Case hardening Aluminium welds with shot blasting

[NTwoO]

Hi all,

Like the topic states, I am looking for information regarding case hardening Alu with shot blasting.

1: does it work/help?
2: What should be used as medium?
3: How much stronger does the structure become?
4: does it affect longevity in any way, positive or negative?

 

[Hitch]

I think the process is called shot peening...
That will give you the term to google for some more info.
I gather its quite a specialist process.

Its well reported that it does work, but not sure about the effect on welds, or if a specific range of materials is relevant.

I have no idea on what shot to use, i guess it depends on the application of the finished product. Possibly glass beads...steel shot would contaminate the aluminium.
Surface finish would be the other consideration. I gather the surface would be quite rough after the process is complete.

 

[pedrobedro]

I would buy a harder grade of aluminium for strength or more wear resistance.

 

[rtbcomp]

Bead blasting gives a satin finish to aluminium.

I don't if it case hardens it though.

 

[piman]

hello all,

shot peening is a different thing to abrasive cleaning, although the application method is the same. It uses round steel shot which 'hammers' the metal surface. I had only heard of it being used for ferrous materials but I suppose it could work with aluminium as it seems to form a skin on the metal?

Alec

 

[R Kraft]

I think the closer thing to case hardening for aluminum, would be anodizing.
Robert

 

[graffian]

The foundry I go to shot blast the castings, I have them heat treated and don't notice any difference in hardness thru the ali. They must use stainless shot as I haven't had any castings rust, I do find some shot,even after the castings have been driven 40 miles each way and been thru heat treatment.
The shot looks like fine steel shot, doesn't look at all like stainless. It's considerably smaller than the shot I was using on con rods.

I have an aluminium catering tray that's used as an oil drain tray and it was in the cabinet when some engine parts were shot blasted, it rusts if it's cleaned;-)

 

[hotrodder]

Shot peening is really about improving fatigue life- places the surface layer in compressive stress which is more resistant to both initiation and propagation of cracks. It's a precise process compared with shot blasting requiring control over shot size, velocity and exposure time

While shot peening gives better fatigue life than a smooth/polished surface, a rough surface reduces fatigue life

Ball burnishing is another fatigue life enhancing process, again not a DIY proposition but maybe more suitable for soft materials like Al. As with shotpeening the compressed surface layer ends up harder than the underlying material due to the cold working

As said, surface hardness can be increased by anodising too, don't think it helps fatigue life though

 

[gordon stephenson]

Hello Guys,
On the same theme, I remember years a go when I was doing a lot of engine work on my XK jags, A local place (Associated Engineers) used to have a service of internally bead blasting slightly worn piston skirts back to size, Can't remembet the name for it but it sounded like "Coterise" or "Cotherise". Does anybody else remember this process and is it still used today,

Regards Gordon.

 

[Shox Dr]

I may well be talking out my butt here, but I'm pretty sure the term "Case Harden" is only used for Ferrous Metals, the only way to "case harden" Ali is to Anodise it.


As Hotrodder has said, it increases wear life, but if any thing will decrease fatigue life.


As my sig states Im all ears

from wiki

Anodizing, or anodising, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, and niobium. This process is not a useful treatment for iron or carbon steel because these metals exfoliate when oxidized; i.e. the iron oxide (also known as rust) flakes off, constantly exposing the underlying metal to corrosion. "Stay-Brite" is sometimes used as a market name for products made from anodised aluminium such as brass replica.
Anodization, or Anodising in British English, changes the microscopic texture of the surface and changes the crystal structure of the metal near the surface. Coatings are often porous, even when thick, so a sealing process is often needed to achieve corrosion resistance. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit. Anodized aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, making them less likely to crack and peel.

 

[NTwoO]

Sorry for the long silence, I had a busy weekend doing everything except following the forum.

Regarding the choice of Alu alloy, 6061 is the easiest to get hold of, so sticking to that is mp what I'll do. With alu, the welds are obviously affected by the temp. Cold working is supposed to improve the metal properties on and around the weld. The intention is thus not to get the benefit of the hardening of anodising, but to improve the fatigue properties.

The feedback of hotrodder is closest to what I was looking at. It sounds like complex stuff and I am keen to learn more. This sounds like a very interesting process.

 

[NTwoO]

Found some more info. searching on "shot peening aluminium" provided quite some information.

 

[hotrodder]

I may well be talking out my butt here, but I'm pretty sure the term "Case Harden" is only used for Ferrous Metals...

I might be too but IIRC case hardening (origionally at least) referred to a specific process for surface hardening ferrous alloys i.e. packing the item to be hardened along with a source of carbon (charcoal, bits of animal etc) in a 'case' and heating it

Fatigue is a harsh mistress but shot peening or burnishing are typically carried out as a final step for components subjected to high loadings and stress reversals. It'd be pointless to shot peen a part without radiusing sharp edges and crack testing it first for example. A component subjected to simple bending stress will live longer than the same part subjected to a reversed bending stress i.e. bending it one way then the other. Torsional stress will kill a part more quickly than simple bending, fully reversed torsion worse still

Most things that break are the result of fatigue. Gross generalisation but failures are usually down to a lack of attention to detail- those crisp edges that some machinists love, bends with little to no radius that many folders produce, welds with poor profiles, crater defects etc. etc.

Obviously it depends on the part but in the case of aluminium (for most of us) it's generally not used for highly stressed critical parts. Most of the failures i've seen are down to general slopiness- cracks growing from weld craters, rigidly mounting radiators, badly designed mounting tabs on tanks and the like.

As highest stress levels typically occur at the surface of a part it's all about allowing a smooth, uninterupted flow of stress really- cutting splines into a shaft can drastically shorten it's life for example. Smoothly increasing the diameter of the shaft so that the splines don't intrude into the cross sectional diameter is much better (some half shafts/driveshafts for example). Nature's a good example, the way tree branches grow from the trunk or a skeleton- the way the ends of bones are shaped etc.