Unfortunately, there is no Excel shortcut key available to merge cells, but you can create one using a macro. Right-click on the main toolbar and select Customize the Ribbon. Check the Developer box from the Main tabs list and click OK. Open the Developer tab, click the icon and then press the first icon below ActiveX Controls. Click anywhere on the worksheet and drag a command button. Next, right-click the button and select View Code: . In the opened window type:
After that, go to Microsoft Excel Options, select Quick Access Toolbar and from the Commands list select Macros. Click on the Merge Cells macro and add it in the customization window. Click OK and then press the Alt key. The shortcut for the Merge Cells command will appear next to its icon in the Quick Access Toolbar: (4 in my case).
Note: Also, you can use Ctrl+1, A, Alt+M, Enter sequence to merge cells.