Abstract:
Using Abrasive Flow Machining (AFM) to finish high performance engine cylinder heads enhances engine efficiency, performance and durability. For this cylinder head application, Al 319 was selected as the material of choice. Two AFM approaches can be utilized on the production line, one-way or two-way. Factors that affect quality are abrasive grain size, medium viscosity, temperature control, pressure and flow rates. Quality control is essential to guaranteeing a capable process, proper gauging and inspection frequencies are crucial. Geometric and thermal factors also play a role in surface finish output. Cost and environmental effect attributes were also considered.
Key words:
Abrasive Flow Machining (AFM) Abrasive Medium Economic Engine Performance Environment Quality Control
Table of Contents
Introduction: Project scope, assumptions and thesis
Abrasive Flow Machining: A brief Definition 1
Thesis: Utilizing AFM to finish High Performance Cylinder Heads: How and Why? 1
Body: AFM - A Deep Dive
Why utilize AFM to finish High Performance Cylinder Heads 2 - 4
Material selection for the cylinder head 4 - 6
AFM factors that affect the finished products' quality 6
Quality control measure for a capable process) 7
AFM: Frictional, geometrical and thermal aspects 8 - 9
The Abrasive medium 10
Process Foot print options: Considerations for cost and the environment 10 - 12
Conclusion: AFM and improved engine performance 12
Introduction:
Abrasive Flow Machining (AFM) is a finishing method of smoothing, deburring and polishing complex geometries like internal passages, bends, cavities edges and producing controlled radii. Areas that are impossible to reach by other machining operations can be machined by AFM process, resulting is a surface finish quality that cannot be matched using conventional machining processes [1].
Having won business for a low displacement high performance four cylinder engine for a major Original Equipment Manufacturer (OEM), my manufacturing process team has been tasked with developing plans to implement AFM on the 16 valve, in-line cylinder head. For this project, the reasons behind the selection of AFM for finishing high performance cylinder heads will be discussed. To be discussed also are the factors that affect the finished products' quality and quality control measures that will guarantee dimensionally accurate, reproducible and cost effective cycles. Material selection for the cylinder head will be visited. Frictional, Geometric and thermal aspects of the AFM process will be explained. In this project, we will also present process footprint options that would allow for an economic and environmentally friendly production plan.
AFM and Engine Performance
On the intake side:
An important factor that governs the power of an internal combustion engine is maximizing the amount of air going intake. More air results in more fuel induction, more fuel is burned and more energy is converted to work. The power output and the mean effective pressure are determined by the quantity of charge (mixture of air and fuel) inducted. Engine induction effectiveness is measured as volumetric efficiency. -------------------------- (1) where is the air mass flow rate per stroke, N is revolutions per second, Vsw is the swept volume and Ïa,i is the inlet air density. [k2]
At rated speed (Max RPM) , Air density, Swept volume and Engine speed are constant so the air mass flow rate from equation (1) is the only variable which can be increased in order to improve the engine's volumetric efficiency. Smoother intake ports can increase the mass flow rate.
On the exhaust side:
The intake pressure is below atmospheric pressure but the exhaust pressure is nearly or above atmospheric. This causes a negative work loop in the entire cycle and hence reduces the net work. However, the exhaust manifold is positive during the stroke.. The exhaust port pressure influences the flow rate of the exhaust gases. Smooth exhaust ports actually increase the flow rate of the exhaust gases and decreases the port pressure. This improves volumetric efficiency [3].
Volumetric efficiency can be increased in several ways. Using larger valves or increasing the number of valves per cylinder. Larger valves increase flow but increase the weight, multiple valves per cylinder also increases the number of mechanical components in the engine, increasing the weight and production cost. Porting is a common approach to increase the volumetric efficiency but is inaccurate and time consuming.
Effect of AFM on Engine performance
Utilizing AFM allows for the increase of the volumetric efficiency using an abrasive media to polish, debur and edge-radius the intake and exhaust ports. The ports are impossible to reach by traditional machining operations. AFM results in a surface finish quality that is superior to that achieved by conventional machining processes
AFM process has ability to increase airflow on all types of internal combustion engines. Reducing the resistance in intake and exhaust passages will increase engine efficiency and performance. Mixing the Fuel and air is more effective with smoother intake passages. This results in higher volumetric efficiency. A 10 % increase in power and torque can be seen for an engine with a cylinder head machined by AFM.
Abrasive flow machining process creates a passage with less restriction to flow, that allows charge and exhaust gases to flow more quickly through the ports, maximizing the flow velocity. These results in a reduced pumping work (negative work) during exhaust stroke and increases volumetric efficiency during Suction stroke. The process increases the flow rate in an aluminum cylinder head by 30% [4].
