PM Parts Assembly Techniques Guide

Assembly Method Selection Matrix

*Threading requires minimum density of 6.8 g/cm3 for internal threads

1. Press-Fitting (Interference Fits)

Overview

Shaft pressed into PM hub or gear bore creates mechanical joint through interference between mating surfaces.

Design Guidelines

Interference Recommendations by PM Density

General Rule: PM parts typically handle 50-70% of wrought steel interference values due to lower hoop strength from porosity.

Hub Design Requirements

• Minimum wall thickness: 3x interference amount
  • Example: 0.06mm interference requires minimum 0.18mm wall thickness
• Lead-in chamfer: 15-30° angle x 0.5mm depth (facilitates assembly, prevents cracking)
• Corner radii: Minimum R0.5mm at stress concentration locations

Shaft Requirements

• Hardness: Shaft should be harder than PM hub (typically HRB 85+ vs hub HRB 70-75)
• Surface finish: Ra 0.8-1.6 um (controlled roughness improves retention)
• Entry chamfer: 30° x 0.3mm (prevents hub damage during pressing)

Assembly Process Best Practices

1. Carefully align shaft and hub (zero tilt tolerance)
2. Press slowly and steadily (5-10 mm/sec; avoid impact loading)
3. Monitor press force (sudden increase indicates potential cracking)
4. Expected force range: 50-150 kN (for Dia. 50mm, 0.06mm interference, 25mm engagement length)

Retention Strength Estimation

• Axial pull-out force: Typically 0.8-1.2 x press-in force (for roughened shaft surface)
• Torque capacity: Calculate using friction coefficient range 0.15-0.25

Example Calculation (Dia. 50mm x 25mm press-fit, 0.06mm interference):

• Approximate press force: ~80 kN
• Approximate pull-out force: ~70 kN
• Estimated torque capacity: ~350 Nm

Common Failure Modes

• Hub cracking: Excessive interference or insufficient wall thickness
• Inadequate retention: Insufficient interference or overly smooth shaft surface
• Hub expansion: PM density too low (<6.6 g/cm3) for specified interference

2. Threading (Tapped Holes)

Thread Strength in PM Parts

Thread holding strength depends on:

• Material density (higher density = stronger threads)
• Thread engagement length (longer engagement = stronger joint)
• Thread pitch (coarse threads generally stronger than fine threads in PM)

Design Guidelines

Minimum Density Requirements

• External threads (on PM shafts): 6.4 g/cm3 minimum
• Internal threads (tapped holes): 6.8 g/cm3 minimum (due to hoop stress)

Thread Engagement Length Recommendations

Example: M8 bolt in 6.8 g/cm3 PM part requires minimum 16mm thread engagement

Thread Design Considerations

• Prefer coarse threads (M8x1.25 vs M8x1.0) for better strength in PM
• Rolled threads preferred for external threads on PM shafts (work-hardening effect)
• Avoid blind holes when possible (chip evacuation challenges)

Tapping Process Recommendations

• Through-holes: Spiral point taps (chip-pushing design)
• Blind holes: Spiral flute taps (chip-extracting design)
• Speed reduction: Operate at approximately 50% of speed used for wrought steel
• Lubrication: Always use appropriate tapping fluid or cutting oil

Thread Strength Example

M8 threads in FN-0408 PM part (density 6.8 g/cm3):

• 10mm engagement: ~8 kN stripping strength (vs ~12 kN in wrought steel)
• 16mm engagement: ~12 kN stripping strength (adequate for most applications)

Alternatives for Low-Density Parts

• Helicoil inserts - Provide full wrought-strength threads
• Press-in threaded inserts - Molded or pressed into place
• Self-tapping screws - Form threads, reducing stripping risk

3. Welding PM Parts

Challenges Specific to PM

• Porosity effects: Entrapped gases can expand during welding
• Contamination risk: Residual lubricants or oxides
• Reduced ductility: Higher cracking susceptibility

Welding Methods Ranked for PM

1. Laser Welding
   • Small heat-affected zone
   • Rapid process limits gas expansion time
   • Best when distortion and local heat input must be minimized
2. TIG Welding
   • Controlled heat input
   • Clean process with inert gas shielding
   • Often preferred when joint access and operator control are good
3. MIG Welding
   • Faster than TIG but more spatter
   • Requires tighter process control on porous substrates
   • Better suited to less delicate assemblies
4. Conventional Arc Welding
   • High heat input increases warping risk
   • Usually a last-choice option for PM applications
   • Not generally recommended for precision or highly porous parts

Pre-Weld Requirements

• Minimum density: 7.0 g/cm3 (higher preferred)
• Pore sealing: Resin impregnation before welding strongly recommended
• Cleaning: Remove all lubricant residue (bake at 400 deg C if necessary)
• Preheat: 150-250 deg C (reduces thermal shock)

Weldable PM Materials (Best to Worst)

