Sintering is a method for making objects from powder, by heating the material in a sintering furnace[1] below its melting point (solid state sintering) until its particles adhere to each other. Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy.

Sintering of metallic powders

Most, if not all, metals can be sintered. This applies especially to pure metals produced in vacuum which suffer no surface contamination. Sintering under atmospheric pressure requires the usage of a protective gas, quite often endothermic gas.[4] Many nonmetallic substances also sinter, such as glass, alumina, zirconia, silica, magnesia, lime, ice, beryllium oxide, ferric oxide, and various organic polymers. Sintering, with subsequent reworking, can produce a great range of material properties. Changes in density, alloying, or heat treatments can alter the physical characteristics of various products. For instance, the Young's Modulus En of sintered iron powders remains insensitive to sintering time, alloying, or particle size in the original powder, but depends upon the density of the final product:
En / E = (D / d)3.4
where D is the density, E is Young's modulus and d is the maximum density of iron.
Sintering is static when a metal powder under certain external conditions may exhibit coalescence, and yet reverts to its normal behavior when such conditions are removed. In most cases, the density of a collection of grains increases as material flows into voids, causing a decrease in overall volume. Mass movements that occur during sintering consist of the reduction of total porosity by repacking, followed by material transport due to evaporation and condensation from diffusion. In the final stages, metal atoms move along crystal boundaries to the walls of internal pores, redistributing mass from the internal bulk of the object and smoothing pore walls. Surface tension is the driving force for this movement.
A special form of sintering, still considered part of powder metallurgy, is liquid state sintering. In liquid state sintering, at least one but not all elements are in a liquid state. Liquid state sintering is required for making cemented carbide or tungsten carbide.
Sintered bronze in particular is frequently used as a material for bearings, since its porosity allows lubricants to flow through it or remain captured within it. For materials that have relatively high melting points, by comparison to other materials of the same type, such as PTFE and tungsten, sintering is one of the few viable manufacturing processes. In these cases very low porosity is desirable and can often be achieved.
Sintered bronze and stainless steel are used as filter materials in applications requiring high temperature resistance while retaining the ability to regenerate the filter element. For example, sintered stainless steel elements are used for filtering steam in food and pharmaceutical applications.
Separation of items within the furnace is achieved using sheets similar to those described in the ceramic process above.

Originally Posted by Ltngdrvr
Ford has added forged pistons so it is all forged now in the boss.

Standard 5.0 is forged, except for the hypereutectic cast pistons.

The sinter/powder forged connecting rods having been improved for strength over standard 5.0 rods.

4. POWDER FORGING

Powder forging produces fully dense PM steel parts , such as the automotive connecting rod used in BMW V8 engines.

The production of traditional PM parts has been expanding at a significantly faster rate than the general growth of engineering production and when it was originally developed in the 1970s powder forging or sinter forging was expected to alter fundamentally the scale of the PM industry.

PROCESS
In this process, a powder blank is pressed to a simple shape halfway between that of a forging billet and the required finished part.

This compact, referred to as a preform, is sintered and then hot forged to finished size and shape in a closed die.

The amount of deformation involved is sufficient to give a final density very closely approaching that of the solid metal , and consequently, the mechanical properties are comparable with those of material forged from wrought bar.

ADVANTAGES
Indeed they may be superior in some respects because of the freedom of the sinter forged part from directionality, the greater homogeneity as regards composition, and a finer microstructure, as well as the absence of internal discontinuities resulting from ingot defects that are possible in forgings made from cast metal.

An additional advantage is the dimensional consistency achievable in consequence of the accurate metering of the quantity of powder used.

LIMITATIONS
There are limitations to the steel compositions that can be successfully produced on a commercial scale.

* Steels containing readily oxidisable elements such as chromium and manganese - which happens to be also the cheaper strengthening elements - cannot easily be used, but special compositions , generally containing as alloying elements, nickel and molybdenum, the oxides of which are reduced in sintering atmospheres, have been developed.

* Powder forged steel parts can be heat treated in the same manner as wrought steels.

Production costs in powder forging are generally higher than in conventional casting or forging due mainly to the high price of the starting material and tooling. However, the high precision achieved in powder forging results in considerable savings on machining costs and hence savings on investments in machining operations.

This has particularly proved to be the case for powder forged connecting rods which are gaining in popularity all over the world due to their improved dimensional accuracy, higher dynamic properties, smoother running in the engine, and significant cost savings.

Many companies in North America, Japan and Europe now have large powder forging installations mainly to produce parts for the automotive industry . Such parts can have inside and outside spline forms, cam forms, and other forms that require extensive machining. In addition to the well known connecting rod other applications include bearing races, torque convertor hubs, and gears.

Actual forging is the strongest possible conventional manufacturing technique for metals. Sintering is not forging.