Part 1 The meaning of thermal spraying
Thermal spraying is a series of processes, in which processes finely dispersed metallic or nonmetallic coating materials with a molten or semi-molten state deposite on the prepared surface of the substrate, forming some coating deposition layer. It is a technology which use some pyrogen (like arc, plasma spraying) heat powdered or wired metal or non-metal materials to a molten or semi-molten state, then inject onto the pretreated surface of the substrate with a constant speed by means of the flame itself or compressed air, deposit and form a surface coating with varies functions.
Part 2 Thermal spraying methods and application industry
Thermal spraying methods include flame spraying, oxygen-B flame spraying (oxyacetylene flame powder, spraying oxyacetylene flame wire spray, oxyacetylene flame spray, High Velocity Oxygen Fuel); arc spraying; plasma spraying (atmospheric plasma spraying, low pressure plasma spraying).
Thermal spraying is widely used in Chemistry (Varies of vessels, reactors, pipes, pumps, valves and the sealing components), transportation (such as gear shaft, mud pump water seal neck, hopper door, paddle wheel of slurry pump, etc), electronic power(Ball mill, steam turbine rotor and the generator rotor shaft cylinder surface),Metallurgy (Guide roller and conveyor roller)industry because of thermal spraying easily operation, strong adaptability and good economic effect advantages.
Part 3 Thermal spraying Process
Process: Surface pretreatment-Preheating-Spraying- spraying post-processing
Surface pretreatment: In order to combine spraying layer with basis materials well, the substrate surface must be clean and rough, there are so many ways purifying and roughening the surface and the methods selection depend on the fit requirement of the spraying layer ,the substrate materials, shape, thickness, original surface condition and construction condition factors.
Purifying Process aims at removing all the dirty on the workpiece surface, such as oxides and grunge, ect. Roughening Process aims at increasing contact surface area and mechanical bite force between spraying layer and substrate materials. For some coating materials which bonds bad, we should choose one kind material which combine well with the substrate material to spray a transition layer, called Adhesive primer layer, thickness usually is 0.8-0.18μm.
Pretreatment:All aims are to eliminate the moisture on the workpiece surface, improve interface temperature between spraying particles and workpiece, improve adhesive strength between spraying layer and substratum.
Spraying: The selection of spraying methods depends on the choices of spraying materials, workpiece condition and the coating quality requirement.
The pretreated workpiece need be spraying within short time, the spray parameters need depend on the spraying materials, spray gun performance and the workpiece conditional details. Optimized spraying conditions can improve spraying efficiency and obtain high quality coating with high density and high bonding strength.
Spraying post-treatment: The coatings after thermal spraying can’t be used directly, it must process a series of post-processing. For workpiece which need obey high stress load or impact wear, in order to improve adhesive strength of the coating ,it need re-melt sprayed layer and change porous sprayer layer combined with substratum only with mechanical bonding to dense coatings which combined with substratum with metallurgy.
It need machining spraying layers for workpieces which has precision requirements. Because spraying layer has some characters different from common metal and ceramics materials. The coating layer itself strength is low ,can’t obey large cutting force; and the coating layer has so many hard spots which abrade the cutting tools quickly, so forming the difficult to machine character of the spraying layer which is different from common materials. So we need choose suitable machining technology and cutting tools materials and corresponding parameters to make sure reach all the dimension precisions.
Part 4 Machining of the workpiece after thermal spraying
As above already mentioned the workpiece which has dimensional accuracy requirements, it need machine to make sure satisfy all the dimension and precision requirements. But the hardness after thermal spraying usually is above HRC 50, some spraying layer hardness can arrive HRC 65,and there are hard particles in the layer , which will require the cutting tools materials not only have high hardness ,but also have good Abrasion resistance and impact resistance.
We can choose the machining methods: turning or grinding, based on the layer thickness of thermal spraying materials, machining allowance and substrate material. Because grinding has low efficiency, we usually choose spraying workpiece which has small machining allowance and spraying thickness; On the contrary, turning instead of grinding process will be cost-effective choice if has large machining allowance.
There are 3 kinds materials for turning thermal spraying materials cutting tools: Carbide tools, Ceramic cutting tools, CBN tools. Now we will introduce how to choose cutting tools materials to machine thermal spraying workpiece based on the hardness.
（1）When the hardness is less than HRC45, we can choose ceramic alloy cutting tools material, and common grades:YS30, YG3X, ect.
（2）When the hardness is more than HRC45,we can choose ceramic alloy or CBN cutting tools; Ceramic inserts and soldering CBN inserts are usually used in finish machining (Cutting Depth better less than 0.3mm )because of their poor impact resistance.; If has large machining allowance ,it can choose Halnn superhard non metal adhesive BN-S20 grade CBN inserts .
When the spraying layer hardness is more than HRC 55, it can not only improve working efficiency and increase the inserts lifetime, but also lower the machining cost with Halnn superhard non-metal adhesive BN-S20 Solid CBN inserts.
Now let us introduce the cutting parameters of Halnn d non-metal adhesive BN-S20 Solid CBN inserts machining thermal spraying layer
Part 5 The cutting parameters of Halnn superhard non-metal adhesive BN-S20 Solid CBN inserts machining thermal spraying layer
For example, machining Cobalt-based alloy spraying materials,
The spraying layer thickness is 2 mm;
The coating hardness is HRC60;
Liner speed Vc=70~130m/min; Feed Rate is 0.7~0.8mm/r,Cutting Depth ap =1.7mm;
Tool lifetime: 150min/edge; it has 8 edges, it will be more economic.
Part 6 Conclusions
With the development of the modern technology, more and more expendable parts use Welding, Spraying and thermal spraying technology to be restored ,make the parts used again. In the restored, the machine manufactures always look for cutting tools materials with high hardness and high quality in order to improve the quality and the working efficiency. For cutting tools industry, as long as continuous comply with the development of the times, research high quality and efficient tool materials, we can promote the development of China's machinery manufacturing industry.