• Best: 316L stainless steel (austenitic, no phase transformation)
• Good: 304 stainless, low-carbon steel (<0.3% C)
• Fair: FN-0408, FC-0808 (preheating essential)
• Poor: High-carbon steels (>0.5% C), parts in hardened condition

Joint Design Recommendations

• Fillet welds preferred over butt welds (easier porosity management)
• Overlap joints provide more joint area and tolerance for defects
• Minimize root gaps (challenging to achieve full penetration in porous material)

4. Brazing

Why Brazing Works Well for PM

Brazing uses filler metal that melts below the base metal melting point, making it particularly suitable for PM parts:

Advantages for PM Applications

• Lower temperature process (no base metal melting = no gas expansion from pores)
• Excellent for dissimilar materials (PM + wrought, PM + ceramic)
• Can seal porosity (filler metal flows into surface pores by capillary action)
• Suitable for leak-tight joints in hydraulic/pneumatic applications

Brazing Methods

Torch Brazing: Manual process for small parts, low volumes Furnace Brazing: High-volume production, complex assemblies Induction Brazing: Localized heating, rapid cycle time

Filler Metal Selection

Joint Design Guidelines

• Gap specification: 0.05-0.15mm (promotes capillary flow)
• Overlap length: 3-5x thickness of thinner part
• Fillet radius: Minimum R0.5mm (improves filler flow)

Example Application

PM gear brazed to steel shaft:

• Material: FN-0408 gear, 4140 steel shaft
• Filler: 45% silver brazing alloy
• Process: Furnace braze at 680 deg C
• Joint strength: 350 MPa (exceeds PM base material strength)

5. Adhesive Bonding

Advantages for PM Parts

Structural adhesives work particularly well with PM because they:

• Require no heat (eliminates distortion risk)
• Fill surface porosity (improves sealing)
• Dampen vibration (elastic adhesive layer)
• Distribute stress over joint area

Adhesive Types for PM Applications

Surface Preparation

1. Degrease: Solvent clean (acetone or isopropyl alcohol)
2. Roughen: Light grit blast or sanding (increases bond area)
3. Apply adhesive: Within 30 minutes of cleaning for best results

Joint Design

• Overlap joints: 10-25mm overlap length (optimizes bond area)
• Avoid peel loading: Design for shear stress
• Gap control: Maintain 0.05-0.2mm bondline thickness

PM-Specific Advantages

• Fills and seals surface porosity
• Distributes stress (avoids PM stress concentration sensitivity)
• No thermal effects on PM microstructure
• Low cost tooling requirements

Limitations

• Service temperature typically limited to <180 deg C
• Difficult or impossible to disassemble
• Cure time can extend assembly cycle (except for fast-cure adhesives)

6. Mechanical Fasteners

Design Considerations for PM

Bolt/Screw Assembly:

• Use washers to distribute bearing loads (PM has lower bearing strength than wrought)
• Torque limitation: Apply approximately 70-80% of wrought steel torque values
• Thread engagement: Follow guidelines in Threading section above
• Lock features: Use lock washers or thread locker to prevent loosening

Riveting PM Parts:

• Solid rivets: Suitable if PM density >7.0 g/cm3
• Hollow rivets: Preferred (lower expansion stress)
• Rivet material: Should be softer than PM part (aluminum or copper)

Dowel Pins:

• Interference: Use approximately 50% of wrought material interference
• Hole tolerance: H7 hole with m6 pin (light press fit)

7. Sintered Joints (Integrated Manufacturing)

Overview

Two PM parts pressed together in same die cavity, then sintered as single assembly.

Advantages

• Strongest possible joint (full metallurgical bond)
• No secondary assembly operation cost
• Complex assemblies achievable in single step

Limitations

• Requires specialized multi-level tooling
• Design constraints (both parts must co-sinter)
• Limited to same material composition
• Tooling development more complex

Example Application

Gear with integral shaft sintered as one piece (common in small motor applications)

Assembly Method Decision Guide

Selection Based on Requirements

High Strength Required?

• Yes -> Press-fit, brazing, or welding (if density >7.0 g/cm3)
• No -> Adhesive bonding or mechanical fasteners

Must Be Disassembled?

• Yes -> Threading or mechanical fasteners
• No -> Press-fit, adhesive, or brazing

Leak-Tight Joint?

• Yes -> Brazing, adhesive bonding, or resin impregnation + press-fit
• No -> Any method suitable

Dissimilar Materials?

• Yes -> Brazing or adhesive bonding (most versatile)
• No -> Any appropriate method

High-Temperature Service (>150 deg C)?

• Yes -> Brazing, welding, or press-fit
• No -> Any method (adhesives have temperature limitations)

Get PM Assembly Design Support

SinterWorks provides assembly engineering consultation:

• Joint design review (interference fits, thread engagement calculations)
• Torque and strength calculations
• Assembly process recommendations
• Prototyping and validation testing

Contact us for assembly design guidance.