The idea of SLM was first proposed by the German Institute of Fraunhofer in 1995. In 2002, the Institute achieved great success in the study of SLM technology. The world's first SLM device by the MCP group of companies under the jurisdiction of the German MCPHEK branch has launched at the end of 2003.
To obtain the compact laser forming, but also benefit from the great progress of rapid prototyping equipment in 2000 after laser (for use, advanced high energy fiber laser powder to improve the precision, etc.) completely melted powder metallurgy mechanism which is used for laser rapid forming of metal components.
For example, EOS, a famous German rapid prototyping company, is the world's earliest specialized company for metal powder laser sintering, which mainly engages in research and development of SLS metal powder, process and equipment. The company's newly developed EOSINTM270 / 280 type equipment, although the continued "sintering" in this statement, but the assembled 200W fiber laser, and the metallurgical mechanism completely melted metal forming component, forming performance can be improved significantly.
At present, as an extension of SLS technology, SLM technology is booming in Germany, Britain and other European countries. Even though the expression of selective laser sintering (SLS) continues to be used, the actual forming mechanism has been changed to the complete melting mechanism of powders.
The Principle of Selective Laser Melting
SLM technology is developed on the basis of SLS, and the basic principle of the two is similar.
SLM technology makes the metal powder completely melted, direct forming metal parts, requiring high power density laser beam to scan the level of powder roller to substrate metal powder tile to the processing chamber, and a laser beam according to the contour information selectively current layer on the substrate melting powder, processing of the current layer the outline, then lifting system decreased a layer thickness of the powder roller rolling distance, then in the current layer on the metal powder have good processing equipment, in the next layer for processing, so the processing layer, until the entire finished parts.
The whole process in the processing chamber vacuum or with gas protection, to avoid the metal reacts with other gases at high temperature. The boundaries between SLM and DMLS are very vague, and the difference is not obvious. Although DMLS technology is translated into metal sintering, most of the metal powder has been melted completely during the actual molding process. DMLS technology uses materials that are mixtures of different metals, and each component compensates for each other in the process of sintering (melting), which is beneficial to ensure the manufacturing accuracy. The SLM technique uses materials mainly consisting of a single component of powder, a laser beam that rapidly melts the metal powder and obtains successive scanning lines.
The development of selective laser melting technology
In selective laser forming Fe based alloy (mainly steel) SLM forming research more, but SLM still need to optimize the forming process, forming performance needs to be further improved; the forming performance of SLM (especially the density accounted for the basic status, forming SLM) steel at present is usually difficult to realize full densification. To solve the densification problem of SLM forming of steel materials is the key bottleneck of rapid prototyping.
The difficulty of laser forming of steel material mainly depends on the chemical characteristics of main elements in steel. The matrix element Fe and the alloy element Cr have strong affinity to oxygen, so it is difficult to avoid oxidation completely under conventional powder treatment and laser forming. Therefore, in the process of SLM steel, the melt surface oxide pollution layer, will significantly reduce the metallurgical defects caused by wettability, balling effect of laser melting and solidification characteristic of the micro cracks, thereby significantly reducing the density of laser forming and corresponding mechanical properties.
On the other hand, the content of C in steel is another key factor to determine the formability of laser. In general, excessive C content will adversely affect the formability of the laser. As the C content increases, the thickness of the C layer on the melt surface will also increase. This is similar to the adverse effects of the oxide layer, and also reduces wettability, which leads to a decrease in melt spreading and a spheroidization effect. In addition, the formation of complex carbides at grain boundaries can increase the brittleness of laser forming parts of steel. Therefore, it is necessary to improve the laser energy density and the forming temperature of SLM in the SLM forming of steel materials. It can also promote the dissolution of carbides and even the alloying elements.
Through powder materials and SLM process optimization, including:
1. strictly control the oxygen content of the original powder material and the laser forming system to improve wettability;
2. reasonable control of the input laser energy density to obtain the appropriate liquid viscosity and rheological properties, can effectively inhibit the spheroidization effect and microcrack formation, and then obtain near full dense structure.
For light alloy parts by laser with Al alloy as the representative of the rapid prototyping, most previous research reports is the mechanism of forming SLS semi-solid sintering based on, but because of the spherical effect and pore defects, so the research has made little progress; while SLM technology is a high performance complex structure of Al alloy parts provide a new technical way near net rapid forming and manufacturing.
SLM forming of Al based alloy parts is highly difficult and depends on the special physical properties of the material itself.
On the one hand, the low power CO2 laser can not make the Al alloy powder melt effectively, but it needs to use the optical fiber or Nd:YAG laser with higher energy density, which undoubtedly puts forward more stringent requirements for the laser performance.
On the other hand, Al alloy material with high thermal conductivity of SLM in the process of forming laser energy input easily along the substrate or in powder bed transfer leads to consumption, laser molten pool temperature, melt viscosity increase and reduce liquidity, so it is difficult to effectively wet base material, resulting in SLM forming spherical effect and pore and crack other defects.
Thirdly, from the angle of forming technology, the density of Al alloy is lower and the flowability of powder is poor.
It must be pointed out that the SLM / SLRM forming mechanism based on laser forming can improve the density and surface finish in a certain extent, but because of the powder forming process complete melting / solidification, obvious shrinkage deformation process in the liquid, resulting in forming in the accumulation of large thermal stress force, and will be released in the cooling process, the stamping deformation, and even cracking.
Because of selective laser melting forming technology of powder of high demand, in the whole forming plane laying metal powder, which is not suitable for forming of precious metals; the whole forming platform is large, poor protective effect of inert gas, so it is not suitable for metal powder forming oxidation at the end.
