Material Challenges and Process Countermeasures in Facing machining

In the field of mechanical machining, the quality and efficiency of facing machining not only depend on the equipment accuracy and process parameters, but are also closely related to the physical and chemical properties of the material being processed. The hardness, thermal conductivity, plasticity, chemical activity and other properties of different materials directly determine the tool wear mechanism, cutting force distribution, thermal deformation law and surface quality formation mechanism during the cutting process.

For example, the high hardness of steel parts may cause severe tool wear, the low melting point of aluminum alloys may easily cause cutting adhesion, the low thermal conductivity of titanium alloys may cause local high temperatures, and the hardening tendency of stainless steel may increase the tool load. This article systematically analyzes the technical challenges caused by material characteristics in response to the facing machining problems of typical engineering materials, and proposes targeted process solutions to provide theoretical and practical guidance for material adaptive processing in actual production.

1. Steel facing machining: hardness and heat-affected zone issues

(1) Material properties and machining challenges

Steel parts (such as 45# steel, alloy steel, bearing steel, etc.) are the most common materials in end surface machining. Their main characteristics are high hardness (150-300HB), medium thermal conductivity (45# steel thermal conductivity is about 50W/m・K), and some materials (such as quenched steel) have a hardness of more than 50HRC. The machining difficulties of this type of material are concentrated in two aspects:

1) Tool wear caused by hardness:

When cutting high-hardness steel parts, the stress in the contact area between the tool and the workpiece can reach 2000-3000MPa, which can easily cause abrasive wear of the tool (grinding wheel abrasive particles or workpiece hard points scratch the tool surface) and adhesive wear (element diffusion between the tool and the workpiece material at high temperature).

2) Heat-affected zone caused by cutting heat:

When cutting steel parts, about 70% of the cutting heat is concentrated on the tool, causing the tool tip temperature to rise to 800-1000℃, which not only accelerates the failure of the tool coating (such as TiN coating oxidation above 900℃), but also may cause thermal deformation of the workpiece end face, affecting dimensional accuracy (such as shaft parts with a length of 100mm, the thermal expansion is about 0.006mm when the temperature rises by 50℃).

(2) Process countermeasures

1) Tool selection

● Ordinary carbon steel (such as 45# steel):

Use carbide coated tools (such as TiAlN coated inserts, hardness 3000HV, anti-oxidation temperature 1100℃), the front angle is 5°-10° to balance the cutting force and tool strength, and the main deflection angle is 90° to avoid excessive radial force.

● Hardened steel (hardness > 50HRC):

PCBN (cubic boron nitride) tools (hardness 8000-9000HV) are selected, whose thermal conductivity is more than 3 times that of cemented carbide, which can effectively reduce the cutting temperature. The blade adopts a negative chamfer design (chamfer width 0.2-0.3mm, angle - 5°) to enhance impact resistance.

2) Parameter optimization and cooling

● Cutting speed:

The rough machining of ordinary steel parts is 80-120m/min, and the fine machining is increased to 150-200m/min (using high-speed cutting to reduce the conduction of cutting heat on the workpiece); the processing speed of hardened steel is controlled at 30-50m/min to avoid tool damage caused by excessive speed.

● Coolant:

Use high-pressure internal cooling (pressure 5-10MPa) to supply emulsion (concentration 5%-8%), and the nozzle is aimed at the contact area between the tool tip and the workpiece, which not only cools the tool but also flushes the chips. For precision machining, oil-based cutting fluids (such as mineral oil) can be used to reduce the rust risk of water-based coolants.

2. Aluminum alloy facing machining: cutting adhesion and burr problems

(1) Material properties and machining challenges

Aluminum alloys (such as 6061, 7075, 2A12, etc.) have the characteristics of low density (2.7g/cm³), low hardness (60-150HB), and good thermal conductivity (200-240W/m・K), but there are also two major machining problems:

1) Cutting adhesion (built-up edge):

Aluminum alloys have a low melting point (about 660℃), and it is easy to form built-up edge at the tool tip during cutting (the hardness can be up to 2-3 times that of the parent material), resulting in periodic scratches on the machined surface (depth 0.01-0.05mm).

