pouring/casting: solidifying trajectory better short or long? 3

[kingnero]

First of all, I hope "solidifying traject" is the correct name: I am talking about the time that it takes from a molten metal or alloy to form a solid mass.

My question is as follows: why is it favourable to try for as small a solidifying traject as possible?
I would think that a longer time it takes, the better the moulds can be filled. Whereas a eutectoidic metal (Fe + 2%C for example)would solidify at one certain temp, which would make for a more uncontrolled solidifying process.

Brass for example has a longer solidifying traject, and is easily cast or poured.

Could someone elaborate on this? or point me towards a meaningful explanation on-line?

I am a bit sorry for the bad explanation, however this is not something I have to deal with so I don't have any experience with this whatsoever. But I am visiting a foundry of high-carbon steel products soon and I have picked up some information from different persons, but I don't really understand the advantages and disadvantages of a solidifying trajectory.

thanks four your insights on this.

 

[salmon2]

I assume "solidifying traject" is similar as casting speed in continuous casting. Somehow. I have learned that a faster casting will reduce alloy segregation, and obviously has higher productivity. But a faster casting will tend to have more common casting defects, which are more concerned by people usually. So your question depends on the specific chemistry. By the way, 2.0%C steel has a range of temperature to solidy rather than a temperature per iron-carbon phase diagram.

 

[EdStainless]

There are a number of factors to balance.
The solidification rate as well as the liquidus/solidus gap, and when you cast into a mold there is the superheat also.
While slow gradual solidification may maximize fill and minimize shrinkage defects it also maximizes negative reactions with the mold materials and leads to greater segregation.

In general solidification that happens as fast as practical without causing voids and tears is the best.
In some complex alloy systems you need to push solidification as fast as possible in order to prevent segregation and the formation of detrimental secondary phases.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[arunmrao]

First of all, I hope "solidifying traject" is the correct name: I am talking about the time that it takes from a molten metal or alloy to form a solid mass.

If,my understanding is correct,the OP refers to solidification range of the alloy,(i.e the difference between liquids and solidus). This would classify the alloy as a long freezing range or short freezing range .

The freezing range of the alloy,determines,the occurrence of shrinkage defects,segregation related issues,proneness to hot tears etc.

Brass has 2 grades 60/40 and 70/30 and they exhibit different solidification mechanisms. Which grade are ou referring to please?


_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[mw1st]

As arunmrao mentioned, you probably mean solidification range.
You see, the temperature at which freezing begins is called the liquidus, and the temperature at which freezing is complete the solidus. Between these temperatures exists an area consisting of both liquid and solid metal caled mushy zone. The extend of the mushy zone is dependent on the difference in temperature between the liquidus and solidus and thermal gradient. For example alloys with narrow freezing range would be steels or 70:30 brass
Alloys with wide freezing range - copper tin or copper lead.
In the range of chemistries normally encountered in commercial steel castings, the freezing range increases with increasing carbon content.
Now, copper alloys can have:
a) short freezing range, ie: aluminium bronze, manganese bronze, silicon bronze
b)long freezing range, ie: gun metals, red brass, yellow brass,phosphor bronze, nickel silver.
Even increasing riser size may not help to eliminate the dispersed shrinkage for long freezing range alloys.
As Eddy mentioned, for those alloys, and in the case with long freezing range aluminium alloys, "pushing solidification" in other words chilling is more effective to increase mechanical properties by reducing segregation. To minimize microshrinkage for these alloys chills are more effective, because they extract heat faster than sand molds and minimize the length over which liquid and solid co-exist. As far as casting/risering is concern, grey cast iron for example, requires less feed metal because graphitization occurs during the final stages of freezing. Steels and white cast iron and many nonferrous alloys, have an extened freezing range and require more extensive and elaborate riser system.

 

[kingnero]

"solidificating range" or "freezing range" indeed will be the better translation. sorry for that!

for the Brass, I don't know which grade (I do not work with that... I just know the system/binary diagram). I was just trying to give an example of a longer range alloy.

@ Salmon, correct about the 2%, I tried to remember the Fe-C diagram by heart. It should have been 4.3%.

I believe the segregation of secondary phases might indeed be a controlling variable for choosing the size of the solidification range.

thanks all for your replies.

 

[Maui]

For any given alloy composition, the difference between the liquidus and the solidus temperatures defines the freezing range of the alloy. Alloys that possess relatively wide freezing ranges, termed long freezing alloys, can be prone to void formation (porosity) during casting, while alloy compositions near eutectic melting points where the freezing range is much narrower can have clear advantages in casting specific shapes or geometries under certain conditions. This is a particularly important consideration in aluminum-silicon alloys, which have been used for decades in the automotive industry for casting engine blocks. Al-Si alloys are used in this application primarily to reduce overall vehicle weight and to improve fuel economy.

The freezing range of Al-Si alloys is strongly dependant upon the silicon concentration in these alloys. For example, for a 10% Al-Si alloy the freezing range is 16 C, while for a 5% Al-Si alloy it is 49 C. If an improper composition is selected where the freezing range is too wide, voids can (and will) form in the casting, rendering it unfit for service. Furthermore, if the liquid is not poured at the correct temperature for the composition that is selected, then it will either fail to fill the mold completely (if the liquid is too cold), or it may adversely react with the mold walls and create a poor casting surface (if the liquid is too hot).

Pouring or teeming temperatures are always selected to be higher than the liquidus temperature of the alloy. This occurs because some heat is invariably lost during the time span when the alloy is transferred to the molds during pouring, and the alloy cannot be allowed to drop below the liquidus temperature before the pouring operation is completed. Otherwise, partially solidified material would be transferred to molds, and this could have very serious consequences during the pouring or teeming operation itself. The number of degrees to which the alloy is heated above the liquidus temperature is referred to as the superheat. The amount of superheat is typically in the neighborhood of 25 to 110 C above the liquidus temperature, depending upon the particular alloy composition and the intended method of casting.

Maui

http://www.EngineeringMetallurgy.com

 

[Metalhead97]

Alloys with large freezing ranges are easier to feed, which keeps pipe-shrinkage from getting too deep into the mold. Alloys with narrow freezing ranges tend to "choke off" the molten metal supply during freezing and as a result the pipe shrinkage tends to penetrate into the mold. A way to get around this is to use a mold that is tapered in the areas that are being fed. Large freezing ranges can result in larger amounts of microporosity due to poor interdentric feeding and as Maui mentioned that's a reason Si is added to cast Al alloys.

Metalhead97