The following report describes the manufacture process of tungsten carbide cutting tools using the effective and versatile method of powder metallurgy. This technology is capable of large scale production of complex, discrete components and is therefore used widely in manufacturing industries. The finding of this technology was largely responsible for the growth of the electrical, electronic, automobile and aerospace industry. The most simplified summary of P/M would be that it is concerned with the use of metal powders and converting them into a useful shape. One of the main successes of this process is that products like tungsten carbide which we will be discussing in detail are either very difficult or impossible to produce by other methods.
The powder metallurgy process dates back to ancient times where it was an art-form, and was used to create artifacts dating back as far as AD 400 (iron pillar of Delhi, India)[i]. Although this process has been around for a long time it was not a well accepted industrial manufacturing process until the twentieth century[ii].
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A good economical and environmental advantage of this process is that it minimizes of eliminates the use of large machining and concurrently produce high volume of target product components, saving energy and raw materials, producing mass volume of precision quality components.
Tungsten is a metal with a very high melting point than platinum, therefore it becomes more difficult to melt and process. One of the early uses of tungsten powder metallurgy came after the famous invention of the electric lamp by the great Thomas Edison in October 1879, where later P/M was used to manufacture filaments at a large scale. Because tungsten was recognized to be the best metal for filaments due to its high melting point in the order of 3653 K along with its good electrical properties.
Tungsten carbide which is 80-95% WC can only shaped by slitting with diamond tools and by grinding. Thereby limiting the shapes to which they can be formed economically and efficiently. Thus the most suitable process that allows the manufacture of cutting tool shapes is process of powder metallurgy (P/M). The following report will describe the powder metallurgy process for the manufacture of tungsten carbide cutting tools.
A very important parameter is the apparent density (AD) of the powder, i.e. the mass of a given volume, since this strongly influences the strength of the compact obtained on pressing. The AD is a function of particle shape and the degree of porosity of the particles.
The choice of powder characteristicsare normally based on compromise, since many of the factors are in direct opposition to each other:
The purity of the powder is critically important. Impurity levels which can be tolerated depend to a large extent on the nature and state of combination of the substances concerned.
For example, the presence of combined carbon in iron tends to harden the matrix so that increased pressures are required during compaction.
Free carbon, however, is often an advantage, acting as a lubricant during the pressing operation.
Most metal powder grains are coated by a thin oxide film, but in general these do not interfere with the process, since they are ruptured during the pressing operation to provide clean and active metal surfaces which are easily cold-welded.
Their final reduction under the controlled sintering atmosphere is essential for complete metal bonding and maximum strength.
Stable oxide films or included oxide particles, such as SiO2 and Al2O3 are more serious, since these are generally abrasive and lead to increased tool wear.
Furthermore they cannot be reduced during subsequent sintering and their presence may adversely affect the mechanical properties especially impact strength of the finished part.
Blending and mixing
The blending and mixing of the preformed tungsten carbide-matrix metal is a very critical step in the formation of the finished hard metal cutting element because of the propensity of contaminants to enter the tungsten carbide powder during this step. Generally the grinding or the milling of these pellets occurs in what is called a steel ball mill, or other type grinding device, and because of the hard nature of tungsten carbide, a substantial quantity of impurities such as iron or steel may be abraded from the grinding machinery by the tungsten carbide particles, and as a result, will end up in the finished tungsten carbide powder. The presence of these iron and steel impurities in the tungsten carbide powder after the powder is combined with the matrix powder and sintered into a finished product will generate defects such as porosities and holes. The blending process mixes powder of the required chemical composition (in this process tungsten carbide). On top of the powder some other ingredients are added, namely,
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· Lubricants - to reduce the particle-die friction.
· Binders - to achieve enough strength before sintering.
· Deflocculants - to improve the flow characteristics during feeding.
