D2 die fatigue? 1

[tbaran]

I have a square D2 die that is experiencing die fatigue in the corners which have a .143R. Here is a picture of the fatigued corner [

http://files.engineering.com/getfil...c0cb-4946-922e-63e1423fec33&file=DSCN0788.JPG

], the die square measures 1.5" AF. The die is used in the reverse impact extrusion process to cold form a square aluminum slug into a square aluminum part and the vertical pressure used to form this part is approximately 290 tons. This die was wire EDM'd with the following notes: (1) 1.2" diameter hole created prior to HT, (2) 1.5" square wire EDM after HT and press fit into casing, and (3) 1.5" is a final polished dimension. The OD of the D2 sleeve is 3.5" and it has a .010" press fit into a H13 48RC die casing. The D2 heat treat instructions are 58-60RC double draw at higher temp. Can anyone give any insight as to why these dies are experiencing fatigue in the corners after running only a couple hundred parts? The aluminum we used is lubricated heavily and it works fine in all our other impact extrusions.

 

[DesignerGuy16]

Is this slug deep drawn in a single step to that depth? That radius to size is pretty small and I would think the stress concentration is pretty high.

On the tool steel: "D2 is very high in wear resistance, but is also low in toughness."
It could be the stress in the corners is so much that it's causing the cracking. Not so much the stress from friction, but just from trying to cold form the material to a small corner.

I'm not a die expert but I worked at a stamping plant for a spell. Have you looked at drawing in multiple steps? Or pre-heating the slugs for increased formability? In the aluminum tooling die business they'd use a nitride coating on the bearing surfaces to decrease friction. Don't know how something like that would work on a deep draw, but reducing your friction and overall forces (by spreading them out) would help.

James Spisich
Design Engineer, CSWP

 

[tbaran]

No impact extrusion is not a deep draw process, so the top corner radius is insignificant. You can view our process or at least get an idea of what is happening at this page

http://www.jlometal.com/the_process/default.asp

We place a billet or slug of aluminum into a die and in one hit we form a can that in this case is about 11" long, the slug thickness we use is 2" thick. The punch hits the aluminum slug with such force that it places the aluminum in its plastic zone for a fraction of a second and the material then flows into the the direction our tooling forces or directs it. We compete directly with deep drawn and cold forged parts.

 

[Compositepro]

Interesting process! If you are getting fatigue cracking you are allowing too much strain to occur in the corners. You need either stiffer containment or a more flexible tool steel. I've used molds where the liner is in segments contained by softer steel (very durable). But if the containment is not stiff enough and preloaded enough the seams will open - which is equivalent to a crack.

 

[DesignerGuy16]

Interesting process. Haven't dealt with that one before.

I wasn't worried about the lead in radius so much as the corners where the fatigue was occuring. I don't know how much better support you can get than an H-13 casing. We used to use that for large bolster and backer plates for extrusions and they held up great.

Is there any possibility of forming using that process in multiple dies? I still think if you could do half in one half in another it'd probably go away.

1.5" Sq x 11" deep is a pretty deep way to go in one step unless it's some kind of casting. Aluminum or not there's going to be enormous forces invoved in forming something that size/length. Just my gut feeling on that one. Then again I'm not experienced in the exact process you're using. Do you have similar parts with that size cross section and length that have no problems? You've got me curious now at least.

James Spisich
Design Engineer, CSWP

 

[tbaran]

Yes we specialize in this process and have many other parts. Predominantly we do round parts, but we have a couple rectangle and square sizes and it is these that give us a hard time. With the round parts the general rule of thumb is we can hit a max length that is 10x the diameter of the part. A very common item we form is the paint marker tubes, these use a .560" dia. x .610" thick slug and in one hit in an enclosed die we make a can that has a .015" wall about 6" long. There are also some larger ones using a 1.750" dia x 2.4" thick slug making a 9" long can with a .060" wall. And we also do larger diameters up to about 6". I'm going to contact our aluminum bar supplier, Sapa (Alcoa), and try to speak with somebody there because when they do a forward extrusion of the bar these corners might also give them problems. Yes, I do believe the forces in the corners are very high and it also might be related to the lowered fatigue strength of the D2 caused by the EDM process for the die. Here's an article in eng-tips related to that

http://www.eng-tips.com/viewthread.cfm?qid=96252.

 

[DesignerGuy16]

That could definetly play a factor. Though I have doubts it will solve the problem.

Any way to use say a S7 tool steel, and heat the die and preheat the slugs? S7 may not have the wear resistance, but it does have excellent shock loading (considering the application is dependant on a lot of force in a short time frame).

Preheating may increase the formability and hopefully reduce the stress concentrations at those corners. That's all I can think of. Hope it works out!

