Application of powder metallurgy process manufacturing in automobile power system

The use of powder metallurgy (PM) processes to manufacture parts for automotive powertrains continues to grow. Parts made with the PM process have a number of important and unique advantages, and the residual porous structure intentionally left in these parts is beneficial for self-lubrication and sound insulation. Using PM techniques can produce complex alloys that are difficult or impossible to manufacture with traditional casting processes, and parts made with such techniques typically have little to no machining, making them cheaper and less wasteful in terms of material. But unfortunately, behind the appeal of these properties, PM parts are difficult to machine.
 
Although one of the original intentions of the PM industry was to eliminate all processing, this goal has not yet been achieved. Most parts can only be "near final shape" and still require some kind of finishing. However, compared to castings and forgings, the small amount of material that needs to be removed from PM parts is typically wear resistant.
 
Porous structure is one of the properties that makes PM parts useful for a wide variety of purposes, but tool life can also be compromised by the porous structure. The porous structure retains oil and provides sound insulation, but also results in microscopically interrupted cuts. The tool tip is continuously impacted as it travels back and forth from the hole to the solid particles, which can cause little fatigue fracture deformation and fine chipping along the cutting edge. To make matters worse, these particles are often extremely hard. Even though the measured macro-hardness of the material is between 20 and 35 degrees Rockwell, the individual particles that make up the part can be as high as 60 degrees Rockwell. These hard particles cause severe and rapid edge wear. Many PM parts are heat treatable, and the material hardness and strength are higher after heat treatment. Finally, due to the sintering and heat treatment techniques and gases used, the surface of the material can contain hard and wear-resistant oxides and/or carbides.
Performance of PM parts.
 
Most of the properties of PM parts, including machinability, are related not only to the alloy chemistry, but also to the porosity level of the porous structure. Many structural parts contain porosity as high as 15% to 20%. Parts used as filtration devices may have porosity as high as 50%. At the other end of the spectrum, forged or HIP (Thermal Ion Die Casting) parts have a porosity of 1% or less. This material is becoming especially important in automotive and aircraft applications as they enable higher levels of strength.
 
Tensile strength, toughness, and ductility of PM alloys increase with density, and machinability may also increase because porosity has a detrimental effect on the tip.
 
An increase in the porosity level can improve the sound insulation performance of the part. Damped oscillations, which are prevalent in standard parts, are significantly reduced in PM parts, which are important for machine tools, air conditioning blowpipes and air tools. High porosity is also necessary for self-lubricating gears.
 
Difficulties in cutting.
While one of the growing goals of the PM industry is to eliminate machining, and one of the main attractions of the PM process is that only a small amount of machining is required, many parts still require post-processing for higher precision or a better surface finish. Unfortunately, machining these parts is extremely difficult. Most of the troubles encountered are caused by porosity. Porosity causes microscopic fatigue of the cutting edge. The cutting edge is constantly cutting in and out, it passes between the particles and the hole, and the repeated small impacts cause small cracks on the cutting edge. These fatigue cracks grow until the cutting edge chipping. These microchippings are usually very small and generally manifest as normal abrasive wear.
 
Porosity also reduces the thermal conductivity of PM parts, which results in high temperatures on the cutting edge and can cause crater wear and deformation. The interconnected porous structure provides pathways for the cutting fluid to drain from the cutting area. This can cause thermal cracking or deformation, which is especially important in drilling.
 
The increased surface area caused by the inherent porous structure also leads to susceptibility to oxidation and/or carbonization during heat treatment. As mentioned earlier, these oxides and carbides are hard and wear-resistant.
 
It is extremely important that the porous structure also gives failure of part hardness readings. When intentionally measuring the macrohardness of a PM part, it includes the hole hardness factor. The porous structure leads to collapse of the structure, giving the false impression of a relatively soft part. The individual particles are much harder. As described above, the difference is dramatic.
 
The presence of inclusions in powder metallurgy parts is also disadvantageous. During machining, these particles are pulled up from the surface, creating a scratch or scratch on the surface of the part as it rubs across the front of the tool. These inclusions are usually large and leave visible holes in the surface of the part.
 
Variations in carbon content lead to inconsistencies in machinability. For example, the FC0208 alloy has a carbon content between 0.6% and 0.9%. A batch of 0.9% carbon was relatively hard, resulting in poor tool life. Another batch of material with 0.6% carbon gave excellent tool life. Both alloys are within the allowable range.
 
The ultimate machining problem is related to the type of cutting that occurs on PM parts. Because the part is close to its final shape, the depth of cut is usually shallow. This requires a free cutting edge. The built-up edge on the cutting edge often causes chipping.
 
Processing Technology
To overcome these problems, several techniques (unique to the industry) are applied. Superficially porous structures are often closed by impregnation. Additional free cutting is usually required. Recently, improved powder production techniques designed to increase powder cleanliness and reduce oxides and carbides during heat treatment have been used.
 
The closed surface porous structure is impregnated with metal (usually copper) or polymer. It has been hypothesized that impregnation acts like a lubricant, and most experimental data suggest that the real advantage lies in closing the superficial porosity and thus preventing microscopic fatigue of the cutting edge. Chatter reduction improves tool life and surface finish. The most dramatic use of impregnation showed a 200% increase in tool life when the porous structure was closed.
 
Additives such as MnS, S, MoS2, MgSiO3 and BN are known to improve tool life. These additives improve machinability by making chips easier to separate from the workpiece, breaking chips, preventing built-up edge, and lubricating cutting edges. Increasing the amount of additive increases machinability but decreases strength and toughness.
 
