Microstructure and Properties of Q890D Steel AZ91 Magnesium Alloy MIG Butt Weld Joints Based on PLC Control

Literature Overview and Research Significance

This 2018 publication by Han Xiaolei, Liu Yu, and Sun Xiaowan from Inner Mongolia Institute of Industry and Technology and Jilin University addresses a highly challenging welding problem: the dissimilar butt welding of Q890D ultra-high strength steel to AZ91 magnesium alloy using MIG welding with PLC-controlled process parameters. The combination of a 960 MPa grade structural steel with a lightweight magnesium alloy represents an extreme metallurgical mismatch that poses significant challenges in terms of intermetallic compound formation, residual stress management, and joint mechanical performance.

Material Characterization and Welding Challenges

Q890D is an ultra-high strength low-alloy steel with a minimum yield strength of 890 MPa, while AZ91 is a cast magnesium alloy containing approximately 9% aluminum and 1% zinc. The fundamental incompatibilities between these two materials include:

Property	Q890D Steel	AZ91 Mg Alloy
Density	7.85 g/cm³	1.81 g/cm³
Melting point	~1450°C	~620°C
Thermal conductivity	50 W/(m·K)	72 W/(m·K)
Coefficient of thermal expansion	12×10⁻⁶/K	26×10⁻⁶/K
Minimum yield strength	890 MPa	175 MPa
Elastic modulus	206 GPa	45 GPa

The enormous difference in melting points, thermal expansion coefficients, and mechanical properties creates severe challenges. The high thermal conductivity of magnesium means that the molten pool on the AZ91 side solidifies rapidly, while the steel side retains heat for longer periods. This asymmetry leads to complex solidification patterns and extensive intermetallic compound formation at the fusion boundary.

PLC-Controlled MIG Welding Process

The researchers employed a PLC (Programmable Logic Controller) system to precisely control the MIG welding parameters throughout the welding sequence. The PLC system enables real-time adjustment of current, voltage, and travel speed based on pre-programmed profiles, ensuring consistent heat input and minimizing the formation of deleterious intermetallic phases. The welding process was divided into distinct stages:

Stage	Current (A)	Voltage (V)	Speed (mm/min)	Purpose
Root pass	120-150	18-20	300-400	Minimal heat input
Fill passes	180-220	22-26	200-300	Building weld volume
Cap pass	150-180	20-24	250-350	Surface quality

The shielding gas employed was pure argon with a flow rate of 15-20 L/min to prevent magnesium oxidation and hydrogen absorption. A preheating temperature of 150-200°C was applied to the AZ91 side to reduce the thermal gradient and minimize cracking susceptibility.

Microstructural Analysis

Metallographic examination of the weld cross-section revealed a complex microstructural gradient. On the Q890D side, the HAZ exhibited tempered martensite and bainite with a hardness of 320-360 HV. The weld metal showed a mixed microstructure of ferrite, pearlite, and intermetallic compounds. At the fusion boundary on the AZ91 side, a continuous layer of Fe-Mg intermetallic compounds was observed, with a thickness of 50-150 μm. The primary intermetallic phases identified were Fe₂Mg₁₇ and Fe₃Mg₂₃, both of which are extremely brittle and significantly reduce the joint's ductility.

The AZ91 HAZ showed a fine-grained microstructure with reduced grain size compared to the base metal, attributed to the rapid cooling rates experienced on the magnesium side. However, the presence of a coarse grain zone adjacent to the fusion line, containing thick intermetallic phases, represents the primary weakness of the joint.

Mechanical Performance and Failure Analysis

Tensile testing of the butt weld joints revealed that failure consistently occurred at the intermetallic compound layer adjacent to the AZ91 fusion line. The average tensile strength of the joints was 180-220 MPa, which is approximately 80-100% of the AZ91 base metal strength but only about 20-25% of the Q890D strength. The elongation was limited to 1.5-3.0%, indicating very low ductility due to the brittle intermetallic layer.

Hardness mapping across the joint cross-section showed a sharp gradient from 340 HV on the Q890D side to 60 HV on the AZ91 side, with a localized peak of 500-600 HV at the intermetallic compound zone. This extreme hardness gradient is a hathe writing systemark of dissimilar steel-magnesium welds and is the primary driver of joint embrittlement.

Engineering Practice Considerations

For engineers considering steel-magnesium dissimilar weldments, this study provides several critical insights. First, the formation of intermetallic compounds is unavoidable and must be minimized through strict control of heat input and cooling rates. Second, the joint strength is fundamentally limited by the magnesium alloy side and the intermetallic layer, regardless of the welding process parameters employed. Third, the use of PLC-controlled welding to precisely manage heat input is a practical approach to optimizing joint performance within the inherent limitations of this material combination.

The practical applications of such joints are limited to scenarios where the load-bearing capacity requirement is modest and weight reduction is the primary design driver. The joints should not be subjected to cyclic loading or impact conditions due to the extremely low fatigue resistance associated with the intermetallic compound layer.

Summary and Reflection

This study provides a thorough characterization of the challenges inherent in welding Q890D steel to AZ91 magnesium alloy. The findings confirm that while butt weld joints can be produced with acceptable static strength, the fundamental metallurgical incompatibility limits their practical applicability. The use of PLC-controlled welding parameters represents a practical engineering approach to optimizing joint quality, but cannot overcome the inherent limitations of intermetallic compound formation. Engineers should exercise caution when considering such dissimilar material joints and should carefully evaluate whether the design requirements can be met within the demonstrated performance envelope.