Advantages of selective laser melting technology
In principle, laser sintering, selective laser melting and selection is similar, but because the laser energy density is higher and more small beam diameter, forming parts of mechanical properties and dimensional accuracy are good, only simple postprocessing can be put into use, and forming the raw materials without special preparation. The advantages of selective laser melting can be summarized as follows:
1. direct manufacture of metal functional parts, without intermediate process;
2. fine beam quality can be obtained by fine focusing facula so that functional parts with higher size accuracy and better surface roughness can be fabricated directly;
3. metal powder is completely melted, and the metal functional parts directly manufactured have metallurgical binding structure, which has higher density and better mechanical performance, without post-processing;
4. powder materials can be either single or multi component materials, and raw materials need not be specially formulated;
5. can directly manufacture complex geometry function parts;
6. especially suitable for single or small batch of functional parts manufacturing. Selective laser sintering parts density, poor mechanical properties; electron beam melting molding and laser cladding manufacturing is difficult to obtain high precision parts; in contrast, selective laser melting technology can obtain metallurgical molding with precision, and good mechanical properties and high dimensional dense tissue, in recent years, rapid prototyping the main research hotspot and development trend.
Prospect of Research on Selective Laser Melting
1) to realize the specialization and specialization of metal powder materials for laser rapid forming.
Pay attention to powder materials to improve the material base performance of laser rapid forming, in-depth quantitative research for powder chemical composition, physical index, selective laser melting process of preparation technology and characterization methods, realize the specialization and serialization of special metal and alloy powder laser rapid forming.
2) to study quantitatively the nature and mechanism of laser forming metallurgy of metal and alloy powders.
The key scientific problems closely related to metal and alloy powder laser rapid prototyping, including laser beam and the metal powder, the interaction mechanism of laser molten pool non-equilibrium heat and mass transfer mechanism, ultra high temperature gradient and rapid solidification of molten metal under internal metallurgical defects and microstructure control, metal powder laser melting process and the various types of stress the evolution of metallurgy, chemical and physical and mechanical problem, provide scientific theoretical basis for improving the metal and alloy powder on Microstructure and properties of laser rapid forming.
3) high performance complex structure, metal and alloy parts, laser control, shape control, net shape manufacturing.
In the laser rapid forming special high fluidity metal powder preparation for the material basis, to laser molten pool of non-equilibrium thermodynamics and kinetics behavior and microstructure of laser forming mechanism, laser forming internal stress evolution of multi-scale prediction as the theoretical basis, through the powder preparation - Part Structure Design - forming SLM process organization and performance evaluation of the integration oriented research, aerospace, biomedicine, mold manufacturing and other fields of application requirements, implementation of key parts of laser high performance complex structure of metal and alloy shape forming direct net shape manufacturing precision.
There are still many breakthroughs in the study of material, process and theory of selective laser melting for metal parts. It is the foundation of the application of laser rapid prototyping technology to the further research and discovery of many new materials, new techniques, new phenomena and new theories in this field.
Research on selective laser melting
A great deal of work has been done by scholars and research teams on selective laser melting. The important parameters of RehmeO on the selective laser melting process are analyzed and classified, studied the influence of scanning line length, scanning distance, thickness, direction of forming parameters such as density and residual stress of the parts.
KozoOsakada study of selective laser melting properties of nickel based alloys, Fe based alloy and pure titanium material, molding analysis of the thermal stress distribution, through the scanning strategy and preheating method to reduce the thermal stress, and directly manufacture metal mold caused by a density greater than 90%.
J.P.Kruth using Rayleigh stability principle explained the spheroidization of Fe based alloys, and puts forward the strategy and control method using oxygen content scanning to eliminate the spheroidization, also influence the different elements of the laser absorption rate, thermal conductivity, melt wettability and spreadability, oxygen content and Rayleigh instability etc..
Selective laser melting characteristics of I.Shishkovsky on aluminum zirconium ceramic materials were analyzed, the research on organizational structure and composition of molding, molding and found in the air of the parts is a structure and rules of dense tissue stable phase distribution.
M.Badrossamay study of the stainless steel and tool steel. The effects of laser power, scanning strategy and other parameters on the forming quality, the study found that the stainless steel and tool steel are similar between the forming rules, and the forming quality and scanning speed is not a linear relationship, suggesting there is the amount of powder bed heat loss scanning speed.
I.Yadroitsev uses stainless steel and other raw materials to carry out a lot of work on the selective laser melting process, studied the effect of scan strategy on impact, the density of the scanning angle on the mechanical properties, using scanning can obtain high density molding after filling filling strategy ", also found that scanning angle on the molding yield impact strength and tensile strength; in addition, through the experiment, the molding thickness of continuous thin walled 140 m optimization of process parameters.
Stability of molten pool and Gusarov by using thermodynamic analysis in selective laser melting process, using Rayleigh stability principle to explain the balling phenomenon and high scan speed, and put forward the suitable for continuous molten pool the optimum shape of weld pool, which decreases the weld pool length width ratio and increase the contact line of the molten pool and the width of the substrate.
KamranAamirMumtaz of the nickel alloy single pass weld pool, analysis of scanning strategy influence on density, and put forward a method to improve the surface quality, that is the "scan can prevent thermal deformation caused by adjacent lap pool strategy" after filling filling, simultaneously forming a relative density of 99.7% alloy parts.
Julio, Rehme, McKown and Yadroitsev also conducted a preliminary exploration on direct forming of selective laser melting of functional materials, and achieved some results, such as: Julio using selective laser melting with heat radiating pipe directly to produce materials; Rehme using selective laser melting directly to produce porous medical implant materials with cellular structure, while the McKown can directly create grid materials; Yadroitsev the study of selective laser melting manufacturing parts filter material with microporous structure directly.