James Spisich
Design Engineer, CSWP

 

[TVP]

tbaran,

Here are some areas to explore concerning your die fatigue problem:

1. Flow stress of the Al billet. Is this particular batch of Al higher in flow stress compared to other/previous batches? Higher flow stress translates into higher forming forces and therefore higher die stresses.

2. Press fit of die. I am unclear on the exact number of tools involved in this die assembly. There is a square die (D2 tool steel) and round stress ring/casing (H13 tool steel). Is there also an intermediate stress ring/sleeve that has an OD of 3.5 inches and a square inside profile? Either way, the exact condition of the press fit will have an enormous influence on the nature of the die corner stresses. Any time there is a cross-section other than round the ability to obtain consistent, uniform compressive stresses during press fitting is hindered. Dimensionally verify that the corner radii, across flat dimensions, etc. are consistent with previous dies. Is one corner of the die cracking before the other sides? Is there any dimensional variation along that corner/side that is preventing a uniform press fit?

3. Microstructure and surface of the die corner. Have you had the cracked die metallurgically evaluated? If not, I recommend that you do so. It only costs ~ $1000 to have an outside lab perform the usual fractography and microstructure analysis necessary to characterize the type of fracture. Three of the most common causes of premature fatigue in tools are (1) cracks caused by grinding process, (2) insufficient removal of EDM white layer and heat affected zone, and (3) Clusters of large, blocky carbide particles.

4. Have you looked into potential misalignment of the press ram/punch? Perhaps the punch centerline is not completely perpendicular to the die face, and some side load is being introduced into the punch/die assembly during forming. I would expect that you would see premature punch fractures if this is the case, but maybe you have really good punches and not so good dies.

Since you already mentioned lubrication and friction, I won’t go into any detail on that. For what its worth, there can be a lot of variation in D2 microstructure, so lot-to-lot consistency may be lacking. Since this is a higher stress application (square die) you just may be on the limit of what is acceptable compared to other round die applications. You might try a “matrix” type tool steel, which were developed from D2 to improve fracture resistance. Daido DC53 and Hitachi SLD8 are examples of 8% Cr matrix tool steels:

http://www.imsteel.com/newsroom_article08.htm

http://www.hitachi-metals.co.jp/e/prod/prod19/pdf/yss_tool_steels_b.pdf

 

[tbaran]

TVP,

The cracking/pitting has happened on all dies from day 1 so it's not a single time occurrence.

1. The flow stress of the billet should be the same, we call out all our material to have the same low temper from Sapa (Alcoa).

2. The die sleeve which is the D2 has a round outer diameter for the press fit. A picture of this is attached so you can see the die ring and H13 case.

3. We have not had the die metallurgically evaluated. This is a good idea once I can get a replacement for this die or it gets to the point we cannot run it anymore.

4. We have had slight misalignment problems with this press, but the cracking is consistent in all 4 corners of the die so I doubt this is the problem.

In the past I have reviewed the variation you mention in D2 micro structure and we have gone directly to carbide dies for some jobs as well as looking at the DC53 material. I will need to greatly consider these alternatives for this die.

Also, I need to find out exactly what our outside die supplier is doing to remove the EDM white layer or to insure that the D2 surface is sound after the EDM process.

 

[dvd]

I am curious about the part geometry.

What is the thickness of the formed part and inside/outside corner radius on the part?

I am suspicious that there is too much material being forced into the corners of the part.

Have you experimented with the blank shape to see if that has an effect?

 

[btrueblood]

"Predominantly we do round parts, but we have a couple rectangle and square sizes and it is these that give us a hard time."

Round parts will not have stress concentrations at the corners like the rectangular parts will (round parts don't have corners). You could put larger fillets on the rectangular parts, but to reduce the stress to levels near the round ones, you would have essentially oval or round parts again.

Do the cracks cause problems with the finished parts (e.g. the image of the cracks show on the parts, and cause them to be rejected)? Or is the problem that the die eventually cracks completely through, and fails?

Someone mentioned a split die, this could help eliminate the corners cracking, but you would have a witness line where the split seam opens up.

Another approach would be to auto-frettage the die, essentially swaging/press-fitting/shrink-fitting it into a mating holder, so that the surface of the inner die is left in compression.

 

[TVP]

tbaran,

Thanks for the additional info. I think the main problem is insufficient press-fit for this application. I would try to increase the press fit and change the sleeve to a higher strength material-- maybe M2 at 59-61 HRC. I would also change the die from D2 to DC53 or one of Hitachi's cold work tool steels. For complex shaped parts you may want to contact Strecon-- they have an excellent system for prestressing dies that allows increased loads and reduced deflection:

http://www.strecon.dk/site/index.php?Precision_forging

Click on the various topics (Products, Technology Notes, Application Notes, etc.) to view the numerous .pdfs on their site. They explain the concept, options, etc.