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Modeling and validation of a forming and welding procedure in alloy 718
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Swerea IVF AB. (Solid Mechanics)ORCID iD: 0000-0002-1432-444X
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The Finite ElementMethod (FE) has enabled important advances in the study and design ofcompetitive manufacturing procedures for metal structures. Predicting the final geometry of a component is a major issue to the manufacturing industry. This is a complex task, especially if the manufacturing involves several types of processes. In order to succeed, the complete manufacturing process chain has to be included in the simulation. This has been done for quite some time in the stamping industry involving sequences of trimming, forming and springback. However, more complex manufacturing procedures that include assembling of formed parts with forgings and castings by welding have been modeled with simplifications limiting prediction accuracy and thereby hindering compensation of accumulated shape distortions based on simulation results. One such example is the fabrication of aero engine structures in which the history from the forming procedure has not been considered in subsequent welding and heat treatment analyses. In this work, coupling of different manufacturing analyses is done using LS-DYNA including cold forming, trimming, result mapping, welding, cooling and springback. The results from the forming analysis are used as input to the welding simulation, performed using the novel material model *MAT_CWM. The anisotropic thermo-mechanical properties of alloy 718 are determined and included at temperatures up to 1000˚C. Comparison of model predictions with forming and welding tests show encouraging agreement. Our findings underscore the importance of including the history and accurate process conditions along the manufacturing chain to the prediction accuracy of shape distortions and the potential to the industry.

Keyword [en]
alloy 718, forming, welding, springback, nickel-based superalloys, superalloys
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-63348OAI: oai:DiVA.org:ltu-63348DiVA: diva2:1095444
Projects
NFFP6 - Virtuell processkedja för plåtformade flygmotorstrukturer i superlegeringar – Validering och demonstrator
Funder
VINNOVA, 2013-01173
Available from: 2017-05-14 Created: 2017-05-14 Last updated: 2017-05-31
In thesis
1. Modelling of Forming and Welding in Alloy 718
Open this publication in new window or tab >>Modelling of Forming and Welding in Alloy 718
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The reduction of fuel consumption and carbon dioxide emissions are currently a key factor for the aviation industry due to major concerns about climate change and more restrictive environmental laws. One way to reduce both fuel consumption and CO2 emissions is by significantly decreasing the vehicle’s weight while increasing engine's efficiency. In order to meet these requirements, the European aero engine industry is continuously focusing on alternative manufacturing methods for load carrying structures in advanced materials, such as titanium and nickel-based superalloys. Alternatively to traditional large-scale single castings, new manufacturing methods involve sheet metal parts, small castings and forgings assembled by welding. These new manufacturing methods allow more flexible designs in which each part is made of the most suitable material state, leading to several advantages such as reduction of product cost and weight while increasing engine's efficiency. Nickel-based superalloys are widely used in the aero engine industry, typically constituting up to 50% of the total weight of the aircraft engine. Due to their excellent material properties at high temperatures in severe corrosive environments, these superalloys are employed most extensively in the hot sections of gas turbine engines for both military and civil aircrafts with running temperatures up to 650°C.

In this thesis, a manufacturing process chain including forming and welding in the nickelbased superalloy 718 is studied. The main focus in the work lies on determining the thermomechanical properties, modelling and simulation of cold forming, study forming limits based on Nakazima tests for forming limit curves (FLC) and applying a damage and failure criterion. The work also comprises a brief study on hot forming. Finally, modelling of a subsequent welding procedure is included where residual stresses from the forming simulation are used to predict shape distortions due to the welding procedure. The results are compared with experimental observations.

The cold forming procedure of a double-curved component made of alloy 718 is studied using FE-analyses and forming tests. The same geometry was used to produce a hot forming tool. During forming tests at room temperature, micro cracks and open cracks were observed in the draw bead regions, not indicated when formability is assessed using a forming limit curve (FLC). Standard material models such as von Mises or Barlat Yld2000-2D were not capable of accurately predict the behaviour of the material after the point of diffuse necking, making the prediction of damage and failure during forming a challenge. The GISSMO damage model was therefore calibrated and used to predict material failure in forming of alloy 718. Tensile, plane strain, shear and biaxial tests at room temperature are performed up to fracture and continuously evaluated using Digital Image Correlation (DIC) by ARAMIS™. In this work, the GISSMO damage model is coupled with the anisotropic Barlat Yld2000-2D material model for forming simulations in alloy 718 at room temperature using LS-DYNA. Numerical predictions are able to accurately predict failure on the same regions as observed during the experimental forming tests. Comparisons of the distribution of damage on one of the draw beads between simulations and damage measurements by acoustic emission indicate that higher damage values correspond to bigger micro cracks. Numerical FE-predictions of the cold forming and subsequent welding procedure shows that the welding procedure further increases the shape distortions. This was found to agree with experimental observations.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keyword
Forming, welding, Alloy 718, damage, GISSMO, failure
National Category
Applied Mechanics Metallurgy and Metallic Materials
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62901 (URN)978-91-7583-870-0 (ISBN)978-91-7583-871-7 (ISBN)
Presentation
2017-06-16, E231, Luleå, 13:00 (English)
Supervisors
Projects
Virtuell processkedja för plåtformade flygmotorstrukturer i superlegeringar – Validering och demonstrator
Funder
VINNOVA, 2013-01173
Available from: 2017-04-06 Created: 2017-04-05 Last updated: 2017-05-31Bibliographically approved

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