2) Burr generation:

Aluminum alloy has high plasticity. When the blunt radius of the cutting edge (such as 0.02mm) is greater than the material thickness, burrs are easily formed on the edge of the end face (the height can reach 0.1-0.5mm), affecting the assembly accuracy of parts (such as burrs on the end face of the bearing causing axial clearance to exceed the tolerance).

(2) Process countermeasures

1) Tool optimization

● Tool material:

Choose diamond-coated tools or single-crystal diamond tools (hardness 10000HV). The surface smoothness (Ra<0.01μm) can reduce material adhesion; if cost is limited, fine-grained cemented carbide (grain size ≤1μm) can be selected, and the cutting edge is ground to a blunt radius ≤5μm.

● Geometric parameters:

The front angle is 15°-25° (to reduce cutting force and reduce plastic deformation of the material), the back angle is 10°-15° (to reduce friction between the tool and the workpiece), and the blade inclination angle is 5°-10° (to make the chips flow to the surface to be processed to avoid scratching the processed surface).

2) Process parameter adjustment

● High-speed cutting:

The cutting speed is increased to 200-500m/min (the optimal cutting speed range for aluminum alloys), and the shear slip effect at high speed is used to reduce the formation of built-up edge. For example, when processing 7075-T6 aluminum alloy, the speed is increased from 1000r/min to 3000r/min, and the incidence of built-up edge is reduced from 40% to 5%.

● Minimum quantity lubrication (MQL):

Use plant-based cutting oil for MQL cooling (flow rate 50-100ml/h), which not only lubricates the blade to reduce adhesion, but also avoids aluminum alloy corrosion caused by water-based coolant (such as pitting).

3) Burr control

● Sharp blade processing:

Ensure that the tool edge is free of defects (check the integrity of the edge through a microscope), and control the back cutting amount to 0.1-0.3mm during fine processing, so that the cutting layer thickness is less than the blunt radius of the blade edge, and realize "scraping" rather than "extrusion" molding.

● Edge chamfering pretreatment:

After rough machining, the end face edge is chamfered at 0.5×45° to destroy the burr generation conditions. The burr height can be reduced by more than 60% during fine machining.

3. Titanium alloy facing machining: elastic recovery and tool wear

(1) Material properties and machining challenges

Titanium alloys (such as TC4, Ti-6Al-4V, Ti-5553, etc.) are one of the most difficult materials to machine due to their high strength (tensile strength ≥ 900MPa), low thermal conductivity (6-8W/m・K, only 1/10 of steel), and high chemical activity (easy to react with tool materials at high temperatures). The difficulties in facing machining include:

1) Tool wear caused by elastic recovery:

Titanium alloys have a low elastic modulus (110GPa). The surface rebound of the workpiece after cutting can reach 0.02-0.05mm, causing severe friction between the back face of the tool and the machined surface, exacerbating wear (the back face wear zone VB can reach more than 0.3mm).

2) Diffusion wear at high temperature:

When the cutting temperature exceeds 600℃, the Al and V elements in the titanium alloy diffuse into the tool (such as the Co element in the cemented carbide is precipitated), resulting in a decrease in blade strength, chipping or peeling.

(2) Process countermeasures

1) Tool optimization

● Tool material:

CVD diamond-coated tools (thickness 5-10μm) are preferred, as their chemical inertness can inhibit element diffusion; if processing intermittent surfaces, metal ceramic tools (such as TiC-based ceramics, hardness 1800HV, oxidation resistance temperature 1200℃) are selected, which have better impact resistance than ordinary ceramics.

● Tool structure:

A large back angle (15°-20°) is used to reduce back face friction, and a negative rake angle (-5°-0°) is used to enhance blade strength. The main rake angle is 45°-60° to reduce radial cutting force (avoid workpiece vibration).