The powers that were blended are pressed in dies under great pressure to form them into the required shape. This compaction is generally brought by hydraulic pistons which generate a great amount of pressure. The resulting shape is not yet fully processed, therefore being given the word green compact. Hydraulic presses enable greater pressures to be used - up to 5,000 tonnes - but speeds are necessarily much lower, 10 parts per minute being a fairly representative high speed for parts of comparatively simple geometry.
When required, external and internal threads can be automatically moulded into the part thereby eliminating the need for mechanical thread-forming operations, like in the figure.
Internal threads are typically moulded by using automatic unscrewing devices, but this route is often not cost-effective and tapping should be considered.
Pressure and density distributions after compaction varies which is shown below in
There are three several methods which can improve the density distribution, namely,
· Application of double acting press and two moving punches in conventional compaction as shown in the figure below,
· Iso-static pressing.
Pressure is applied from multiple directions against the powder mixture, which is placed in a flexible mold:
Attention must be given to the following design factors in the light of limited lateral flow and also of the necessity of ejecting the green part in the direction of pressing:
Compressed metal powder is heated in a controlled-atmosphere furnace to a temperature below its melting point, but high enough to allow bounding of the particles:
The primary driving force for sintering is not the fusion of material, but formation and growth of bonds between the particles, as illustrated in a series of sketches showing on a microscopic scale the changes that occur during sintering of metallic powders.
Inoder to end up with the finished product a number of secondary and finishing operations can be applied after sintering namely,
· Sizing - cold pressing to improve dimensional accuracy.
· Coining - cold pressing to press details into surface.
· Impregnation - oil fills the pores of the part.
· Infiltration - pores are filled with a molten metal.
· Heat treating, plating, painting.
Advantages of Tungsten Carbides
High hardness at both room and high temperatures makes cemented carbides particularly well suited for metal cutting. The hardness of even the softest carbide used for machining is significantly higher than the hardest tool steel.
Hot hardnessis the capacity of WC-Co to maintain a high hardness at elevated temperatures that permits the use of higher cutting speeds. These cemented carbides can also be distinguished by high compressive strength. The compressive strength is influenced by Cobalt content. Cemented carbides are classified into two categories,
Straight Grades. They comprise tungsten carbide (WC) with a cobalt binder (Co) and are best suited for workpieces materials normally associated with abrasion as the primary failure mode, i.e., cast iron, nonferrous, and nonmetals.
Complex Grades. They comprise tungsten carbide, titanium carbide (TiC), tantalum carbide (TaC) and often niobium carbide (NbC) with a cobalt (Co) binder. Complex grades of cemented carbide are best suited for “long chip” materials such as most steels.
Titanium carbide provides a resistance to cratering and built-up edge. Hot hardness is improved with the addition of TiC. TiC reduces the transverse rupture, compressive, and impact strengths of the carbide.
Tantalum carbide provides a resistance to thermal deformation. TaC has lower hardness than TiC at room temperature but greater hot hardness at higher temperatures. The coefficient of thermal expansion for TaC more closely matches that for WC-Co, resulting in better resistance to thermal shock.
Carbide Grade Design.
The cutting-tool grades of cemented carbides are divided into two groups depending on their primary application. If the carbide is intended for use on cast iron, which is a nonductile material, it is graded as a straight carbide grade. If it is to be used to cut steel, a ductile material, it is graded as a complex carbide grade. Cast-iron carbides must be more resistant to abrasive wear. Steel carbides require more resistance to cratering and heat. The tool- wear characteristics of various metals are different, thereby requiring different tool properties. The high abrasiveness of cast iron causes mainly edge wear to the tool.
Economic Advantages of Powder Metallurgy
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The expansion of the powder metallurgy industry during the past few decade is greatly due to the cost savings associated with net shape processing compared to other metalworking methods, such as casting or forging. In certain cases, the conversion of a cast or wrought component to powder metal provides a cost savings of 40% or higher.
The powder metallurgy process typically uses more than 97% of the starting raw material in the finished part and is specially suited to high volume components production requirements.
There are two primary reasons for using a powder metallurgy product, namely,