Surface quality in arc spraying - optimising the parameters
TWI Bulletin, May/June 1991
Since 1966 when Amin joined TWI he has earned a reputation as an authority on synergic control in MIG welding. The results of his work are incorporated in MIG welding power sources the world over.
He has also studied the arc and metal transfer behaviour of various materials used in the MIG process and has originated a patent for a shielding gas mixture for optimum weld quality. More recently his attention has been directed to the arc spray process and TWIs arc spraying equipment.
Recent months have seen intense interest in Abington's arc spraying work. For applying corrosion and abrasion protection to large outdoor structures the process is ideal; fast, cheap and effective. Mohammed Amin investigates the influence which the process parameters have on the roughness and porosity of the sprayed surface.
The ARC spray process is used for various industrial applications to deposit a metallic coating on a substrate for corrosion and abrasion protection. The effectiveness of the coating is determined in part by the level of porosity produced in the coating and by the bond strength between the coating and substrate.
Another essential feature of the coating is the degree of roughness of its surface. For example, production of a smooth surface is normally desired to eliminate or reduce subsequent machining but a rough surface can be required to prevent slip, or to transmit drive by friction between surfaces.
Coatings produced by the arc spray process often contain a high level of porosity and have a low bond strength with the substrate. Porosity is detrimental because it allows corrosive fluids to pass through the coating and thus reduces its ability to protect the substrate.
Coatings with low bond strength can break or flake off prematurely in service. Therefore, the arc spray process is usually restricted to those applications where the high level of porosity can be tolerated, where unpredictable coating failures do not cause serious damage and where the failed coatings can be replaced easily.
The arc spray process has a number of key parameters which determine the characteristics of a coating. In this study, effects of arc current, arc voltage, air pressure and standoff have been determined on the level of porosity and surface roughness of aluminium-silicon coatings on mild steel. Effects of these parameters on bond strength will be reported in a future article. However, the coatings discussed below were firmly attached to the substrate, as indicated by the microscopic examination of the coating/ substrate interface.
Experimental
Effects of arc spray process parameters were examined using the equipment and methods described below.
Spray rig
The spray rig, (Fig. 1), provided movement of the spray gun relative to the substrate surface to be coated. The standoff distance, the coating speed and spray run overlap could be adjusted as desired.
Arc spray system
The arc spray system used was a commercial type (Metallisation Energiser - 300) comprising essentially a power source and a spray gun.
Power source
The power source included a power supply unit, pneumatic control circuit and monitoring instruments, together with an overall electronic control circuit. The unit was designed to provide a current range of 0-300A at 100% duty cycle, with voltage range of 0-45V.
The pneumatic circuit controlled the air input from the mains supply to the spray gun nozzle through a solenoid valve. The specified maximum air supply was 0.85 M 3/min at 5.5bar.
Spray gun The spray gun (Arcspray 528 CG16) included a wire feeder designed to pull two wires of 1.6mm diameter from the reels and feed them into the arc (Fig. 2). An air jet impinged on the arc, so that the molten wire material was projected as a spray of droplets to form a coating on the substrate.
Materials
The substrate was bright mild steel plate 6 x 150 x 200mm. This was coated with spray from two aluminium - 5% silicon wires. The detailed compositions of the wires are given in the Table.
Table - Chemical composition of the two wires used for providing coating material, wt %
| Wire | Mg | Fe | Zn | Si | Mn | Ni | Cr | Mo | V | Cu | Nb | Ti | Al | Pb | Sn | Zr | As |
| A | 0.01 | 0.35 | 0.02 | 5.04 | <0.01 | <0.01 | 0.01 | - | - | 0.02 | - | 0.01 | Ba1 | <0.01 | <0.01 | <0.01 | - |
| B | 0.01 | 0.34 | 0.01 | 5.16 | <0.01 | <0.01 | <0.01 | - | - | <0.01 | - | 0.01 | Ba1 | <0.01 | <0.01 | <0.01 | - |
Substrate surface preparation
The substrate surface was degreased with petroleum ether and then grit blasted using chilled cast iron grit of grade G24.
The degree of roughness was estimated as 'coarse grade' according to the International Standards Organisation Standard ISO 8503- Pt 1.
Coating procedure
For each test, coating was carried out as soon as possible (within 5 min) after surface preparation of the substrate, to reduce growth of the oxide layer and to avoid deposition of dust and moisture. For a given combination of process parameters, constituting a spray condition, five layers were deposited to complete a coat.
Each layer was completed in a number of spray runs deposited by traversing the spray gun back and forth in the horizontal direction at a constant speed of 3 m/min and indexing the spray gun as required in the vertical direction at the end of each run. Each coating was completed in a single operation, i.e. with a continuous series of spray runs from the start to the finish of the test.
Measurement of surface roughness and porosity
The roughness of the coating texture was measured with a commercial roughness measuring instrument (SURFCOM).
The porosities of the coatings were measured, using a computer vision and image analysis system, from photomacrographs of sections cut from the coated plates.
