Statistical Process Control Application to Weld Process

SUMMARY

Statistical Process Control

Application to Weld Process

By:

Kumari Neha

PGDIE 42

A-42

A statistical weld process monitoring system is de- scribed. Using data (voltage, current, wire feed speed, gas flow rate, travel speed, and elapsed arc time) collected while weld- ing, the welding statistical process control (SPC) tool provides weld process quality control by implementing techniques of data trending  analysis,  tolerance  analysis,  and  sequential  analysis. The SPC system computes the mean, standard deviation, and range of each of the parameters sampled by the data collection system.  Changes  in  the  mean,  standard  deviation, and  range are displayed using control (or trend) charts. The control chart displays a function of a parameter with respect to the ordering of  the  weld  records  (for  a  single weld) or  weld  number (for multiple welds). The SPC tool also permits plotting tolerance charts of the mean, standard deviation, and range for each of the sampled parameters. The tolerance chart is plotted versus the record number (or weld number) and consists of a vertical line for each record (or weld number) showing the minimum and maximum value of that parameter for that record (or weld number). The upper control limit (UCL), lower control limit (LCL), and nominal value may also be displayed on the tolerance chart printout. The SPC also performs sequential analysis, which allows the user to examine the process as it goes along, which in turn may permit the user to locate a possible change in the process before it goes out of control. Sequential analysis makes use of the running average and running standard deviation. Simply stated, the mean and standard deviation are constantly being updated as  new values are read into the list. By examining each new value versus the accumulated information concerning previous values, a determination can be made of any suspect nature of the weld just completed. Work directed toward developing an expert interpreter of the voluminous statistical output generated by the SPC is also described.

THE  ABILITY  to  monitor  welding  performance  is  of crucial importance to manual welding and is of major importance to mechanized and automatic welding as well, par- ticularly on those applications that demand stringent adherence to quality assurance specifications and documentation of the welding results. The consumable electrode welding arc, under steady-state conditions, is maintained at the gap between the tip of the melting electrode and the molten pool of the workpiece [2]. The electrode is continuously fed into the arc and is melted by the heat of the arc. The molten metal of the electrode transfers across the arc gap to the workpiece, where it is deposited and upon solidification becomes the deposited weld metal.

There are various types of control charts, each having its own specific purpose [8]. Since the objective here is to monitor sampled values of the indirect weld parameters, e.g., voltage, current, travel speed, wirefeed speed, etc., the (Shewhart) control charts [8] for measurement of characteristics is chosen. These charts are known as control charts for variables. They are known as the  X-bar (average) chart, the R (range) chart, and the s (sample standard deviation) chart.For purposes of quality control, the control charts provided by the SPC program may be specified in the weld procedure acceptance specifications.

A statistical process control tool has been described that provides weld quality control and documentation by im- plementing techniques of data trending analysis, tolerance analysis, and sequential analysis. The SPC tool has been used in combination with an arc data acquisition and monitoring system for industrial weld quality assurance. Rules have also been developed for providing equipment/materials diagnostic assistance based on  observations of  the  SPC  control chart trends.