MolaKule
Staff member
Member Trav, on another thread, suggested we comment on two referenced two papers regarding porosity in cast metals.
His two very interesting references were:
1.) https://www.jim.or.jp/journal/e/pdf3/47/09/2259.pdf
Tribological Behaviour of Lotus-Type Porous Cast Iron,
by Takeshi Kato1, Takuji Nakahata and Hideo Nakajima,
found in Materials Transactions, Vol. 47, No. 9 (2006) pp. 2259 to 2263
and
2.) http://www.google.com/patents/US4173500 a patent, Oxidation of flake graphite in atmosphere of carbon monoxide and dioxide, and water and hydrogen
US 4173500 A.
There are also other papers on this topic such as:
3.) Compressive properties of lotus-type porous iron, by Matej Vesenjak, Aljaz Kovac, Masakazu Tane, Matej Borovinšek, Hideo Nakajima, and Zoran Ren, in Computational Materials Science, Vol. 65, (2012) 37–43,
4.) Fabrication of lotus-type porous iron and its mechanical properties, by Soong-Keun Hyun, Teruyuki Ikeda, Hideo Nakajima in Science and Technology of Advanced Materials, Vol. 5, (2004) 201–205.
5.) Fabrication, properties, and applications of porous metals and directional pores, by Hideo Nakajim,
in Proc Jpn Acad Ser B Phys Biol Sci. Nov 11, 2010; 86(9): 884–899.
So let’s examine the two major forms of porosity for automobile engine castings.
A. Internal porosity in metal castings can be modeled as little spheres or voids with solid boundaries and hollow internal spaces.
B. Surface porosity in metal castings (such as in cylinder bores) can be modeled or idealized as little wells with three solid sides and an opening at the top which allow other metal particles and oil films, with dimensions smaller than themselves to enter, Ref. 1, Fig 7.
In basic engine block finishing or refinishing, the cast iron block is first bored for cylinders and bearing saddles, then the cylinders are finished with a hone of approx. 280 grit in the usual cross-hatch pattern. In first machining the bore to basic dimensions, the bore surface is too smooth to trap oil films. The honing process produces an intersecting pattern of hills and valleys readily seen with the naked eye. This intersecting pattern of hills and valleys (cross-hatching) produces valleys which trap oil films, so when the piston ring passes by, there is a thick enough oil film for lubrication.
Castings with large voids in them (casting defects) can lead to problems such as cracking and poor structural strength, poor heat conduction (less cooling), and even leakage, as in the case of poor aluminum transmission case castings in the past.
In most of the papers referenced, Lotus means, “a new type of porous material which is distinguished by elongated pores resembling a lotus root.”
The basic process of producing Lotus (unidirectional) pores is: 1.) controlling the alloying content, 2.) introducing various gases, and 3.) controlled heating and cooling cycles.
Depending on the process, an elongated unidirectional pore can be produced, Ref. 1 and 3, or pores perpendicular to the solidification direction, Ref. 1.
In reference 1, Figure 3, they show two porous surfaces, one elongated and one perpendicular. In the left photograph is an elongated pore, whereas the picture on the right is a perpendicular pore.
For the picture on the left, the smallest pore “track” is about 0.02mm in width, and the largest is about 0.07mm in width. For the picture on the right, the smallest pore is about 0.005mm in diameter, and the largest pore is about 0.06mm in diameter.
If one can produce a casting structure with directional pores to hold oil films, AND with the necessary structural strength, the production process can be simplified, the weight can be reduced, the friction coefficient is reduced, and wear resistance is increased.
In one of the papers, it was suggested that the casting of porous bearings could entrain oil when heated to a specific temperature and under a specific pressure. This would be similar to the Oilite process where the sintered metals entrain oil because of their large inter-granular voids. Except here, the pores in the casting would entrain the oil. If used as engine bearings, the problem I see is this: When the oil in the pores become oxidized from continual heating and turn to sludge and then to solid carbon, what happens to the COF and the wear? The oil in those pores can never be “refreshed” with new oil. If you increased the porosity such that the pressurized oil supply could refresh it, then it appears the structural strength of the bearing would decrease and be unable to support the loads, which then takes us back to the old multi-layer, grooved, solid bearing.
