment was also found to result in precipitation of needle-like δ-phase around partially dissolved Laves particles. Solution treatment at 1080 °C was found to result in complete dissolution of Laves phase, but also in significant grain coarsening in the base metal, which led to considerable deterioration in base metal properties.

Microstructural refinement of weld fusion zones in α-β titanium alloys using pulsed current welding

Abstract: Pulsing of the welding current is one approach for refining the fusion zone grain structure in α – β titanium alloy welds. This paper reports work in which gas tungsten-arc welds were produced in two α – β titanium alloys under a variety of conditions including direct current (d.c.) pulsing and alternating current (a.c.) pulsing. The results show that, while d.c. pulsing did also refine the weld metal β grain structure, the effect of a.c. pulsing was much greater. Current pulsing enhances fluid flow, reduces temperature gradients and causes a continual change in the weld pool size and shape. These effects, which are believed to be responsible for refining the solidification structure, are much stronger in a.c. pulsing than in d.c. pulsing. The observed grain refinement was shown to result in an appreciable increase in fusion zone tensile ductility. Post-weld heat treatment improved ductility both in pulsed and unpulsed welds, but pulsed welds showed greater tensile elongation even in the heat treated condition.