Engine durability:
Engine components are subjected to complex thermal and mechanical loading conditions. AFM induces compressive residual stresses in the cylinder head; residual stresses relax during thermal expansion and prevent thermal cracking thus increasing the durability and the service life of the cylinder head [5].
Part material selection
Material selection for a parts is key to design and manufacturing process success. "Availability of materials and their costs are important considerations, but it is the performance which is the most important criterion for selection of any engineering material for a specific application" [6]. In our case, high hardness media is required. A wide range of materials can accommodate this requirement. Due to the complicated internal shape of cool passage and inlet and exhausted port, materials with good castability should be chosen. The high strength -especially under high temperature conditions - is also necessary to ensure that head withstands of combustion force resulting from thermal expansion and contraction when fixed rigidly to the cylinder block. The cylinder head is exposed to low cycle fatigue (LCF) due to the thermal deformation during start/shut-off engine cycles. LCF can cause cracks in thin-walled areas. Higher ductility materials are advantageous in avoiding low cycle fatigue and improving the engine's life [7].
Table 1: Comparison of common use cast alloys for castability and machinability [8]
From table 1, the excellent castability and machinability characteristics of Aluminum make it a great candidate for our cylinder head. Its high thermal conductivity, good corrosion resistance and light weight density (at least 50% less than the cast iron) cylinder heads make it favorable to other cast alloys [9].
Cast aluminum's major chemical composition is aluminum along with different other metal elements. Major elements include silicon, copper and magnesium. Silicon is very hard phase thus contributes to the alloy's wear resistance and to its lower thermal expansion coefficient due to the specific gravity. Copper allows for an increased strength and hardness at both room and elevated temperatures; however, one of copper's disadvantages is its low corrosion resistance. Magnesium combines with silicon to form a hardening phase that strengthens and hardens the alloy [10].
Table 2: Cast aluminum alloy series [11]
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For our application, we have chosen Al 3xx alloy family (alloying elements presented in table 2 above). It takes advantage of multi-elements to get better balance regards to castability, machinability, and surface hardenability, corrosion and wear resistance and cost of manufacturing. For our application, the best combinations of strength and ductility are offered by cast alloy with low iron such as A319 (AlSi8Cu3) can be selected. This Cu and Ni-containing alloys can provide better high strength at elevated temperatures while maintain ductility and fatigue performance. Mg in this alloy is kept in low level to decrease the oxides formation but still can combine with Si to improve hardness and machining characteristics. The concentration by mass percentage for AL 319 is shown in table 3 below:
Table 3: The composition of Al 319 [12]
Al
Si
Fe
Cu
Mn
Mg
Ni
Zn
Ti
90.5%
5-7%
1%
3-5%
0.2-0.65%
0.55%
0.45%
2%
0.25%
AFM factors that affect the finished products' quality:
The quality of the finished surface is based on a number of Abrasive Flow Machining parameters like the work material, design of tooling, the number of cycles, the abrasive grain size, the concentration and the flow speed of the abrasive medium. The number of cycles at any given period of time depends on the velocity of the abrasive medium. More material is removed as the thickness of the abrasive medium increases, however, the surface roughness value of under this condition would be higher in comparison to that during the sue of finer grains. The surface roughness and material removal rate are affected by the extrusion pressure and medium's viscosity. The viscosity is affected by the temperature. With a small increase in temperature the viscosity of the medium reduces drastically thus causing a reduction in material removal effectiveness. As the concentration of the abrasives increases, the viscosity increases. As the size of the abrasive medium increases, the viscosity decreases. The material removal and surface roughness reduction increases with the percentage concentration of abrasive in the medium.
Silicone rubber is a good abrasive medium to polish the surface and give it a smooth surface finish since it has low flow rate and high viscosity. The material removal rate and surface roughness can be improved upon "by applying a magnetic field around the workpiece" [13]; [14].
Quality Control:
Manufacturing teams can design the best process out there, if means to controlling this process are not taken in consideration amid the design phase, the process will not be capable. Process failure mode effects analysis (PFMEA) is a basis for determining what parameters and what operations need to be tightly controlled and monitored. PFMEA is a structured method that allows for the identification of potential failures that may result in a manufacturing process. Quality control is a process in which all the factors that are identified by the PFMEA monitored and checked. The frequency of inspection is established according the dimensional or structural importance.
