For those who aren’t familiar with manufacturing, and metal forming, in particular, the difference between melting and sintering may seem largely semantic. However, it is important to know that there is are definite differences between a process that melts an object and a process that uses sintering.
Liquefaction and Use of Temperature
One of the biggest differences is the liquefaction of the material.
A material that is melting is being brought to a temperature sufficient to turn it from a solid into a liquid—the material is being imbued with enough thermal energy to make the transition between energy states. A material that is being sintered is instead being compacted using a combination of heat and pressure without actually crossing the energy threshold required for turning into a liquid.
Why is this an important difference? It's important because sintering can occur at low temperatures in the right conditions. Take, for example, snow. Snow, when compacted into a ball by a child, would be an example of pressure-based sintering.
Basically, sintering can be an operation of either pressure or temperature, while melting is primarily an operation of temperature.
Uses of Sintering and Melting
Sintering has a few different uses. One of the key uses of sintering is to join metal particles together—sintering is often used on metals with high melting points, since it doesn’t rely on reaching melting temperatures to work.
Some 3D printing devices operate by sintering metals one layer at a time to create custom metal forms. Sintering a metal for 3D printing could help to save energy compared to melting the same metal, and allows for greater control and consistency, since the material isn’t being completely liquefied. However, this leaves more microscopic gaps than the full liquefaction caused by melting would.
Sintering can also be used to reduce the porosity of an object’s surface—which can enhance the properties of certain materials.
Melting, on the other hand, has many uses depending on the material being melted and why it's being melted. The process of liquefying metal is commonly used when joining two metals—similar to sintering. In fact, some processes that describe themselves as “sintering” (such as Direct Metal Laster Sintering) are actually melting materials—which can be a contributing factor to the confusion surrounding the two processes.
Another use of melting is to completely liquefy a metal alloy to reform it into a new shape or to change some of its physical characteristics.
For example, heating up magnetized steel can cause the steel to lose its magnetism by disrupting the alignment of the metal’s atomic structure. However, for most magnetized metals, the magnet doesn’t have to be fully melted to remove its magnetic properties—it merely has to reach it’s “Curie temperature,” which is the point at which the magnetized atoms lose their alignment.
In most manufacturing applications, melting is more commonly used to fuse two objects or to reform a metal than it is to change object properties.
So, while sintering and melting have similarities, they are in fact two different processes that have different applications.