Daniel G. Sanders

Examination of Superplastic Forming Combined with Diffusion Bonding for Titanium: Perspective from Experience

Superplastic forming (SPF) combined with diffusion bonding (DB) has been used successfully for the fabrication of titanium aerospace hardware. Many of these applications have been for military aircraft, whereby a complex built-up structure has been replaced with monolithic parts. Several methods for applying the two- and four-sheet titanium SPF/DB processes have been devised, including the welding of sheets prior to forming and the use of silk-screened stop-off (yttria) to prevent bonding where it is undesirable. Very little progress has been made in the past few years toward understanding and modeling the SPF/DB process using constitutive equations and data by laboratory testing. Concerns that engineers face in designing for fatigue life, acceptable design loads, and damage tolerance are currently being studied, but the database is very limited. This is a summary of past work found in the literature and forms the foundation for additional research.

Superplastic Forming Manufacturing Technology Moves Towards the Twenty-First Century

Superplastic Forming (SPF) of titanium alloys for military aviation hardware became a viable manufacturing technology in United States (U.S.) the early 1970s as an outgrowth of the Rockwell B-1 Bomber for the U.S. Air Force and in the United Kingdom during the development of the Concorde supersonic transport. Many early metallurgical studies of the Superplastic phenomenon were made prior to these efforts. However, the Built up Low cost Advanced Titanium Structure (BLATS) program sponsored by the U.S. government generated renewed interest and launched an entire new field of study for both the academic and industrial communities. The BLATS SPF related efforts were targeted primarily at the discovery and development of new superplastic titanium and aluminum alloys for structural aerospace applications. A limited amount of manufacturing development was accomplished, but the program did result in a SPF process that was commercially successful, albeit somewhat archaic and inefficient. Military airframes, such as the Boeing F-15E and Euro-Fighter 2000 have reported tremendous design gains by using SPF structures. Complex designs which make use of superplastic formed 6Al-4V titanium have now found their way into the mainstream of commercial aviation. Superplastic formed parts are now flying on every model of aircraft that is currently produced by Boeing. In general, SPF industrial manufacturing technology has lagged behind the development of advanced SPF materials. This has led to the current situation, in which the factories that must produce SPF and SPF/DB components are struggling to overcome a host of challenges. As we move towards the twenty-first century, the focus of SPF technology innovation is shifting. Commercial SPF research and development activities are moving away from the traditional objectives of advancing new materials and structural design development. This paper has been written to identify the many new categories of research that will be explored in the coming years. These areas include the following: ○ Development of High Temperature Oxide resistant and creep resistant CRES alloys for use in cast/machined dies ○ Lead Time: Fast die change methods, setup reduction ○ Inexpensive SPF press design and components ○ Cast ceramic tooling (fused silica and other materials).

A Production System Using Ceramic Die Technology for Superplastic Forming

The recurring costs of producing sheet metal parts using the Superplastic Forming (SPF) and/or SPF combined with Diffusion Bonding process (SPF/DB) have diminished greatly over the past two decades. Today, the most significant roadblock for implementing SPF as a mainstream metal forming technique, for both aerospace parts and other manufactured goods, has become the non-recurring cost and long lead times that are required for the metal dies. This paper examines over a decade of work at Boeing to develop a robust ceramic die production system for the fabrication of aerospace components. Castable ceramic dies are now used extensively by Boeing to produce a broad range of structural airframe parts.

Superplastic Behavior and Microstructure of Titanium (Ti-6Al-4V) Friction Stir Welds Made under a Variety of Processing Conditions

Friction Stir Welding of Ti-6Al-4V was performed on 5 mm thickness plate in order to assess the affect of welding conditions on the resulting microstructure and superplastic forming behavior of the joints. A variety of welding conditions were tested and all welds were subsequently Superplastically formed. It was found that the weld parameters do influence the microstructure and degree of superplastic performance of the joints. Spindle speed was found to have the most dominant affect on the resulting microstructure and superplastic forming behavior. Low spindle speed welds lead to fine grained microstructures and highly superplastic welds, relative to the base material, while high spindle speed welds larger grained microstructures and less superplastic welds.

Reinforced ceramic dies for superplastic forming operations

Ceramic dies have been developed to meet the need for a dimensionally stable tool, which can withstand the temperatures (425 to 950 °C) and high forming pressures (up to 7 MPa) that are required for superplastic forming (SPF), superplastic forming with diffusion bonding (SPF/DB), and hot sizing of metal parts. With the improvements that have been made to strengthen fused silica based ceramics, the performance of ceramic tools is slowly closing in on meeting the same forming complexity as corrosion-resistant steel (CRES) dies can achieve. Boeing has successfully superplastically formed jet engine wide chord fan blades using ceramic dies, and many production aircraft parts are being built with Boeing’s patented ceramic die technology.

Optimization of the Friction Stir Welding Process for Superplastic Forming and Improved Surface Texture for Titanium Aerospace Structures

In friction stir welding (FSW), the semi-circular shaped FSW pin tool feed marks that are left behind varied in depth and shape which are detrimental not only in fatigue performance but also in further processing such as superplastic forming (SPF). Experimental investigation was conducted to determine the effects of changes to the FSW process parameters on the surface roughness of the weld of fine grain 2 mm thick titanium alloy, Ti- 6Al-4V. In addition to optimizing the surface texture of the welds, the superplastic performance of the weld nugget was made to be equal to the superplasticity of the parent metal by altering the spindle speed and feed rate used during FSW to identify the quality in terms of cold weld or hot weld. FSW process conditions of spindle speed of 500 RPM and a feed rate of 150 mm/min was found to produce a uniform deformation in both weld and parent metal when the joint was superplastc formed.

Friction Stir Welding Combined with Superplastic Forming for Monolithic Titanium Aircraft Structure: Influence of Post Welding Thermal Treatments on Weld Nugget Residual Stress

This paper includes a review of the manufacturing technologies related to the combination of friction stir welding (FSW) and superplastic forming (SPF) to fabricate large monolithic titanium structures for aircraft. The particular focus is on the post FSW residual stress measured on the weld nugget after thermal treatments and SPF forming.

Simulation of tensile behavior in friction STIR welded and superplastically formed titanium 6 AL-4V alloy

A hybrid numerical and experimental study was undertaken to evaluate the performance of as Friction Stir Welded (FSW) and Superplastically Formed Friction Stir Welded (SPF-FSW) Titanium joints. This paper presents the numerical models which were developed to simulate mechanical response of as FSW and SPF-FSW joints. The simulation results were then compared to experimentally determined behavior characteristics of the joints to assess the validity of the modeling approach. It was found that the numerical modeling approach presented here have simulated successfully the tensile behavior of a FSW joint agreeing with the experimental results. This method also adequately simulated the tensile behavior of a SPF-FSW joint, but due to geometrical influences, there are discrepancies between the numerical results and experimental observations.Copyright