For the AFM process, Pressure qauges sould be installed and attached to alert systems where a drop below certain values or an increase above a certain value would shut off the system. Hysteresis should be taken into consideration where the pressure has some room to fluctuate (to avoid frequent interruption of the production line) while still not affecting the surface finish quality. Thermocouples need to be installed in order to monitor the work pice's temperature as well as the medium's temperature. As mentioned earlier, a small rise in the abrasive's temperature would alter the surface finish drastically. Fluid viscosity is to be checked on a frequent manner as well. Cycle times are to be monitored and number of cycles are to be counted and reviewed.
The abrasive medium needs to be inspected. The grain sizes should be measured and recorded in order to keep track of the medium's production life. If the medium is wearing out too soon and lasting for too long, the quality team needs to investigate. Production parts needs to be pulled on a regular basis for dimensional inspection and material properties. This data needs to be recorded and charted in a manner that presents the lower limit allowed for the operation, the upper limit allowed for the operation and the mean targeted. Six Sigma analysis determine whether the process is under control or if any modifications are required.
Proper procedures need to be established in regards to cleaning and recycling the abrasive media. These procedures would include inspector signatures that verify that recycle periods are being adhered to and that logs are being updated. A tainted medium would have severe impact on finished product quality. All of these parameters and their associated values (with tolerances) are to be organized in a quality control chart that is posted on the production line and in reach of the operators. This allows operators to check for any suspected errors such as verifying the piston pressure for example. All of the gauges used on this production lines need to calibrated from time to time. Go-No-Go gauges can also be utilized.
AFM in Frictional, Thermal, and Geometrical Aspects:
AFM is similar to stone grinding where the abrasive medium is a "self-deforming" stone. Figure 1 below is a schematic that shows the interaction forces of a single grain on the work piece's surface.
Figure : Free Body Diagram of a simple grain actin on the Work Piece Surface [16]
The sheer force "F" shown above will act on the work piece by the grain (assumed to be a sphere for simplification) and allow the removal of material via a single pass (assumed to be linear for simplification purposes also). The thickness of the removed material will be "d - wear in grain". Two components constitute the force "F", ploughing and frictional forces. The frictional force is the transformation of normal forces due to friction coefficient values that are around 0.2 for AFM.
Figure 1 could be characterized as a kinetic energy transfer between the moving grain and the static work piece. After all, the abrasive compound is mobilized under a specified pressure with a specific volumetric flow. This could be interpreted using Bernoulli's Equation for Fluid Kinetic Energy where assuming laminar flow (where v is fluid velocity and can be deduced from the volumetric flow rate). The frictional component of the force "F" (due to rubbing) will result in a temperature increase of the work piece as well as the abrasive fluid.
As in any balanced energy equation, the input energy will result in output in addition to heat generation due to efficiency losses [17]. The following is a model for the heat exchanged between the work piece and abrasive medium:
Figure : Thermal Model for Heat Exchange between Work Piece and Abrasive [18]
A detailed derivation of the thermal exchange during AFM has been studied by Rajendra K. Jain and V.K. Jain in their paper titled "Specific Energy and Temperature Determination in Abrasive Flow Machining Process". The paper concluded that rise in work piece temperature is directly proportional to the abrasive's pressure and number of cycles. The authors noted that empirical analysis showed that the abrasive medium's viscosity drops significantly with minimal increases in temperature. This minimal change could be as low as 2°C [19]. This viscosity drop would alter the finishing characteristics of the medium and thus create variation in the finished products.
To avoid this issue, the quality control plan should allow for abrasive change overs where multiple abrasive containers (all featuring the same characteristics - density, grain size, viscosity, etc…) are rotated to allow for cooling time in order to avoid viscosity reduction effects on the finished product due to increase in temperature. A proper utilization of the down time for the abrasive container is to clean out the medium from material chatter and recycle it. In Larry Rhoades' " Abrasive Flow Machining: a Case Study", he states that larger passages require very high viscosity (nearly solid) media while smaller passages requires lower viscosity media.
The Geometry of the work piece is a major parameter in the AFM process capabilities and surface finish output. The AFM fixture serves as a clamp for the work piece and as a guide for the abrasive. Improper fixturing could lead to non-uniform Material Removal Rates (MMRs) with roughness errors [20]. The geometry of the product dictates the parameters of the process (pressure, grain size, viscosity, fixturing, etc…). Geometry also dictates the number and order of operations in the machining process. Our work piece is a 16 valve, SOHC Aluminum cylinder head. We will be finishing the intake and exhaust ports.
