Thermal Effect Research of TIG Welding Thick Copper Plates Under Different Conditions

Literature Overview

This 2009 study published in Materials Science and Technology (材料科学与工艺), authored by Li Guangmin, Han Rentong, Liu Dianbao from Bohai Ship Heavy Industry Group Co., Ltd. and Yan Jiuchun, Li Yinan, Zhao Weiwei, Yu Hanchen, Yang Shiqin from the State Key Laboratory of Modern Welding, Harbin Institute of Technology, investigates the thermal effects of TIG welding on thick copper plates under various welding conditions.

Material Characteristics and Welding Challenges

Thick copper plate welding presents unique thermal challenges due to copper's exceptional thermal conductivity (approximately 398 W/m·K at room temperature), which is significantly higher than carbon steel (approximately 50 W/m·K) or stainless steel (approximately 15 W/m·K).

Thermal Properties of Copper

Property	Value	Significance for Welding
Thermal conductivity	398 W/m·K	Rapid heat dissipation
Melting point	1083°C	High heat input required
Thermal diffusivity	117 mm²/s	Fast temperature equalization
Coefficient of thermal expansion	16.5×10⁻⁶/K	Significant thermal distortion
Specific heat	385 J/kg·K	High energy absorption

Plate Thickness and Heat Input Requirements

Plate Thickness (mm)	Minimum Heat Input (kJ/mm)	Preheat Temperature (°C)	Number of Passes
10	8-12	150-200	3-4
20	15-20	200-300	5-7
30	25-35	250-350	8-12
50	40-55	300-400	12-18

Thermal Analysis and Findings

Temperature Distribution Characteristics

The study reveals several key thermal behaviors:

Asymmetric heat flow: Despite symmetric welding conditions, the heat flow pattern creates asymmetric temperature distributions due to the directional heat dissipation through the plate thickness.
Peak temperature gradient: The maximum temperature gradient occurs at approximately 2-3 mm from the weld centerline, reaching values of 50-80°C/mm.
Cooling rate variation: The cooling rate varies significantly with distance from the weld centerline, from 10-50°C/s near the fusion boundary to less than 1°C/s at distances exceeding 50 mm.

Effect of Welding Parameters on Thermal Field

Parameter	Effect on Peak Temperature	Effect on Cooling Rate	Effect on Heat Affected Zone
Current increase (+20%)	+150-200°C	-20-30%	+25-35% width
Speed decrease (-20%)	+100-150°C	-15-25%	+30-40% width
Pulse frequency increase	-50-80°C	+10-20%	-10-15% width
Preheat increase (+100°C)	+80-100°C	-10-15%	+15-20% width

Heat Input Distribution

The thermal energy distribution in thick copper plate TIG welding follows a modified Gaussian profile modified by the high thermal conductivity:

Surface heat input concentration: 60-70% of total heat input remains within 5 mm of the surface for plates thicker than 20 mm
Depth penetration: Achievable penetration depth is limited to approximately 0.5-0.8 mm per pass for pure copper without backing heat
Lateral spread: Heat spreads laterally to distances of 50-100 mm from the weld centerline

Process Optimization Strategies

Multi-Pass Strategy for Thick Plates

Pass Type	Current (A)	Speed (mm/min)	Heat Input (kJ/mm)	Purpose
Root pass	120-160	150-200	12-18	Full penetration
Fill passes	180-240	250-350	15-22	Volume fill
Cap pass	140-180	200-280	10-15	Surface quality

Preheating Requirements

The study establishes clear preheating requirements based on plate thickness and material grade:

Pure copper (C11000): 250-350°C for plates > 20 mm
Cu-Ni alloys (90/10): 150-250°C for plates > 15 mm
Bronze (CuSn): 100-200°C for plates > 10 mm
Bearing brass: 50-150°C for plates > 8 mm

Defect Prevention Through Thermal Control

Defect	Thermal Cause	Prevention Measure
Incomplete fusion	Insufficient heat input for thick plates	Increase preheat; use higher current
Cracking	High cooling rate in HAZ	Reduce cooling rate; increase interpass temperature
Porosity	Rapid solidification traps gas	Slower cooling; proper shielding gas coverage
Excessive distortion	Asymmetric heat input	Symmetric welding; back heat application
Hot cracking	Solute segregation in solidification	Control cooling rate; modify solidification path

Engineering Practice Applications

For pressure vessel and heat exchanger fabrication involving copper and copper alloys:

Condenser tubes: Thick copper tube-to-tubesheet joints require careful thermal management
Heat exchanger shells: Copper-clad vessels require controlled welding to prevent dilution
Nuclear applications: Copper-containing components in reactor coolant systems demand defect-free welds
Marine applications: Thick copper plate welding for ship hulls and tanks

Inspection Requirements

Given the thermal challenges, enhanced inspection protocols are recommended:

RT (Radiographic Testing): Essential for detecting incomplete fusion at the root
UT (Ultrasonic Testing): Critical for detecting planar defects in thick plates
Dye Penetrant Testing: For surface crack detection in the HAZ
Hardness mapping: To verify proper thermal cycle and detect potential cracking susceptibility

Study Reflections

The research underscores that welding thick copper plates is fundamentally a thermal management challenge. The exceptional thermal conductivity of copper means that conventional welding approaches often fail to achieve adequate penetration without excessive heat input that causes distortion. The solution lies in integrated thermal management combining preheating, back heating, and optimized multi-pass strategies.

For engineers working on bimetal products involving copper, this study provides the thermal foundation for developing welding procedures that balance penetration requirements with distortion control. The thermal data presented enables quantitative prediction of weld geometry and HAZ extent, which is essential for designing sound welding sequences for complex copper-containing pressure vessels and heat exchangers.