Social

A review of the literature on welding aluminum alloys to steels suggests that resultant performance is heavily influenced by the formation of intermetallic phases.  Joining of aluminum to steel can result in a number of these intermetallic phases, including FeAl, Fe3Al, and Fe2Al5.   Previous observations have suggested that these phases form along the bond line of welds through interdiffusion.  As these intermetallic phases grow, their inherent low ductility compromises the integrity of the joint.  Historical work going back 20 years, confirmed by more recent studies at EWI, has suggested that when friction welding 6XXX alloys to steels, heating times in excess of 200-ms can result in extensive intermetallic formation.  Alternately, shorter (than 200-ms) cycle times appear to minimize that intermetallic formation.  EWI has recently conducted a study examining the kinetics of intermetallic formation in inertia friction welds.  In this program, welds were made at an intermediate burn-time and sectioned to assess the morphology of the resulting intermetallics.  The macro-section of the weld under consideration is shown in Figure 1, and the associated temperature profile is shown in Figure 2. 

Temperature cycles for aluminum and steel during FRW-I. Actual heating times here are on the order of 700-ms

Macrostructure of an aluminum to steel FRW-I. All deformation is on the aluminum side of the joint

 

The weld shows an effective heating time on the order of 700-ms, significantly longer the 200-ms times reported for intermetallic free joint formation.  Several locations along the bond line were then examined using scanning electron microscopy.  The analysis was done in backscatter mode to reveal changes between the relative phases between the aluminum and steel sides of the joint.  Backscatter images typically show lighter for heavier elements, and darker for the lighter ones.  Clearly, intermediate phases appear in gray tones reflecting the composition of the particular intermetallic formed.  Examples Taken at three locations are shown in Figures 3 – 5.  The micrograph of Figure 3 shows a clean interface, indicating no intermetallic.  Alternately, Figure 4 shows an area of apparent moderate intermetallic formation, while Figure 5 demonstrates a thicker intermediate layer.  Also of interest, areas that form intermetallics (Figures 4 and 5) show relatively uniform thicknesses of the intermediate phase, extending a length along the bondline.  In short, at these intermediate heating times, intermetallics can be characterized as coarsely intermittent, and of nominal uniform thickness over an extended area when they do form.  These results relate to the nature of nucleation and growth for the intermetallic itself.  It appears that intermetallic formation is a discontinuous process, but that growth is rapid once that nucleation occurs.  For areas lacking an intermetallic between the aluminum and steel, nucleation has simply not had sufficient incubation energy to occur.  In areas where this nucleation has occurred, it appears that growth is largely down the bond line rather into the bulk materials themselves.

Area of the FRW-I showing no intermetallic formation. The dark material is the steel and the light area aluminum

Area of the FRW-I with limited intermetallic formation. The intermetallic shows as an intermediate shade between the aluminum (left) and steel (right).Area of the FRW-I with extensive intermetallics.

 

Area of the FRW-I with extensive intermetallics.


One Response to “Intermetallic Formation in Aluminum to Steel Welds”

  1. Great article, Jerry. We often see friction welding applications to join tube-to-tube or tube-to-solid. With aluminum to stainless steel we prefer to work with solid bar stock and then drill the inner bore. Otherwise the extreme weld-flash that we see generated in your images would block the inner bore, and prove harder to remove than drilling through solid material. How do you handle tube applications?

Leave a Reply

Latest News

ALMMII Opens Innovation Acceleration Center in Detroit
The new LIFT center is officially opened in Detroit. The…
Read more...
Energy Savings: The Results Are In
It’s all in the numbers. Let’s start with 2013. That’s…
Read more...
EWI In-House Drafting & Modeling Expertise
All of the specialties at EWI rely on the Design, Controls,…
Read more...
Why would I want to be a welder?!
It's been a while since my last post, but I…
Read more...
Robotic GMAW-P RP2Z Pipe Welding
The oil & gas industry routinely uses the American Petroleum…
Read more...
Dissimilar Materials Workshop in Columbus on January 26th
Do you face the challenges of dissimilar materials joining in…
Read more...
Welcome New Member – The Schaller Group
We would like to welcome The Schaller Group as a new…
Read more...
Welcome New Member – The NanoSteel Company
We would like to welcome The NanoSteel Company as a new…
Read more...