Ramakrishna Koganti

Resistance Spot Welding (RSW) of Advanced High Strength Steels (AHSS) for Automotive Body Construction

There has been a substantial increase in the use of advanced high strength steel in automotive structures in the last few years. The usage of these materials is projected to grow significantly in the next 5–10 years with new safety and fuel economy regulations. Advanced High Strength Steels (AHSS) are getting popular with superior mechanical properties and weight advantages compared to mild steel materials. These new materials have significant manufacturing challenges, particularly for welding and stamping. Proper understanding of the weldability of these materials is critical for successful application in future vehicle programs. Due to high strength nature of AHSS materials, higher weld forces and longer weld times are needed to weld AHSS materials. In this paper, weld lobe development for DP600, and DP780 steels are discussed. DP600 steels were joined with two different weld equipments and three different electrodes and their influence on mechanical properties are discussed. Development work on the effect of weld tips on button size, and shrinkage voids due to different welding variables is discussed. DP780 EG steel (1.0 mm) is also joined to itself. The weld lobes, mechanical properties (tensile shear and cross tension), cross-section examination, and microhardness of 1.0 mm DP780 EG to 1.0 mm DP780 EG weld joint results are discussed.Copyright

Metal Inert Gas (MIG) Welding Process Optimization for Joining Aluminum 5754 Sheet Material Using OTC/Daihen Equipment

The development of lightweight vehicles, in particular aluminum intensive vehicles, require significant manufacturing process development for joining and assembling aluminum structures. Currently, 5xxx and 6xxx aluminum alloys are being used in various structural applications in a number of lightweight vehicles worldwide. Various joining methods, such as MIG, Laser and adhesive bonding have been investigated as technology enables for high volume joining of 5xxx, and 6xxx series alloys. In this study, metal inert gas (MIG) welding is used to join 5754 non-heat-treatable alloy sheet products. The objective of this study is to develop optimum weld process parameters for non-heat-treatable 5754 aluminum alloys. The MIG welding equipment used in this study is an OTC/Daihen CPD-350 welding systems and DR-4000 pulse power supply. The factors selected to understand the influence of weld process parameters on the mechanical properties and metallurgy (weld penetration) include power input (torch speed, voltage, current, wire feed), pulse frequency, and gas flow rate. Test coupons used in this study were based on a single lap configuration. A full factorial design of experiment (DOE) was conducted to understand the main and interaction effects on joint failure and weld penetration. The joint strengths and weld penetrations are measured for various operating ranges of weld factors. Post weld analysis indicates, power input and gas flow rate are the two signficant factors (statistically) based on lap shear load to failure and weld penentration data. There were no 2-way or 3-way interaction effects observed in ths weld study. Based on the joint strength and weld penetration, optimum weld process factors were determined.Copyright

Resistance Spot Welding (RSW) Evaluation of 1.0 mm Usibor® 1500 P to 2.0 mm Usibor® 1500 P Steel for Automotive Body Structure Applications

There has been a substantial increase in the use of advanced high strength steel (AHSS) in automotive structures in the last few years. The usage of these materials is projected to grow significantly in the next 5–10 years with the introduction of new safety and fuel economy regulations. AHSS are gaining popularity due to their superior mechanical properties and use in parts for weight savings potential, as compared to mild steels. These new materials pose significant manufacturing challenges, particularly for welding and stamping. Proper understanding of the weldability of these materials is critical for successful application on future vehicle programs. Due to the high strength nature of AHSS materials, higher weld forces and longer weld times are often needed to weld these advanced strength steels. In this paper, the weld current lobes, mechanical properties (shear tension and cross tension), metallographic cross-section and microhardness profile of 1.0 mm Usibor® 1500 P and 2.0 mm Usibor® 1500 P joint in a two-metal stackup are discussed. Weld lobes were developed with Medium Frequency Direct Current (MFDC) equipment, ISO-type B16 tips, weld force of 3.42 kN and hold time of 5 cycles. The weld times were varied at 12, 15 and 18 cycles, with each producing current ranges at or below 3.0 kA. Tensile shear and cross tension samples were made at weld time of 15 cycles, with samples showing average loads of 15.73 kN and 4.41 kN, respectively. Also, microhardness assessment using metallographic cross-sections were analyzed at three different weld cycles (12, 15, and 18 cycles). Voids were observed at 12 and 15 weld cycles, however there was no void at 18 cycles. Similar heat affected zones (HAZ) and weld zones were observed for three different weld cycles.© 2010 ASME

