Ana Sofia C. M. D’Oliveira

Impact of dilution on the microstructure and properties of Ni-based 625 alloy coatings

Nickel-based alloy IN 625 is used to protect components of aircrafts, power generation and oil refinery due to an association of toughness and high corrosion resistance. These properties are associated with the chemical composition and microstructure of coatings which depend on the processing parameters and the composition of the component being protected. This paper assessed impact of dilution on the microstructure and properties of the Ni alloy IN 625 deposited by Plasma Transferred Arc (PTA) on two substrates: carbon steel API 5L and stainless steel AISI 316L. Differences due to the interaction with the substrate were maximized analyzing single layer coatings, processed with three deposition current: 120, 150 and 180 A. Correlation with a cast Nickel-based alloy sample contributed to assess the impact of dilution on coatings. Dilution was determined by the area ratio and Vickers hardness measured on the transverse section of coatings. Scanning electron and Laser confocal microscopy and X-ray diffraction analysis were carried out to characterize the microstructure. Results indicated the increasing dilution with the deposition current was deeply influenced by the substrate. Dilution ranging from 5 to 29% was measured on coatings processed on the API 5L steel and from 22 to 51% on the low thermal conductivity AISI 316L steel substrate. Differences on the microstructure and properties of coatings can be associated with the interaction with each substrate. Higher fraction of carbides account for the higher coating hardness when processing on API 5L whereas the low thermal conductivity of AISI 316L and the higher Fe content in solid solution contributed to the lower hardness of coatings.

Analysis of PTA hardfacing with CoCrWC and CoCrMoSi alloys

CoCrWC alloys are widely used to protect components that operate under wear and high temperature environments. Enhanced performance has been achieved with the CoCrMoSi alloys but processing this alloy system is still a challenge due to the presence of the brittle Laves phase, particularly when welding is involved. This work evaluated Plasma Transferred Arc coatings processed with the Co-based alloy CoMoCrSi - Tribaloy T400, reinforced with Laves phase, comparing its weldability to the CoCrWC - Stellite 6, reinforced with carbides. Coatings were also analyzed regarding the response to temperature exposure at 600°C for 7 days and subsequent effect on microstructure and sliding abrasive wear. Coatings characterization was carried out by light and scanning electron microscopy, X-ray diffraction and Vickers hardness. CoCrWC coatings exhibited a Cobalt solid solution dendritic microstructure and a thin interdendritic region with eutectic carbides, while CoCrMoSi deposits exhibit a large lamellar eutectic region of Laves phase and Cobalt solid solution and a small fraction of primary Laves phase. Although phase stability was observed by X-ray diffraction, coarsening of the microstructure occurred for both alloys. CoCrMoSi showed thicker lamellar Laves phase and CoCrWC coarser eutectic carbides. Coatings stability assessed by wear tests revealed that although the wear rate of the as-deposited CoCrMoSi alloy was lower than that of CoCrWC alloy its increase after temperature exposure was more significant, 22% against 15%. Results were discussed regarding the protection of industrial components in particular, bearings in 55AlZn hot dip galvanizing components.

Deposição por plasma com arco transferido

Em virtude do Processo de Soldagem Plasma com Alimentacao de Po ter similaridades com o Processo de Soldagem Plasma com Alimentacao de Arame, foi realizado um estudo comparativo entre ambos os processos utilizando-se a liga a base de cobalto comercialmente conhecida como Stellite 6, como material de adicao na forma de po e arame. A pesquisa foi realizada com a expectativa de ser aplicada nas operacoes de revestimentos de superficies, em especial em pas de turbinas hidraulicas desgastadas por cavitacao. A selecao do material de adicao a ser empregado depende da natureza do mecanismo de desgaste encontrado. No Labsolda, a liga Stellite 6 vem sendo uma das mais utilizadas, por apresentar uma excelente resistencia ao desgaste erosivo por cavitacao. Foi avaliada a influencia da vazao de gas de plasma a partir dos valores de diluicao, dimensoes do cordao, dureza e microestrutura. O Processo de Soldagem Plasma com Alimentacao de Po foi o que produziu o melhor acabamento superficial, menor diluicao, melhor molhamento e maior largura. Com isto abre-se uma nova perspectiva para revestimentos metalicos e neste contexto se insere a recuperacao por soldagem de partes erodidas de turbinas hidraulicas.

