Cast Aluminium Tooling Plate 4

[drawoh]

I have a piece of cast aluminium tooling plate that has broken in two. I have what appears to be a brittle fracture. There are minimal shiny spots in the fracture face that would indicate fatigue. There is no bending of the material around the fracture face showing ductility. The load almost certainly was in bending. I suspect there has been some rough handling.

Most cast aluminium tooling plate is quoted at 15ksi yield stress and 24ksi ultimate stress. This would indicate a fairly ductile material, but I not seeing a ductile failure here.

Has anyone else here broken this stuff? Is this how it normally behaves?

I have attached a photo of the broken face. The shiny parts may be from handling after the fact.

--
JHG

 

[berkshire]

Drawoh,
What was the material , was it 6061, or some other alloy ?
B.E.

You are judged not by what you know, but by what you can do.

 

[CoryPad]

The shiny spots on the fracture surface are contact damage after the fracture occurred. Your fracture surface is typical of this material. You can try to look for defects (e.g. porosity), but it looks like overstress to me.

 

[metengr]

Most cast aluminium tooling plate is quoted at 15ksi yield stress and 24ksi ultimate stress. This would indicate a fairly ductile material, but I not seeing a ductile failure here.

The relatively low yield and ultimate tensile strengths are not an indicator of ductility, you need to look at elongation or reduction of area values.

Second, agree with what CoryPad mentioned provided this is not subjected to low cycle fatigue. I have seen similar fracture surface features in cast and wrought aluminum alloys from low cycle fatigue.

 

[drawoh]

metengr,

I was comparing the yield and ultimate strengths. If they are not close, the material should be ductile, should it not?

In answer to berkshire's question, the material is not 6061-T6.

This part gets a fair bit of abuse. It was covered in dings and scratches. We mount it in a four point adjustment mount. I will be strongly recommending three points, only. I am trying to figure out what else people do to it.

--
JHG

 

[Compositepro]

A ductile material will have a very flat stress-strain curve which means that there may be very little difference between yield and ultimate strength.

 

[FennLane]

A 'flat' curve between yield and UTS is an indicator of the material's lack of work hardening capability. Its ductility is a measure of the plastic strain which it can support. They are not always related.

The difference between yield point and UTS is more a measure of the 'n' as opposed to ductility.

15ksi is not particularly strong and an increase to 24ksi at relatively limited elongation is hardly exceptional.

Low Cycle Fatigue also seems unlikely unless this part stressed in both directions and reversed plastic deformation occurs.

The fracture surface you have posted looks like a typical brittle elastic fracture with little or no plastic deformation having occurred and is not untypical of an LM4. In a sand cast condition LM4 would only have 2-3% elongation and so the failure you have experienced isn't too surprising if you overload the plate.

 

[mrfailure]

This is an instance where you should really have the part evaluated by a lab specializing in failure analysis of industrial componentry. There is not enough information in the picture for me to comment.

 

[drawoh]

Thanks everyone. I did a crude analysis of this thing being cantilevered from one corner. The bending stress is way over anything aluminium cast tooling plate, or 6061-T6 can manage. It was marginally within the capability of 7075-T6. This may be what actually happened. I am recommending that we change our mounting fixture and that we make this thing out of 7075-T6.

--
JHG