His two very interesting references were:
1.) https://www.jim.or.jp/journal/e/pdf3/47/09/2259.pdf
Tribological Behaviour of Lotus-Type Porous Cast Iron,
by Takeshi Kato1, Takuji Nakahata and Hideo Nakajima,
found in Materials Transactions, Vol. 47, No. 9 (2006) pp. 2259 to 2263
and
2.) http://www.google.com/patents/US4173500 a patent, Oxidation of flake graphite in atmosphere of carbon monoxide and dioxide, and water and hydrogen
US 4173500 A.
There are also other papers on this topic such as:
3.) Compressive properties of lotus-type porous iron, by Matej Vesenjak, Aljaz Kovac, Masakazu Tane, Matej Borovinšek, Hideo Nakajima, and Zoran Ren, in Computational Materials Science, Vol. 65, (2012) 37–43,
4.) Fabrication of lotus-type porous iron and its mechanical properties, by Soong-Keun Hyun, Teruyuki Ikeda, Hideo Nakajima in Science and Technology of Advanced Materials, Vol. 5, (2004) 201–205.
5.) Fabrication, properties, and applications of porous metals and directional pores, by Hideo Nakajim,
in Proc Jpn Acad Ser B Phys Biol Sci. Nov 11, 2010; 86(9): 884–899.
So let’s examine the two major forms of porosity for automobile engine castings.
A. Internal porosity in metal castings can be modeled as little spheres or voids with solid boundaries and hollow internal spaces.
B. Surface porosity in metal castings (such as in cylinder bores) can be modeled or idealized as little wells with three solid sides and an opening at the top which allow other metal particles and oil films, with dimensions smaller than themselves to enter, Ref. 1, Fig 7.
In basic engine block finishing or refinishing, the cast iron block is first bored for cylinders and bearing saddles, then the cylinders are finished with a hone of approx. 280 grit in the usual cross-hatch pattern. In first machining the bore to basic dimensions, the bore surface is too smooth to trap oil films. The honing process produces an intersecting pattern of hills and valleys readily seen with the naked eye. This intersecting pattern of hills and valleys (cross-hatching) produces valleys which trap oil films, so when the piston ring passes by, there is a thick enough oil film for lubrication.
Castings with large voids in them (casting defects) can lead to problems such as cracking and poor structural strength, poor heat conduction (less cooling), and even leakage, as in the case of poor aluminum transmission case castings in the past.
In most of the papers referenced, Lotus means, “a new type of porous material which is distinguished by elongated pores resembling a lotus root.”
The basic process of producing Lotus (unidirectional) pores is: 1.) controlling the alloying content, 2.) introducing various gases, and 3.) controlled heating and cooling cycles.
Depending on the process, an elongated unidirectional pore can be produced, Ref. 1 and 3, or pores perpendicular to the solidification direction, Ref. 1.
In reference 1, Figure 3, they show two porous surfaces, one elongated and one perpendicular. In the left photograph is an elongated pore, whereas the picture on the right is a perpendicular pore.
For the picture on the left, the smallest pore “track” is about 0.02mm in width, and the largest is about 0.07mm in width. For the picture on the right, the smallest pore is about 0.005mm in diameter, and the largest pore is about 0.06mm in diameter.
If one can produce a casting structure with directional pores to hold oil films, AND with the necessary structural strength, the production process can be simplified, the weight can be reduced, the friction coefficient is reduced, and wear resistance is increased.
In one of the papers, it was suggested that the casting of porous bearings could entrain oil when heated to a specific temperature and under a specific pressure. This would be similar to the Oilite process where the sintered metals entrain oil because of their large inter-granular voids. Except here, the pores in the casting would entrain the oil. If used as engine bearings, the problem I see is this: When the oil in the pores become oxidized from continual heating and turn to sludge and then to solid carbon, what happens to the COF and the wear? The oil in those pores can never be “refreshed” with new oil. If you increased the porosity such that the pressurized oil supply could refresh it, then it appears the structural strength of the bearing would decrease and be unable to support the loads, which then takes us back to the old multi-layer, grooved, solid bearing.
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