Evaluation of Friction Stir Weld Process and Properties for Aerospace Application
Dr. Dwight Burford, Director, NIAR Advanced Joining Lab, WSU
Dr. Christian Widener, Research Scientist, NIAR Advanced Joining Lab Bryan Tweedy, Sr. Research Engineer, NIAR Advanced Joining Lab, WSU
Dr. Dale Cope, Director, NIAR Aging Aircraft Research,WSU
Friction Stir Welding (FSW), patented by The Welding Institute in 1991, is a solid-state joining process that is gaining acceptance as a viable manufacturing process in the aerospace industry. Based on the interest of local aviation industry, the National Institute for Aviation Research at Wichita State University established the Advanced Joining Laboratory for FSW research and development. Projects conducted in the lab are primarily for aerospace applications and specifically relate to the efficient and innovative uses of FSW as an aircraft manufacturing technique.
The WSU-NIAR research consists of the following focus areas: material properties testing, destructive and non-destructive inspection techniques, development of pin tools and process parameters, modeling of the FSW process and manufacturing, testing prototypes of complex aircraft components, and standards and specifications development.
Introduction of FSW into aerospace applications requires standard material and joint property data, as well as industry-accepted standards for process and performance control. Therefore, a key research objective is to develop data and standards for use in the design of FSW aerospace structures. Information on strength, fatigue life, fracture toughness, fatigue crack propagation, corrosion fatigue and environmentally assisted cracking will be developed.
Specific evaluations of the FSW process for aerospace applications will include:
- Determination of tensile strength properties of butt and lap joints
- Characterization of fracture properties
- Development of fatigue (S-N) curves
- Assessment of fatigue-crack growth rates in the joint and in the heat affected zone
- Determination of exfoliation and stress corrosion cracking characteristics and methods to enhance their resistance
- Fabrication and testing of flat and curved stiffened panels for comparison with a riveted structure
- Development of finite element models for predicting the strength of FSW structures
- Development of standards and specifications for performance and processing
The initial phase of the research program focused on butt and lap-welded sheets of 2024 and 7075 aluminum with thicknesses ranging from 0.040-in to 0.125-in. The research plan includes investigating additional alloys and thicker materials. Material property testing began with basic joint configurations at the coupon level. Built-up structures such as flat and curved stiffened panels are now included in the investigation. Evaluations of panel strength in tension, compression and shear, along with damage tolerance, are also being made and compared to similar riveted panels as shown in the following figure.
Above: Load displacement curves from the stiffened panel shear test program compare FSW panels with riveted panels. The FSW panels demonstrated a greater extent of displacement and ultimate achieved load. Below: Stiffened shear panels tested to failure. (a) The FSW shear panel failed less abruptly and with much less final distortion/destruction compared to the riveted shear panel (b) This evident lower abrupt energy release on failure is expected to prove significant in terms of the ability of a given structure to sustain damage (i.e. absorb energy prior to failure).
Detailed information developed in this research will lead to the efficient and appropriate use of FSW joining technology. These areas include standardizing the FSW process and performance requirements for qualifying a structure, generating FSW data for inclusion into handbooks and determining FSW joint properties of various configurations. This project will also provide comparisons with riveted panels in tension, compression, shear, damage tolerance and corrosion resistance.
The use of FSW has already begun in general aviation applications and large transport aircraft are currently developing applications. The information generated from this project will be used to develop new aircraft structures that are less complex and less expensive. This research will establish criteria that provide industry with reliable design standards that assure the FSW process is safe.
To learn more about the Advanced Joining Lab at Wichita State University's National Institute for Aviation Research visit the website.
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