2) Cryogenic cutting technology

● Liquid nitrogen cooling (-196℃):

Liquid nitrogen is sprayed into the cutting area through an external nozzle to temporarily increase the hardness of titanium alloy by 10%-15%, reduce elastic recovery, and inhibit the chemical reaction between the tool and the workpiece. Tests show that the tool wear rate is reduced by 50% in a low temperature environment, and the cutting temperature is reduced from 800℃ to below 400℃.

● Cutting parameters:

Low cutting speed (50-100m/min, avoid high temperature areas), medium feed rate (0.1-0.2mm/r, reduce unit cutting edge load), back cutting amount ≥0.5mm (make the cutting edge quickly cut into the material to avoid friction on the hardened layer surface).

3) Surface quality control

● Vibration-assisted processing:

Apply high-frequency vibration (such as 20kHz ultrasonic vibration) to the tool or workpiece, and use the principle of "intermittent cutting" to reduce the contact time between the tool and the workpiece, reduce friction heat, and improve the chip morphology (from strip chips to debris, which is convenient for chip removal).

4. Stainless steel facing machining: work hardening and heat dissipation challenges

(1) Material properties and processing challenges

Stainless steel (such as 304, 316, 17-4PH, etc.) has the characteristics of high toughness (elongation ≥ 40%), strong work hardening tendency (hardening index 0.3-0.5), and poor thermal conductivity (16-20W/m・K). The following problems are faced during end face processing:

1) Work hardening layer:

During the cutting process, the hardness of the surface material can be increased by 50%-100% (such as 304 stainless steel from 200HB to 350HB), causing the tool to slide on the hard point during subsequent cutting, aggravating the edge wear (especially at the arc of the tool tip).

2) Cutting heat concentration:

It is difficult for heat to dissipate through the workpiece and chips. More than 70% of the cutting heat is concentrated on the tool, resulting in coating failure (such as TiCN coating decomposition above 700℃) and blade softening (the hardness of high-speed steel tools drops significantly when the temperature exceeds 600℃).

(2) Process countermeasures

1) Key points of tool design

● Sharp cutting edge and large rake angle:

The rake angle is 12°-18° (reduce cutting force and reduce the depth of hardened layer), the radius of the blunt circle of the cutting edge is ≤10μm (avoid hardening caused by extruded materials), and the spiral blade design (such as the helix angle of 35°-45° for end mills) is used to enhance chip removal ability.

● Coating selection:

TiAlN coating (anti-oxidation temperature 1100℃) or CrN coating (excellent anti-adhesion performance), coating thickness 3-5μm, can effectively isolate the direct contact between the tool and the workpiece and reduce element diffusion.

2) Parameter optimization and cooling

● Medium cutting speed:

Avoid severe hardening at low speed (<50m/min) and overheating at high speed (>200m/min), the recommended speed is 80-150m/min (such as 120m/min for finishing of 304 stainless steel).

● Large feed rate:

Roughing feed rate is 0.2-0.4mm/r, so that the thickness of the cutting layer exceeds the depth of the hardened layer (usually 0.05-0.1mm), avoiding the tool cutting on the hardened layer; finishing feed rate is 0.1-0.2mm/r, with a small back cut (0.5-1mm) to remove the hardened layer.

● High-pressure cooling:

Use water-based cutting fluid (concentration 8%-10%), pressure ≥8MPa, directly flush the cutting area, take away heat and inhibit the formation of hardened layer.

3) Prevention of work hardening

● Staged processing:

Roughing removes 80% of the allowance and reserves 1-2mm finishing allowance; semi-finishing uses a larger back cut (1-2mm) to remove possible hardened layers; finishing uses "light cutting" (back cut 0.2-0.5mm) to ensure processing on unhardened base materials.

5. Summary

Facing machining may seem simple on the surface, but it actually contains complex variables. Material properties are undoubtedly the core factors that affect machining strategies. Different materials exhibit completely different machining behaviors in facing machining. Tool wear patterns, heat treatment responses, and cutting force performance all require tailored strategies. For manufacturing companies, establishing a material-process database, accumulating experience data, and introducing intelligent monitoring will be an effective way to improve facing machining efficiency and accuracy.