The Abrasive Medium:
Figure : an example of The Abrasive Media in AFM
A semisolid polymer acts as a carrier to the abrasive grains. This composition is referred to as the abrasive media (see figure 4 above for an image) [21]. An arc furnace is used to produce Silicon carbide (SiC). It is composed of "60% silica sand and 40 percent finely ground petroleum coke. The silicon carbide crystals are further processed into abrasive grains." The grain size and media viscosity are direct contributors to the surface finish. Ports with large diameter to length ratio require a medium with a different grain size and viscosity (large grains, higher viscosity) than those with smaller diameter to length ratios ports; these require smaller grains and lower viscosity media [22].
Process Foot-Print Options:
The following picture is a cross section of a 16 valve cylinder head obtained from the website billzilla.org.
Figure : Cross section of a 16 Valve Cylinder Head
For this project, two different machining approaches can be utilized. Approach A would utilize a Double Piston tooling (see figure 4 below for illustration) where the abrasive medium is pumped through ports one and three on the intake side and ports two and four on the exhaust side. Valve stem bushing ports five and six would be plugged during this operation in order to direct the abrasive flow in a manner that would allow for the uniform machining of corners seven and eight.
Figure : Double Piston Tooling Schematic [23]
Approach B would be to use a single pressure piston where the abrasive would be channeled through ports one and five on the intake side and ports two and six on the exhaust side. The pressure from the single piston would allow for a higher flow velocity in ports five and six thus diverting the abrasive flow towards corners seven and eight allowing for a uniform finished product. The diameter of inlets six and seven needs to be reduced during the casting process in order to compensate for the material removal during the AFM process. Although passages five and six have a smaller diameter, a common abrasive medium would be used, hence it a single piston operation.
Multiple factors are to be considered before selection an approach. These factors include production volumes, cycle durations and numbers, tooling cost (single vs. double piston tooling and piece cost amortization) amongst other factors.
Environmental and Cost Considerations:
During AFM, the abrasive media and the aluminum chips which removed from the work piece will mix together. As we extend the life of the abrasive medium, we lower operational cost and reduce effects on the environment. In simple words, recycled abrasive medium means fewer landfills. To do so, we have to devise an operation that would allow us to separate the aluminum from the abrasive medium post the finishing operation. Since Aluminum is non magnetic, inducing chemical reactions provides us with a route that we can use to recycle the abrasive medium. Two approaches can be followed here. Approach A would be to add chemical ingredients that would target the Aluminum only (keeping the abrasive medium intact). These chemicals would dissolve the aluminum and allow for separation of the 2 components.
Approach B would be similar to approach A except that we would target the abrasive medium with chemicals to disintegrate it. Once the aluminum is separated from the medium, the chemical is removed and the abrasive medium is restored. And since Aluminum ore extraction requires high levels of energy (higher than iron), recycling aluminum saves 95% of the energy usually used to extract aluminum from bauxite. Another reason that aluminum recycling is efficient and economic is the high corrosion resistibility [24], so it can be simply melted and molded to new product with minimum to no additional operations. In contrast, extracting Aluminum from bauxite could be quite expensive, consumes large amount of energy and emits high levels of pollutants into the environment [25]. The reused aluminum ingot for casting generates lower CO2 emission than these from a same amount of ingot from bauxite. Same with other related air such as NOx, SO2 and other organic compounds emissions [26]. Above all, the aluminum recycle proved to be environment friendly and energy-efficient way.
According to the EPA: "Emissions generated in the production of bonded abrasive products may involve a small amount of dust generated by handling the loose abrasive, but careful control of sizes of abrasive particles limits the amount of fine particulate that can be entrained in the ambient air. Devices such as scrubbers and electrostatic precipitators can be used to control particle matter emissions from abrasives grain and products manufacturing" [27].
The material selection of the abrasive medium is extremely important for environmental reasons. Material with high decomposition rates should be favorable when it comes to abrasive medium material selection. Materials should also be selected based on their water solubility levels as well there acidity levels. Materials that are not water-soluble with PH levels closer to 7.0 should be utilized during material selection in order to minimize the effect on the environment. To reduce landfill effects as well as operational costs, large grain size size abrasive medium should also be considered as a starting medium. Once the grain size is too small to perform effectively in the designed process, the media can be utilized in finishing other parts in the plant where smaller size grains are needed to finish lower diameter to length ratio components.
Conclusion: AFM and improved engine performance
In this paper, the Abrasive flow machining process was discussed in regards to its effects on high performance engine cylinder heads. AFM allows for the increase of engine efficiency making it very desirable for high performance engines where application overthrows cost concerns. Aluminum 319 was chosen for the cylinder head application. Factors that affect the finished products' quality and quality control measures were presented. These factors will guarantee dimensionally accurate, reproducible and cost effective cycles. Frictional, Geometric and thermal aspects of the AFM process were also explained and process footprint options that stress on economic and environmentally friendly production plans were discussed.