Results
Coating structure
In general, the structure of the coatings was inhomogenous as shown typically in Fig. 3a. There was porosity or voids between some of the solidified droplets, and oxide films were also a significant feature. Individual solidified droplets etched differently giving a layered appearance as shown in Fig. 3b. Isolated droplets had a well-developed Al-Si eutectic structure (Fig. 4), but most droplets either had a very fine background structure (Fig. 5) (possibly a fine eutectic), or showed very little second phase resulting in dark and light-etching and gave the layered appearance. Energy dispersive X-ray analysis of the dark and light areas indicated no apparent difference in silicon level. It is possible that these differences in the microstructure may be due to changes in cooling rates.
Fig. 3. Typical microstructure of the coating deposited at 150A arc current, 25V arc voltage, 4.5bar air pressure and 150mm standoff a) General appearance
b) Dark and light layered appearance
Fig. 4. Typical well-developed Al + Si eutectic structure of an isolated droplet in the coating, deposited at 150A, 20V, 4.5bar and 150mm standoff
Fig. 5. Typical fine background structure (possibly a fine eutectic) deposited at 150A, 35V, 4.5bar and 150mm standoff
Effects of arc voltage
For a given spray condition, the surface texture produced on a coating comprised a uniformly distributed mosaic of solidified flattened droplets or rounded granules, as shown typically in Fig. 6. This texture remained substantially unaffected when the arc voltage was varied from 20 to 35V.
Fig. 6. A typical surface texture of the coating, deposited at 150A, 27.5V, 4.5bar and 150mm standoff
Over the voltage range, the surface roughness was essentially constant within the range 38 to 48µm, as shown in Fig. 7.
Fig. 7. Relationship between surface roughness and arc voltage for coatings deposited at 150A, 4.5bar and 150mm standoff
In general, the porosity comprised small to large pores or voids distributed uniformly throughout the coatings, and varied substantially with arc voltage (Fig. 8). Typically, the porosity is about 15% at 2OV and 18% at 3OV, but only 3% at the intermediate 27.5V. The relationship between porosity and the arc voltage has been plotted in Fig. 9. The main feature of the relationship is that the porosity produced in the coating is minimum, at about 3 to 4%, relevant to the voltage range from 26.5 to 29V. Towards the lower voltage range, the porosity increased gradually at a rate of about 1.5% per volt, but it appeared to increase more sharply as the voltage increased from 29 to 3OV.
Fig. 8. Typical photomacrographs showing the effect of arc voltage on porosity for coatings deposited at 150A, 4.5bar and 150mm standoff: a) Arc voltage 20V
Fig. 9. Relationship between porosity and arc voltage for coatings deposited at 150A, 4.5bar and 150mm standoff
Effects of arc current
For spraying at 120A, the surface texture of the coatings was similar to that produced at 150A, except that the size of the granules was reduced. However, at this lower current surface roughness decreased gradually with increasing voltage, as shown in Fig. 10.
Fig. 10. Relationship between surface roughness and arc voltage for coatings deposited at 120A, 4.5bar and 150mm standoff
The porosity produced at 120A was uniformly distributed in a coating for a given voltage but varied substantially with different voltages. Typically, the porosity was about 16.5% at both 25V and 30V, but only 2.1% at the intermediate voltage of 28V. This behaviour is similar to that found for the arc current at 150A, shown in Fig. 9.
Effects of air pressure
The surface textures produced at the air pressure set at 4 bar and 3.5 bar, using arc current at 150A and arc voltage at 27.5V, were similar to that produced at 4.5bar, (Fig. 7). However, surface roughness decreased as pressure increased (Fig.11).
Fig. 11. Relationship between surface roughness and air pressure for coatings deposited at 150A, 27.5V and 150mm standoff
The porosity produced in the coatings at different air pressures was about 8.1% at 3.5bar, 5.3% at 4.0bar and 3% at 4.5bar. That is, the porosity formed at 4.5bar is approximately one third that at 3.5bar. The relationship between porosity and air pressure is plotted in Fig.12.
Fig. 12. Relationship between porosity and air pressure for coatings deposited at 150A, 27.5V and 150mm standoff
Another effect of air pressure is that the scatter in porosity, which is an indication of non-uniformity of porosity distribution in the coating, is reduced at higher pressures.
Effects of standoff
In general, for a given standoff the texture comprised a mosaic of rounded granules deposited uniformly over the coated surface. The main effect of varying the standoff from 75 to 150mm was that the size and density of the rounded granules were substantially larger at 75mm standoff. Roughness decreased linearly with increasing standoff (Fig.13).
Fig. 13. Relationship between surface roughness and standoff for coatings deposited at 150A, 27.5V and 4.5bar
Porosity
The relationship between porosity and standoff has been plotted in Fig. 14. The porosity level is low and it is contained within narrow limits for a standoff less than 150mm.
Fig. 14. Relationship between porosity and standoff for coatings deposited at 150A, 27.5V and 4.5bar
Discussion
In general, the arc spray process has the capability of depositing Al-5% Si coatings with a surface roughness varying between 30 and 56µm, and with a porosity level varying between 2 and 18%. These variations can be achieved simply by variation of the process parameters.