Influence of Processing on the Microstructure and Properties of CoCrMoSi Alloy PTA Coatings

Wear performance as well as the low toughness of CoCrMoSi alloys is associated with the presence of Laves phase. In light of this, alloying elements have been altered in order to reduce the brittleness of newly-cast alloys. This study evaluated coatings by Plasma Transferred Arc (PTA) with different interactions with the AISI 316L substrate. The higher the dilution, it was hypothesized, the higher Iron, Chromium and Nickel contents proceeding from substrate and, therefore, the lower hard Laves phase fraction. Coatings were characterized by light and scanning electron microscopy, X-ray diffraction and Vickers hardness. Wear behavior was assessed by pin-on-disc and ball-on-flat tests. Laves phase and Cobalt solid solution eutectic lamellar microstructure was observed for coating processed with 120A (18% dilution). The chemical composition was displaced to hypoeutectic, showing Cobalt solid solution dendrites and interdendrictic eutectic lamellar for the coatings processed with higher current intensity (150 / 180A), due to the higher interaction with the substrate (26 / 38% dilution). Dilution increased with the deposition current, causing hardness to decrease from 702 – 526 HV0.5. Wear mass loss rate increased by up to 41.7% and friction coefficient (μ) ranged from 0.45 – 1.06 as the chemical composition changed.

Microstructural control of Co-based PTA coatings

Cobalt-based alloys are widely used as hardfacing materials when wear resistance is required at room temperature or high temperature applications. However, their performance is a consequence of their microstructures that depends on the processing conditions. This work focused on the influence of solidification rate on the structure development by processing the alloys with and without the interference of the substrate. The coatings were characterized by scanning electron microscopy, energy dispersive spectrometer, optical microscopy and instrument indentation tests. Results showed that despite the same phases developed in tested conditions, differences in the solidification microstructure and the influence of Fe diffusing from the substrate accounted for the measured variation in hardness. Higher hardness values were obtained for the samples processed free-standing (mini billets) with respect to the coatings and they were independent of the processing parameters, indicating that the substrate compromise the properties of hardness, as expected.

Processamento de Ligas de Níquel com Técnica de Manufatura Aditiva Utilizando Plasma por Arco Transferido

Additive manufacturing (AM) is a process used to build and repair complex shape components or whenever a property gradient is required. In this manufacturing procedure, multiple layers are deposited to fabricate a component. The success of the procedure is strongly dependent on the deposition technique, processing parameters selection and chemical composition of the material being deposited. Also pre-heating and the use of inert atmosphere impact on cracking, wettability and oxidation of the deposited layers. In this study, the potential of plasma transferred arc for additive manufacturing was assessed by the fabrication of “thin walls”. Two Ni superalloys were used, a solid solution hardening and a gamma prime precipitation hardening alloy. The analysis of processing parameters that allowed to process “thin wall”, included pre-heating at 300°C. Results showed that the chemical composition and the using of pre-heating impact on the layers geometry. In both alloys, a fine dendritic solidification structure with epitaxial growth between layers was identified. However, each alloy determined the hardness profile along the cross section. The precipitation hardened alloy is strongly influenced by the deposition thermal cycle of each layer. The solid solution hardened alloy is mainly influenced by dilution of the first layers with the substrate.

