Analysis of the possibility of using additive technologies in basic parts manufacturing

. The purpose of the article is to choose a selective laser sintering method suitable for basic parts manufacturing, as well as to analyze mechanical characteristics of parts obtained using selective laser sintering, depending on the technology of their production. Basic parts manufacturing is a complex and expensive process which includes casting or welding, milling of base surfaces, and boring of holes. As a rule, body is the largest part of the structure, and accuracy of its base surfaces manufacturing directly affects performance of the entire mechanism. Method of producing blank parts is of paramount importance in body manufacturing. Parts of this type must ensure precision of the relative position of individual elements, both at rest and in the dynamic mode of machine operation. At the same time, it is known that all methods of blank parts production are characterized by the presence of residual stresses in the blank part. Gradually relaxing, these stresses "come out" in the form of deformations, which leads to displacement of parts and units of the entire mechanism, and a change in their interaction. This directly affects performance and resource of the entire machine.

Depending on the purpose and design features, body parts can be divided into groups. The first group includes box-shaped. To increase rigidity in these parts, walls, ribs and partitions are used. There are one-piece and detachable bodies. The parting plane runs along the axes of the main holes to enable assembly.
The second group includes body parts with smooth inner cylindrical surfaces, which are subject to increased requirements for the accuracy of diameters and geometric shape.
The third group comprising bodies of complex geometric shape. The group includes bodies of steam and gas turbines, pumps, collectors, valves, cranes. Their complex shape is explained by the need to form flows of liquids or gases.
The fourth group is formed by parts with guiding surfaces, such as tables, carriages, calipers. These parts ensure movement of the cutting tool and the blank parts being processed, performing reciprocating or rotational movements along the guiding surfaces. Most machines have parts of this type. Rigidity of bodies from the fourth group is provided by ribs and internal partitions.
The fifth group comprises body parts such as brackets, corners, racks of plates and covers. These parts have a simple design and perform supporting functions to ensure exact position of individual mechanisms, gears, shafts.
The following general technical requirements implying geometric accuracy of their design are imposed on bodies.
1. Accuracy of the geometric shape of flat base surfaces. Deviations from parallelism and plane up to 0.07 mm are allowed for surfaces no larger than 500 mm and up to 0.005 for critical bodies.

Materials and methods
Malleable cast iron and gray cast iron, non-ferrous metal alloys, alloy and carbon steels are used for basic parts manufacture. Generally, the blanks of machine bodies are produced by welding and casting [3][4][5]. Cast blanks are made by casting into sand-clay molds, injection molding, casting by melting models and shell molds. Materials for castings are cast iron, steel, bronze, brass, magnesium and aluminum alloys. Small-sized parts can be made by forging, hot volumetric stamping or cold sheet stamping. Welding is used in small-scale production, in cases where casting is impractical.
In general, bodies are manufactured of cast irons grades SCh 12, SCh 18 and SCh 35, cast steel grades 40L, 40KL and malleable cast irons. Small weight bodies are made of aluminum and magnesium alloys.
Recently, aluminum alloys such as AL4, AL8, AL9, AL19 have been increasingly used. These materials have high casting qualities, corrosion resistance, machinability, and lightness.
Technological process of part processing includes the following operations: 1. base planes processing; 2. main holes processing; 3. roughing and finishing of other surfaces and mounting holes; Surfaces from which the position of most other surfaces is set are selected as bases. Bed Deposition technology is best suited for manufacture of aluminum alloy body parts. The technology is based on the preliminary formation of a uniform layer of powder material using a roller or knife, followed by selective sintering of powder particles in accordance with the current section of the 3D model of the product. Powder material is applied to a special platform. During product manufacture, the position of the construction plane does not change, instead, the platform moves in the vertical direction.
Among the methods of Bed Deposition technology, the following methods are best for use in this case: selective laser melting (SLM), layer-by-layer spraying (FDM), deposition of metal particles from a gas-powder jet ( SLM (Selective Laser Melting) is selective laser melting of metal powder according to mathematical CAD models under the action of powerful laser radiation. As a result of this effect, the powder particles fuse, forming a homogeneous structure. The process is carried out in the chamber of the SLM machine (Fig. 1) filled with an inert gas. (https://blog.iqb.ru/cjp-technology/). Product construction scheme by SLM method: 1camera with material; 2 -recouter; 3 -laser system; 4 -scanning mirror; 5 -product; 6 -construction camera; 7 -construction platform FDM (Fused Deposition Modeling) is a method of forming an object by layer-by-layer fusing of a filament made from the desired alloy. Product construction scheme by FDM method is shown in Fig. 2.
Before printing, the model is automatically divided into horizontal layers and then the ways of moving the print head are calculated. The material is fed into an extruder (heating head), which squeezes a thin thread of molten material onto the cooled platform, forming a layer according to the model data. Then the platform is lowered to the thickness of one layer to apply the next.
After completion of the construction process, auxiliary structures are removed from the product and, if necessary, final processing is carried out.
DMD (Direct Metal Deposition) is a method of direct deposition of material from a gaspowder jet of metal particles. Laser beam, controlled by a computer, moves according to the geometry of the part. A jet of particles is focused into the same area where laser energy is supplied. The laser melts the area of the grown product, forming a local bath of liquid melt. A portion of metal powder is blown into the melt by a jet of inert transporting gas. The product obtained is homogeneous and has high strength.
Material for metal products manufacture is finely dispersed metal powder with a particle size ranging from 4 to 80 microns. Grain size determines thickness of the future product. Table 1 shows chemical composition of aluminum alloy powders used in 3-D printing.  Manufacturing parts by selective laser sintering has much in common with powder metallurgy. It involves fundamentally similar processes of substance transfer: movement through the gas phase; surface or bulk diffusion; flow caused by external loads or viscous flow.
It is possible to distinguish some features of laser sintering process. For example, structure parameters can be controlled in each layer by adjusting process parameters. These include all the characteristics that affect change in temperature, speed and direction of heat removal during crystallization.
Residual porosity is a characteristic feature of parts made by selective laser melting, since porosity reduces mechanical properties of the material.
Pores can form as a result of insufficient laser energy; therefore, metal powder particles do not melt completely. To prevent defects of this type, it is necessary to optimize the SLM process individually for each type of alloy. It is also important to select printing parameters and powder particle size and take into account its chemical composition and gas which fills the working chamber.

Results and Discussion
Equation of mechanical properties dependence of powder material on its porosity: where σ i is some mechanical property of porous material; σ i0 is the same property of material with nonporous structure; П is proportion of pores in the material; B is the coefficient determined by the conditions for obtaining and testing the material. Let us consider the effects of porosity on mechanical properties of materials on the example of powder steel grade SP60HN3M. We obtained graphs of the dependence of such characteristics as strength, elongation and contraction, as well as impact strength on the pore size of the material from the reference data. Graphs in Fig. 3-6 also show characteristics of materials made by the traditional method.
As can be seen in the graphs, when the minimum porosity is reached, mechanical characteristics of products made of powder materials are practically not inferior to those of products manufactured using traditional technology.   Figure 5 shows the graph of the dependence of relative narrowing on the porosity fraction of the material. The graph of the dependence of impact strength of the material on its porosity is shown in Fig. 6.

Conclusions
It can be seen from the above data that porosity of their structure has a significant impact on the mechanical properties of parts obtained by additive technologies.
It is possible to estimate the volume of pores in the structure of the material obtained by selective laser fusion by analyzing images at high magnification.
It should be noted that use of additive technologies does not exclude further mechanical processing using metal-cutting equipment.