Influence of Lubricants on Advanced High Strength Steel Tubes in Bending Process

To reach safety, emissions, and cost objectives, manufacturers of automotive body structural components shape thin gauge, high strength steel tube using a series of manufacturing steps that often include bending, preforming and hydroforming. Challenging grades and bend severity require a sensitive optimization of the tubular bending process. Lubricants play a significant role in establishing a successful bending process. In this study, the performance of two lubricants, Hydrodraw 551 and HFO 20, were investigated for bending Dual Phase 780 (DP780) and High Strength Low Alloy 350 (HSLA350) thin-walled steel tubes. Formability success was evaluated in terms of wrinkling, thinning strain and final geometry. Lubricant performance was found to be sensitive to grade and application site. HFO 20 was found to be a poor choice for bending DP780 tube.Copyright

Structural Optimization Techniques for Developing Efficient Lightweight Vehicles and Components

Developing automotive vehicles and components to achieve light-weight designs and to meet design targets on structural stiffness, modal frequencies, durability, and crashworthiness, can no longer be driven by a “trial-and-error” strategy. Structural optimization tools provide the necessary analyses during the initial stages of vehicle development to arrive at the most efficient and effective designs. In this paper, we illustrate the importance of topological and gage optimization in achieving mass reduction without compromising on the structural requirements through two design examples.Copyright

Static Tensile Strength of Gas Metal Arc Welded (GMAW) Joints of Uncoated Dual Phase 600 (DP600) Steels

With the increasing demand for safety, energy saving and emission reduction, Advanced High Strength Steels (AHSS) have become very attractive steels for automobile makers. The usage of AHSS steels is projected to grow significantly in the next 5–10 years with new safety and fuel economy regulations. These new steels have significant manufacturing challenges, particularly for welding and stamping. Welding of AHSS remains one of the technical challenges in the successful application of AHSS in automobile structures due to Heat affected Zones (HAZ) at the weld joint. In this study Gas Metal Arc Welding (GMAW) of a lap joint configuration consisting of 1.5 mm uncoated DP600 to itself was investigated. The objective of the study was to understand the wire feed rate and torch speed influence on lap joint strength. A two factor, two level, full factorial design of experiment (DOE) was conducted to understand the wire feed and torch speed influence on tensile strength. In order to understand the curvature effect, center point was also considered in the experiment. Based on the statistical analysis, wire feed rate was the only significant factor on static tensile strength. Metallurgical properties of the lap joints were evaluated using optical microscopy. Significant hardness drop of 40% was observed at the HAZ.Copyright

Gas Metal Arc Welding (GMAW) Process Optimization for Uncoated Dual Phase 600 Material Combination With Aluminized Coated and Uncoated Boron Steels for Automotive Body Structural Applications

With the increasing demand for safety, energy saving and emission reduction, Advanced High Strength Steels (AHSS) have become very attractive steels for automobile makers. The usage of AHSS steels is projected to grow significantly in the next 5–10 years as new safety and fuel economy regulations are enacted. These new steels pose significant manufacturing challenges, particularly for welding and stamping. Welding of AHSS remains one of the technical challenges in the successful application of AHSS in automobile structures, especially when durability of the welded structures is required. In this paper, Gas Metal Arc Welding (GMAW) of uncoated DP 600 and boron (coated and uncoated boron) steels were investigated. In the first study, 2.0 mm DP 600 and 2.0 mm uncoated boron lap joints (Joint #1 and #2) were investigated. In the second study, 1.00 mm DP 600 and 2.0 mm USIBOR (aluminized coated boron) lap joints (Joint # 3 and #4) were investigated. Static and fatigue tests were conducted on the four joint configurations. The effects of steel stack-ups and microhardness distribution along the tensile stress flow direction of the joints on fatigue performance defined by fatigue life as well as crack initiation site and propagation path were analyzed. Metallurgical properties of the dissimilar metal lap joints were evaluated using optical microscopy. The boron steel shows a significant drop in hardness at the heat affected zone (HAZ) as compared to the DP600 steel side. It was found that for the 2.0 mm DP600 and 2.0 mm boron steel dissimilar joint, fatigue life of the joint is better when boron steel was on the top of the joint (Joint #2). However, in the case of 1.0 mm DP 600 and 2.0 mm USIBOR lap joint, the fatigue life of the joint is better when 1.0 mm DP 600 was on the top of the joint (Joint # 3). Ductility of boron steel and significant HAZ softening in boron steel are believed to be the key factors for the fatigue failure at the boron steel side (in all four joint configurations).Copyright

