7075 aluminium 2

[blindheim]

I have some questions about T6 heat treated 7075 aluminum. I am very thankful for any input that can help me understand the materials limitations better.

1. What will happen to the microstructure of 7075 aluminium if I keep it at elevated temperature (600C) for 24 hours?

2. If I have a finished machined part from say 7075 T6. Is it possible to homogenize it back to 0-temper, and do a T6 heat treatment from scratch again and achieve the same strength as I had initially?

3. How much does grain size contribute to the outcome of precipitation hardening?

4. Is there a way to reduce grain size in the part?

5. I have heard welding of 7075 aluminium makes it prone to microcracking. Would this also apply if I heated the parts to just below melting temperature before welding, and use the filler of same properties as the base material?

6. If I hit a cold blank of 7075 alu with a hammer. What temperature and holding time do I need to get rid of the induced stresses from the deformation.

 

[CoryPad]

1. You will liquefy a portion of the part, thereby ruining it for structural purposes.
2. Yes.
3. Not too much, this alloy frequently has large grains and still has high strength.
4. Thermomechanical processing, coupled with alloying (e.g. zirconium), are methods to reduce grain size.
5. Yes.
6. 405 degrees C for 2 to 3 hours

 

[EdStainless]

Grain size has more impact on toughness and fatigue than on strength

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P.E. Metallurgy, Plymouth Tube

 

[blindheim]

@CoryPad
2 and 5. Why yes on these? I thought the block once was cast from liquid aluminium. What mechanism is actually causing the microcracking in welded 7075? I thought if I kept it at elevated temp I could relief all stresses caused by the welding...

@EdStainless
Is it possible to find any data showing the relation between grain size and toughness and fatigue strength? How much reduction are we talking about?

Also:
If we look at other high strength aluminium alloys, which of them has the shortest solidification interval?

Thank you

 

[CoryPad]

What more information do you need regarding question 2? It is possible to re-dissolve precipitates with a solutionizing step, then quench and artificially age to produce new precipitates.

Regarding question 5, here is a nice summary:

Weld cracking in aluminum alloys is of concern due to aluminum's relatively high thermal expansion, large change in volume upon solidification, and wide solidification-temperature range. The weld crack sensitivity of heat-treatable aluminum alloys is especially of concern due to the greater amounts of alloying additions used for these alloys. Because of the detrimental effect of weld cracks on joint properties, the weldability of aluminum alloys is defined as its resistance to weld cracking. Weld cracking in aluminum alloys may be classified into two primary categories based on the mechanism responsible for cracking and crack location. Solidification cracking takes place within the weld fusion zone and typically appears along the center of the weld or at termination craters. Liquation cracks occur adjacent to the fusion zone and may or may not be readily apparent.

 

[blindheim]

Sorry, my mistake. It should be 1. Why is it ruined because parts of the metal has been liquefied? I mean it was once liquefied when it was cast, why wasn't it already ruined then?

 

[SwinnyGG]

Sorry, my mistake. It should be 1. Why is it ruined because parts of the metal has been liquefied? I mean it was once liquefied when it was cast, why wasn't it already ruined then?

It's a chemistry problem. The answer to your question is based on the crystal-level physics of what happens when alloys cool and solidify, and how certain elements effect the crystal structure of the base material in which they live.

Just do yourself a favor and don't weld anything made of 7075.

 

[EdStainless]

1. If you heat to a range where you have incipient melting you will cause internal defects to form and create huge segregation. You will be turning a good wrought part into a bad casting.
3. Do you own a book on mechanical metallurgy? It does depend on the deformation mode that leads to fatigue cracking, but in general having many regions with small slip produces better results than fewer regions with greater slip.
5. In general you run into this with any age hardening material. In most alloy systems there are lower strength versions (ones that age harden less) that are used when welding is required. 7075 was never intended to be welded.

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P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

Blindheim...

In general 7075-xxx can be readily resistance spot-welded and friction-stir-welded.

NOTE. Unsure about resistance seam-welding, brazing and diffusion-welding [diffusion-bonding].

NOTE. Stir-welding alters temper... and, obviously, the grain structure... and still requires added treatments such as full SHT/A and/or shot-peening to be functionally adequate for structural purposes.

Otherwise all fusion welding methods... IE: methods that develop a weld-melt-pool... obliterate all solid state [wrought] features of the alloy and return it to the raw cast metal state... which cannot be restored to the wrought state by any conventional methods.

The AWS and the Aluminum Association have some pretty decent handbooks/manuals/tech documents regarding welding aluminum alloys [construction and aerospace].

I think You would be wise to 'stick-to' well-documented 'weldable' and fully heat-treatable, aluminum welding alloys such as 6061, 6063, 2219, most casting alloys, etc.
-------
Personal NOTE. In the 1980s I was inserted into the middle of a modification for a large helicopter. A retractable boom Assy for spraying water in icing conditions was being designed for it from 7075-T6 extruded tubes welded together. When I examined the design reports, all I could come-up with dozens of assumptions... including "that special welding methods would have to be developed and tested for this project for the exact materials involved". On deep analysis of these reports, it became obvious that the preliminary design had been based solely on static loads and the static strength of the un-welded 7075-T6 tubes. What a nightmare! The design relied heavily on welded joints: but there no practical or technical welding considerations! The design was to be attached under a helicopter airframe [dual rotors] and was to be flown in real-world dynamic and aerodynamic conditions! The design had no considerations for in-flight safety such as secondary mechanisms or a jettison system [etc]. I [a lowly reserve engineer] had to convince the project lead to authorize a redesign effort to (a) eliminate welding and/or change alloys; (b) add design/analysis criteria for structural [and mechanical] dynamic flight factors, including the helo vibration spectrum environment; and (c) evaluate options in-case of mechanical/structural malfunctions or failure. NOTE: I left the organization shortly after that and never looked back.

Regards, Wil Taylor

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[blindheim]

Thanks. I am struggling with understanding what actually is the difference between a cast and a wrought alloy. If I purchase a block of 7075, hasn't it already been cast from a liquid state, or how did they make it? What makes one alloy a cast alloy and another a wrought alloy?

 

[metengr]

Typically, the difference is a cast alloy is melted and cast in a mold to final shape.
Wrought means the metal has started as a casting but was subjected to subsequent forming operations (rolling, forging, extruding, etc.,) that results in a product form for either further processing or near final shape. The key is that the original cast structure of the metal has been altered by forming.

 

[EdStainless]

and alloys that are intended to be used in the cast condition (or at least shape) are often a very different chemistry from the wrought counterpart. Cast grades require higher fluidity so that intricate shapes can be cast.
When the ingots or slabs for wrought alloys are 'cast' all that matters is that they are solid. The huge amounts of cold work and the intermediate heat treatments will assure uniform composition, fine grain size, and reliable properties (strength as well as toughness and fatigue).

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P.E. Metallurgy, Plymouth Tube