Surface roughness
Surface roughness is not significantly sensitive to change in arc current or arc voltage. But the entire operating arc voltage range, say 20 to 35V, cannot be used to achieve a required degree of roughness. This is because, for the minimum porosity formation in the coatings which is desirable for most applications, voltage must be within 26.5-29V.
Surface roughness could be varied substantially with a variation in both air pressure and standoff distance; the roughness decreases with an increase in each parameter.
This is because the particle size decreases with an increase in both the air pressure and standoff, and furthermore, the particle velocity increases particularly with an increase in the air pressure. The smaller particles impinging on the substrate with greater impact produce a smoother surface.
However, for minimum porosity in the coatings, the maximum standoff must be restricted to less than about 150mm because, at this point, a transition occurs from low porosity (2-3%) to high porosity (about 14%) (Fig. 14).
Such a transition in porosity has not been observed for variation in air pressure. With increase in air pressure, not only is the surface roughness reduced, but the porosity is decreased and its distribution becomes more uniform.
However, the maximum air pressure which could be obtained has been limited, probably by the mains air supply used, to only 4.5bar. This reduces the surface roughness to about 38µm. For a further reduction in the roughness, ie. to achieve a very fine and smooth surface, it is desirable to extend the pressure range of the air supply, say to about 7bar.
Effects on porosity
Porosity has been measured over the entire spectrum of sizes contained in the coatings. The large size porosity would provide continuous paths for corrosive fluids to reach the substrate; fine porosity might not allow ingress of fluids. Therefore, large porosity is expected to be detrimental but fine porosity might not influence the effectiveness of a coating. This needs to be determined.
a) Arc voltage
There is a specific voltage (about 27.5V) which gives minimum porosity. However, in view of the limited number of tests, scatter in the data and relative 'flatness' of the relationship around the minimum point, the optimum voltage could be taken as from 26.5 to 29V.
b) Air pressure
Porosity decreases as air pressure increases (Fig. 12). In general the velocity of the molten metal droplets, generated in the arc and projected on to the substrate, increases with increasing air pressure. This implies that the porosity is basically voids between the solidified droplets, which remain unfilled and become embedded in the coating. With increase in air pressure, the droplets strike the substrate with greater impact. The molten metal is then driven firmly into the voids on the substrate surface or that of the coating layer deposited previously. Consequently porosity is reduced.
c) Standoff
In this work, a transition in porosity level has been identified at a standoff of about 150mm (Fig. 14). For distances shorter than 150mm the porosity is low and remains practically constant at less than 3%. For the greater standoff, porosity increases suddenly to about 14%.
The behaviour of the porosity/standoff relationship appears to correspond to the effect of the standoff distance on particle velocity. That is, the particle velocity is low (about 10 m/s) in the proximity of the arc, it increases to a maximum (about 50 to 100 m/s) as the distance from the arc increases to about 40mm, and in some cases to about 150mm, and then decreases.
A low particle velocity causes high porosity, and a high produces low porosity. If the correspondence between the two relationships is found in practice, then to deposit coatings with minimum porosity, the standoff distance should be maintained between say 30 to 150mm where the velocity is expected to be a maximum.
Practical considerations
To produce aluminium - 5%Si coatings with minimum porosity, arc voltage, air pressure and standoff should be closely controlled. Arc voltage should be maintained at the optimum (27.5V) with limits 26.5-29V. This can be achieved easily with a power source operating in the constant voltage mode because it provides self-adjustment of arc voltage, and voltage fluctuations are controlled within narrow limits.
The air pressure should be set as high as possible, say 5bar (or more) which is the predicted value for zero porosity. If this pressure causes unstable arc operation it should be reduced gradually.
The standoff should not be allowed to exceed the transition point. In a mechanised operation to coat multi-surface or complex components, the traverse for the spray gun, or the component, should be programmed to follow the contour of the component so that standoff remains essentially constant.
Process improvement
The maximum air pressure available has been limited to 4.5bar. But the predicted air pressure for zero porosity (Fig. 12) is about 5bar. In practice, however, there will be some porosity formed due to gas evolution on solidification of the molten metal. Nevertheless, the major portion of porosity due to voids would be substantially reduced at the predicted pressure. Therefore, to reduce the level of porosity further than that achieved in this work (about 2-3%), the range of the air pressure provided on an arc spray system should be extended to about 7bar with some allowance for the margin.
Conclusions
- Coatings with a surface roughness of 30 to 56µm with a porosity level as low as 3% can be produced by arc spraying.
- Surface roughness is not sensitive to changes in arc current or arc voltage, but it decreases substantially and linearly as the air pressure or standoff increase.
- For low porosity coatings of approximately 3-4%, the arc voltage must be limited within the range 26.5 to 29V, with 27.5V the optimum.
- To deposit coatings with minimum porosity, the standoff should not be allowed to exceed about 150mm.
- Porosity decreases linearly with increasing air pressure. For zero porosity, the predicted pressure is about 5bar. Therefore, the range of air pressure available should be extended to about 7bar.
Acknowledgements
The work was funded jointly by Industrial Members of TWI and the Minerals and Metals Division of the UK Department of Trade and Industry.
