1. Working Process
SLM works on the principle of melting. The working is same as SLS where as the power is melted instead of sintering. Melting actually creates a pool where the materials can consolidate before reforming and hardening to create new solid structure. The final part won’t be porous, thereby making it suitable for a greater range of applications. In SLM, powdered material is melted, whereas in SLS the powder is heated below its melting point (sintering). When laser heats the powder material to below melting point, it forms solid by fusion. The working principle is almost same in both printing method.
2. Process Variations
Some SLM industries are works with some other material like magnesium for light weight construction. Varieties of materials are tried in SLM process; however very little work has done on gold and its alloys. God and alloys are used to manufacture dental crowns. The gold power is cohesive and non- flowing in nature. SLS has some limitations when compared to SLM. However SLS also has variety of methods like DMSL (Direct Metal Laser Sintering), which use powerful laser to micro-weld the raw material. Yb-fibre or CO2 will be used up to 100W and DMLS used up to 200W.
3. Processing materials
The normal materials used in SLM are stainless steel, aluminium or titanium alloys. SLM materials are selected depending on the properties like strength, ductility and biocompatibility based on the application. Titanium based materials are used in biomedical application due to their good mechanical properties, high bio compatibility and corrosion resistance. SLM works with metals, where as SLS generally works with plastics, glass, ceramics. SLS don’t restrict metals; the same can be used in DMLS. Aluminium filled and glass filled nylon material is used in SLS. Nylon is the ideal material, because of its light weight, high durability and heat resistant etc.,
4. Design aspects
Unlike SLS, SLM is very difficult to control, because of large energy input of laser beam. SLM parts can further processed as many welding part. SLM also requires support structures however i can be act as heat transfer medium which can reduce thermal stress and prevents wrapping. SLS is easy to control and better than FDM. Thinner layers also can be achievable by using DMLS. The un-sintered powder will act as a support and there is no need to provide additional supports. The layer thickness depends on type of material used and post processing can be done by traditional process.
5. Quality aspects
SLM is a proven technology to produce near net-shape parts up to 99.9% relative density. It has good mechanical properties. Tolerance and surface finishes are limited. However it can be improved through post processing. The tolerances are ±0.05-0.2mm, however this will vary machine to machine. SLS parts may require stress relieving and annealing to reduce internal residual stress. SLS chamber is maintained with N2 atmosphere which will prevent explosion of powder. Additional curing may also required when using ceramic parts. The machine needs thorough cleaning to avoid powder contamination. The achievable tolerance is ±0.05-0.25 mm; may vary material to material.
6. Economical aspects
SLM machines are very economical, minimal materials used; time saving and offers reduced cost/cycle time. 100% usage of lasers cannot be achieved in SLM. It is very compact machines and minimum layer thickness also achievable. The process of final approval takes longer time for development and qualification of this technology. SLS has high energy requirement. It is an automated process but needs skilled man power. The initial investment is huge; however the direct labour costs are low. The leftover powder can be re-used and the material utilization also higher. The powder material cost vary in the market depends on its properties.