NiCrSiBC alloy: microstructure and hardness of coatings processed by arc and laser

Abstract The processing technique is decisive for the characteristics of a coating. This is because the heat supply, which depends on the technique and on the parameters, has an influence on the dilution and the solidification rate. In alloys with low metallurgical complexity, the effect of processing with deposition techniques that give a higher cooling rate may be translated into refining of the microstructure. A more refined microstructure is expected to result in higher mechanical strength of the coating. However, in the deposition of alloys that are more complex metallurgically this does not always occur, because the high cooling rate may suppress formation/precipitation of phases responsible for strength. The influence of processing on the microstructure and hardness of coatings of alloy Colmonoy-6® was assessed in this study. The alloy was processed by plasma transferred arc and high-power diode laser on plates of AISI 304 with two levels of dilution. In both cases, good-quality, defect-free coatings were obtained. Increase in Fe content (dilution) and different cooling rates result from processing with different parameters and techniques. This leads to significant changes in microstructure and hardness of the coatings, associated with the distribution, morphology and chemical composition of the carbides and particularly of the borides.

Additive manufacturing: the role of welding in this window of opportunity

Abstract Additive manufacturing (AM) can be considered as an evolution from rapid prototyping as it allows us to manufacture a component from a computer file (CAD 3D), though its applications extrapolate the production of prototypes. This technique involves the layered design of a component and subsequent welding deposition of the multilayer structure to produce parts without the need of moulds or other tools. Although AM is frequently associated with the use of high density processes, the need for higher competitiveness expanded its range of technologies to include arc welding processes. This article aims to summarize up-to-date information on AM, particularly involving arc welding processes. Emphasis is given on the challenges associated with the building up of components during multilayered deposition and on post-deposition procedures.

Assessment of the effect of different forms of Inconel 625 alloy feedstock in Plasma Transferred Arc (PTA) additive manufacturing

Additive manufacturing (AM) is an emerging fabrication technology, which can offer a faster, versatile, flexible, easily customizable, and a cheaper production path, when compared to conventional manufacturing. There are different additive manufacturing techniques differentiated by the way of material is made available and melted in each layer. The Plasma Transferred Arc (PTA) is a promising AM technique because of its superior energy efficiency compared to laser-related processes. PTA allows to process materials both in powder and wire forms. Recent evidences suggested that the characteristics of the feedstock will impact the final microstructure and consequently the mechanical properties of the deposit. This work carried out a detailed analysis on effects of the feedstock form using PTA to build multilayers with wire and powder IN625 alloy. It considered the specific features of each feedstock for the processing and explored the relationship with the characteristics of the deposited multilayers. Powder plus arc and wire plus arc multilayers were analyzed using confocal microscopy, scanning electron microscopy, X-ray diffraction, and microhardness profile. It is shown that the microstructure and mechanical properties of a multilayer component are influenced on choice of the feedstock, mainly because of the differing parameters that will be required in their processing.

Liga NiCrSiBC: Microestrutura e Dureza de Revestimentos Processados a Arco e a Laser

A tecnica de processamento e determinante sobre caracteristicas de um revestimento. Isso porque o aporte termico, que depende da tecnica e dos parâmetros, tem influencia sobre a diluicao e a taxa de solidificacao. Em ligas com baixa complexidade metalurgica, o impacto do processamento com tecnicas de deposicao que impoem maior taxa de resfriamento podem se traduzir em refino da microestrutura. Espera-se que quanto mais refinada a microestrutura maior a resistencia mecânica do revestimento. Entretanto, na deposicao de ligas de maior complexidade metalurgica isso nem sempre ocorre, porque a elevada taxa de resfriamento pode suprimir formacao/precipitacao de fases responsaveis pela resistencia. Neste estudo, a influencia do processamento sobre microestrutura e dureza de revestimentos da liga Colmonoy-6® foi avaliada. A liga foi processada por plasma com arco transferido (PTA) e laser de diodo de alta potencia (HPDL) sobre chapas de AISI 304 com dois niveis de diluicao. Em ambos os casos, revestimentos de boa qualidade e livres defeitos foram obtidos. O aumento do teor de Fe (diluicao) e as diferentes taxas de resfriamento decorrem do processamento com diferentes parâmetros e tecnicas. Em consequencia, ocorrem mudancas significativas na microestrutura e na dureza dos revestimentos que estao associadas a distribuicao, morfologia e composicao quimica dos carbonetos e, principalmente, dos boretos.