Powder atomization technology with controlled sintering and heat treatment furnace gases enables the production of clean powders and parts, which minimizes the occurrence of inclusions and surface oxide carbides.
 
Tool material
The tools most widely used in the PM industry are those that are resistant to wear, edge cracking and no built-up edge while producing a good surface finish. While these properties are useful for any machining operation, they are especially important when machining PM parts. Tool materials included in this category are cubic boron nitride (CBN) tools, uncoated and coated cermets, and modified coated cemented carbides.
 
CBN tools are attractive due to their high hardness and wear resistance. This tool has been used for many years in steel and cast iron machining with a Rockwell hardness of 45 and above. However, due to the unique properties of PM alloys and the significant difference in microhardness and macrohardness, CBN tools can be used for PM parts as soft as 25 Rockwell. The key parameter is the hardness of the particles. When the hardness of the particles exceeds 50 degrees Rockwell, CBN cutters are available regardless of the macrohardness value. The obvious limitation of such knives is their lack of toughness. Edge reinforcement including negative chamfering and heavier honing may be required for interrupted cuts or high porosity. Simple light cutting can be done with honed cutting edges.
 
There are several materials of CBN that are effective. The toughest material is mainly composed of monolithic CBN. They are very tough and can therefore be used for roughing. Their limitations are usually related to surface finish. To a large extent it is determined by the individual particles of CBN that make up the tool. Particles have an effect on the surface of the workpiece material as they fall off the cutting edge. A fine particle tool shedding a particle is less of a problem.
Commonly used CBN materials have high CBN content and medium particle size. CBN finishing inserts are fine grained and low in CBN. They are effective where light cutting and surface finish are required or where the alloy being machined is particularly hard.
 
In many cutting applications tool life and grade are independent. In other words, any CBN grade can achieve similar tool life. In these cases, grade selection is primarily based on the lowest cost per cutting edge. A round insert has an entire CBN top face and can provide four or more cutting edges and is less expensive than a four-insert CBN insert.
 
When the hardness of PM parts is lower than 35 degrees Rockwell, and the hardness of the particles themselves does not exceed the range, cermet is usually one of the choices. Cermet is hard, resists built-up edge effectively and can withstand high speeds. In addition, because cermets have historically been used for high-speed, finishing machining of steel and stainless steel, they often have ideal geometries for approximating formed parts.
 
Today's cermets are metallurgically intricate, with as many as 11 alloying elements. Usually they are sintered from titanium carbonitride (TiCN) particles and a Ni-Mo binder. TiCN provides the hardness, built-up edge resistance and chemical stability that are important for the successful use of cermets. In addition, these knives usually have a high binder content, which means they have good toughness. All in all, they have all the properties for efficient machining of PM alloys. Cermets of several materials are effective, like tungsten carbide sintered cemented carbide, the higher the binder content, the better the toughness.
 
A relatively recent advancement known is that medium temperature chemical vapor deposition (MTCVD) also offers advantages in the PM industry. MTCVD retains all the wear resistance and crater wear resistance of conventional chemical vapor deposition (CVD) and improves toughness very objectively. This increase in toughness mainly comes from the reduction of cracks. The coating is deposited at high temperature and then cooled in a furnace. Due to inconsistent thermal expansion, the coating contains cracks when the tool reaches room temperature. Similar to scratches on flat glass, these cracks reduce the edge strength of the tool. The lower deposition temperature of the MTCVD process results in lower crack frequencies and tougher cutting edges.
 
Differences in toughness can be demonstrated when CVD-coated and MTCVD-coated substrates have the same properties and edge grinding. When used in applications where edge toughness is required, MTCVD coatings outperform CVD coatings. Through analysis, edge toughness is important when machining PM parts with porous structures. MTCVD coatings outperform CVD coatings.
 
Physical vapor deposition (PVD) coatings are thinner and less resistant to abrasion or crater wear than MTCVD or CVD coatings. However, PVD coating applications can withstand significant impact. PVD coatings can be effective when the cutting is abrasive, where CBN and cermet are too brittle and an excellent surface finish is required.
 
For example, the C-2 carbide cutting edge machined FC0205 at a line speed of 180 meters per minute and a feed of 0.15 mm per revolution. Built-up edge can cause microchipping after machining 20 parts. When using PVD titanium nitride (TiN) coatings, built-up edge is suppressed and tool life is extended. When TiN coatings were used for this test, the abrasive wear characteristics of PM parts predicted that TiCN coatings would be more effective. TiCN has almost the same built-up edge resistance as TiN but is harder and more wear-resistant than TiN.
 
The porous structure is important and it affects the machinability of the FC0208 alloy. A variety of different tool materials offer corresponding advantages when the porous structure and properties change. When the density is low (6.4g/cm3), the macrohardness is low. In this case, MTCVD-coated carbide provides the best tool life. The micro fatigue of the cutting edge is very important, and the toughness of the edge is very important. In this case tough cermet inserts provide maximum tool life.
 
When producing the same alloy with a density of 6.8g/cm3, abrasive wear becomes more important than edge cracking. In this case, the MTCVD coating provides good tool life. PVD-coated cemented carbides are tested on both types of extremely hard, oil-free bearings with cracked edges.
 
When the speed increases (the linear speed is more than 300 meters per minute), the cermet and even the coated cermet will also produce crater wear. Coated cemented carbide is more suitable, especially when the cutting edge toughness of the coated cemented carbide is good. MTCVD coating of cemented carbides to substrates with cobalt-rich regions is particularly effective.
 
Cermets are commonly used for turning and boring. Because lower speeds may be expected and more focus on built-up edge, PVD-coated carbide is ideal for threading.