Gas Metal Arc Welding (GMAW) Process Optimization of 2.0 mm Uncoated Boron Steel to 1.0 mm Usibor® 1500 P Steel Joint for Automotive Body Structural Applications

With the increasing demand for safety, energy saving and emission reduction, Advanced High Strength Steels (AHSS) have become very attractive steels for automobile makers. The usage of AHSS steels is projected to grow significantly in the next 5–10 years with new safety and fuel economy regulations. These new steels have significant manufacturing challenges, particularly for welding and stamping. Welding of AHSS remains one of the technical challenges in the successful application of AHSS in automobile structures due to heat affected zones (HAZ) at the weld joint. In this study, Gas Metal Arc Welding (GMAW) of a lap joint configuration consisting of 2.0 mm uncoated boron steel and 1.0 mm Usibor® 1500 steel was investigated. The objective of the study was to understand the wire feed rate (WFR) and torch (or robot) travel speed (TTS) influence on lap joint tensile strength. Design of Experiments (DOE) methodology was used to understand the process parameter influence on the joint strength. Based on the statistical analysis, wire feed rate and torch travel speed were significant factors on static tensile strength. The interaction effect between wire feed rate and torch travel speed was not significant. Metallurgical properties of the lap joints were evaluated using optical microscopy. Significant drops in hardness at the HAZ were observed on both Usibor® 1500 P and boron steels.Copyright

Resistance Spot Welding Evaluation of 1.4 mm Electro Galvanized (EG) Dual Phase 780 (DP780) and 1.6 mm Electro Galvanized Transformation Induced Plasticity 780 (TRIP780) Steel for Automotive Body Structural Applications

The usage of advanced high strength steel (AHSS) in automotive body structures is projected to grow significantly in the next 5–10 years with the introduction of new safety and fuel economy regulations. This is due to their superior mechanical properties and weight savings potential. These new materials pose significant manufacturing challenges, particularly for welding and stamping. Due to the high strength nature of AHSS materials, higher weld forces and longer weld times are often needed to weld these advanced steels. In this paper, the weldability of 1.4 mm electro galvanized (EG) Dual Phase 780 (DP780) welded to a 1.6 mm Electro Galvanized (EG) Transformation Induced Plasticity 780 (TRIP780) is discussed. Also, weld current lobes, mechanical properties (shear and cross tension), metallographic cross-section and micro-hardness profile in a two-metal stack-up are discussed. Weld Lobes and a full factorial Design of Experiment (DOE) was conducted. Weld current and hold time are the chosen factors for the DOE. Based on the DOE data analysis, weld current was the significant factor for tensile load and hold time was the significant factor for the cross tension load. There were no interaction effects observed between weld current and hold time o for tensile and cross tensions loads.Copyright

Static Tensile Strength and Process Optimization of Gas Metal Arc Welding (GMAW) for 1.5 mm Electro-Galvanized Transformation Induced Plasticity 780 (TRIP780) Steel Joint for Automotive Body Structural Applications

With the increasing demand for safety, energy saving and emission reduction, Advanced High Strength Steels (AHSS) have become very attractive steels for automobile makers. The usage of AHSS steels is projected to grow significantly in the next 5–10 years with new safety and fuel economy regulations. These new steels have significant manufacturing challenges, particularly for welding and stamping. Welding of AHSS remains one of the technical challenges in the successful application of AHSS in automobile structures due to Heat affected Zones (HAZ) at the weld joint. In this study Gas Metal Arc Welding (GMAW) of a lap joint configuration consisting of 1.5 mm Electro Galvanized (EG) Transformation Induced Plasticity 780 (TRIP780) to itself was investigated. The objective of the study was to understand the wire feed rate and torch speed influence on lap joint strength. Design of Experiments (DOE) was conducted to understand the wire feed and torch speed influence on tensile strength. Based on the statistical analysis, wire feed rate and torch speed were significant factors on static tensile strength. Two way interaction effect between wire feed and torch speed was significant. Metallurgical properties of the lap joints were evaluated using optical microscopy. No significant drop in hardness at HAZ, however, significant hardening was observed at the base metal and weld fillet interface.Copyright